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Overview
DRBD bsr implements block devices which replicate their data a block device that replicates data from the local node to all other nodes of a in the cluster. The Here, the actual data and associated related metadata are usually stored redundantly on "ordinary" block devices on stored individually (usually in the case of external metadata) on the “generic” block device volume of each cluster node. Replicated Replication block devices are called must be named by default in /dev/drbdminor by default. They are grouped into resources, with one drbd<minor> format or directly as a symbolic link (letter) to the device. One or more devices per resource . Replication among the devices in a resource takes place in chronological order. With DRBD, we refer to the devices inside a resource as volumes.In DRBD 9, a resource can be replicated are grouped and each device is replicated in parallel. The device inside the resource is defined as a volume, and resources can be duplicated between two or more cluster nodes. The connections between cluster nodes Cluster node-to-node connections are point-to-point links , and use the TCP or a TCP-like protocol. All nodes must be directly connected.DRBD consists of low-level user-space components which interact with the kernel and perform basic operations ( drbdsetup, drbdmeta), a high-level user-space component bsr consists of the basic components bsradm, which understands and processes the DRBD configuration and translates it into basic operations of files, and the low-level components ( drbdadm)bsrsetup, bsrmeta, and a kernel componentbsrcon. The default DRBD basic bsr configuration consists of of /etc/drbd.conf and of and any additional files included from there, usually it contains (typically global_common.conf and all and all * .res files inside /files in the /etc path). Usually each resource is in etc/drbdbsr.d/. It has turned out to be is useful to define each resource in a separate separate * .res filefiles in the path. The configuration files are file is designed so that each cluster node can contain an identical contains the same copy of the entire cluster configuration. The host name of each node determines which parts of the configuration apply ( uname -n). It is highly recommended to keep the cluster configuration on all nodes in sync by manually copying it to all nodes, or by automating the process with csync2 or a similar tool.
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However, sometimes it may be necessary to have the contents of different configuration files for each node, so this is not absolute.
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This example defines a resource r0 which contains a single replicated device with volume number 0. The resource is replicated among hosts alice and bob, which have the IPv4 addresses the volume of letter e as the resource r0 containing a single replication device. This resource replicates between IPv4 addresses 10.1.1.31 and and 10.1.1.32 and the and hosts alice and bob with node identifiers 0 and 1, respectively. On both hosts, the replicated device is called /dev/drbd1The actual data is volume e, and the actual data and metadata are stored on the lower-level device /dev/sda7. The connection between the hosts uses protocol C.Please refer to the DRBD User's Guide[1] for more examplesis stored in volume f. Protocol C is used for connections between hosts.
File Format
DRBD The configuration files consist file consists of sections , which contain other containing different sections and parameters depending on the section typestype. Each section consists of one or more keywords, sometimes a section name, an opening brace (“"{”"), the contents of the section's contents, and a closing brace (“"}”"). Parameters inside within a section consist of a keyword , followed by and one or more keywords or values , and and a semicolon (“";”"). Some parameter values have have a default scale which applies applied when specifying a plain regular number is specified (for example Kilo, or 1024 times the numeric value). Such (e.g. Kilo). These default scales can be overridden by using a suffix (for example, M for Mega). The common suffixes K = 2^10 e.g. Mega for M). Common suffixes are K = 2 ^ 10 = 1024, M = 1024 K, and and G = 1024 M are supported. Comments start can be written beginning with a hash sign (“#”"#") and extend to ending at the end of the line. In addition, any section can be prefixed with the keyword skip, which causes the section and any sub-sections to be ignoredYou can also prefix the keyword skip to all sections to ignore sections and subsections. Additional files can be included with the include file-pattern statement (see glob(7) for the expressions supported in file-pattern)in the include file pattern statement. Include statements are only allowed outside of sectionsthe section.
The following sections are defined (indentation indicates in which context):described below are defined. Indicates that the indented section is a subsection.
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Sections in brackets parentheses affect other parts of the configuration: inside the common section, they composition. The contents of the common section apply to all resources. A disk section inside a resource or on section The disk section of a resource or resource section applies to all volumes of that resource, and a net section inside a resource section the network section of the resource section applies to all connections of that resource. This allows to avoid repeating identical options eliminates the need to repeat the same option for each resource, connection , or volume. Options can be overridden in a more specific resource, connection, on, or volume section.You can override more specific options in the Resources, Connections, Volumes or Volumes section. The peer-device - options are are defined as resync-rate, c-plan-ahead, c-delay-target, c-fill-target, c-max-rate and and c-min-rate. Due to backward comapatibility they can be specified in any disk options section as well, and all disks for backward compatibility. Sections can also be specified. They are inherited into by all relevant connectionslinks. If they are given on connection level they are inherited to all volumes on granted in the connection section, it is inherited by all volumes in that connection. A The "peer-device-options section is started with the disk keyword" section begins with the "disk" keyword.
Sections
common
This section can contain each a disk, handlers, net, options, and startup section. All resources inherit the parameters in these sections as their default values.
connection [name]
Define a connection between two hosts. This section must contain two host parameters or multiple path sections. The optional name is used to refer to the connection in the system log and in other messages. If no name is specified, the peer's host name is used instead.
path
Define a path between two hosts. This section must contain two host parameters.
connection-mesh
Define a connection mesh between multiple hosts. This section must contain a hosts parameter, which has the host names as arguments. This section is a shortcut to define many connections which share the same network options.
disk
Define parameters for a volume. All parameters in this section are optional.
floating [address-family] addr:port
Like the on section, except that instead of the host name a network address is used to determine if it matches a floating section. The node-id parameter in this section is required. If the address parameter is not provided, no connections to peers will be created by default. The device, disk, and meta-disk parameters must be defined in, or inherited by, this section.
global
Define some global parameters. All parameters in this section are optional. Only one global section is allowed in the configuration.
handlers
Define handlers to be invoked when certain events occur. The kernel passes the resource name in the first command-line argument and sets the following environment variables depending on the event's context: •For
For events related to a particular device: the device's minor number in
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BSR_MINOR, the device's volume number in
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BSR_VOLUME.
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For events related to a particular device on a particular peer: the connection endpoints in
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BSR_MY_ADDRESS,
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BSR_MY_AF,
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BSR_PEER_ADDRESS, and
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BSR_PEER_AF; the device's local minor number in
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BSR_MINOR, and the device's volume number in
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BSR_VOLUME.
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For events related to a particular connection: the connection endpoints in
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BSR_MY_ADDRESS,
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BSR_MY_AF,
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BSR_PEER_ADDRESS, and
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BSR_PEER_AF; and, for each device defined for that connection: the device's minor number
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in BSR_MINOR_ volume-number.
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For events that identify a device, if a lower-level device is attached, the lower-level device's device name is passed in
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BSR_BACKING_DEV (or
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BSR_BACKING_DEV_volume-number).
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All parameters in this section are optional. Only a single handler can be defined for each event; if no handler is defined, nothing will happen.
net
Define parameters for a connection. All parameters in this section are optional.
on host-name [...]
Define the properties of a resource on a particular host or set of hosts. Specifying more than one host name can make sense in a setup with IP address failover, for example. The host-name argument must match the Linux host name ( uname -n). Usually contains or inherits at least one volume section. The node-id and address parameters must be defined in this section. The device, disk, and meta-disk parameters must be defined in, or inherited by, this section. A normal configuration file contains two or more on sections for each resource. Also see the floating section.
options
Define parameters for a resource. All parameters in this section are optional.
resource name
Define a resource. Usually contains at least two on sections and at least one connection section.
stacked-on-top-of resource
Used instead of an on section for configuring a stacked resource with three to four nodes. Starting with DRBD 9, stacking is deprecated. It is advised to use resources which are replicated among more than two nodes instead.
startup
Stacking is deprecated in bsr, we recommend using a 1:N replication configuration.
startup
The parameters in this section determine the behavior of a resource at startup time.
volume volume-number
Define a volume within a resource. The volume numbers in the various volume sections of a resource define which devices on which hosts form a replicated device.
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host name [address [address-family] address] [port port-number]
Defines an endpoint for a connection. Each host statement refers to an on section in a resource. If a port number is defined, this endpoint will use the specified port instead of the port defined in the on section. Each connection section must contain exactly two host parameters. Instead of two host parameters the connection may contain multiple path sections.
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host name [address [address-family] address] [port port-number]
Defines an endpoint for a connection. Each host statement refers to an on section in a resource. If a port number is defined, this endpoint will use the specified port instead of the port defined in the on section. Each path section must contain exactly two host parameters.
Section connection-mesh Parameters
hosts name...
Defines all nodes of a mesh. Each name refers to an on section in a resource. The port that is defined in the on section will be used.
Section disk Parameters
al-extents extents
DRBD automatically maintains a "hot" or "active" disk area likely to be written to again soon based on the recent write activity. The "active" disk bsr manages active and recently rewritten areas based on recent disk write operations. When write I / O occurs, the active area can be written to disk immediately, while "but the inactive " disk areas area must be " activated " first, which requires a meta-data write. We also refer to this so metadata write is required here. This active disk area as the "is called activity log". The activity log saves meta-data writes, but the whole log must be resynced upon recovery of a failed node. The .
If you save the metadata write to the activity log, but recover the failed node, you will need to resynchronize over the entire activity log. Therefore, the size of the activity log is a major factor of in how long a resync it will take to resynchronize after the primary crash and how fast a replicated disk will become consistent after a crash. The activity quickly the consistency of the clone disk is achieved. Activity log consists of a number of 4-Megabyte segments; the several 4 MiB unit segments. The al-extents parameter parameter determines how many the number of those segments that can be active at the same timesimultaneously. The default value for for al-extents is 1237is 6001, with a minimum of 7 and a maximum of 65536. Note that the effective maximum may be smaller, depending Depending on how you created generated the device meta data, see also drbdmeta(8) The effective maximum metadata, the maximum valid value may be smaller (see bsrmeta).
The maximum effective value is 919 * (available on-disk activity - log ring - buffer area / 4kB -1), the default 32kB ring-buffer effects a maximum of 6433 (covers more than 25 GiB of data) We recommend to keep this well within the amount your backend storage and replication link are able to resync inside of which is up to 6433 (including 25 GiB or more data) in the default 32KB ring buffer. It's a good idea to keep the size of the activity log within an amount where the backend storage and replication links can be resynchronized in about 5 minutes.
al-updates {yes | no}
With this parameter, the activity log can be turned off entirely (see the al-extents parameter). This will speed up writes because fewer meta-data writes will be necessary, but the entire device needs to be resynchronized opon recovery of a failed primary node. The default value for al-updates is yes.
disk-barrier,
disk-flushes,
disk-drainDRBD
bsr has three methods of handling the ordering of dependent write requests:disk-barrierUse disk barriers to make sure that requests are written to disk in the right order. Barriers ensure that all requests submitted before a barrier make it to the disk before any requests submitted after the barrierways to handle the order of write requests.
disk-flush Performs write I / O to disk and forces flush to write all data to disk. Depending on the platform or drive vendor, the implementation of flush may be different. In the old way, it was used as a technique to bypass the disk cache called 'force unit access', but recently, it is basically implemented as a method to ensure disk writes by emptying the cache. This option is enabled by default.
disk-barrier Use this option to ensure that requests are written to disk in the correct order. The barrier ensures that all requests submitted before the barrier are all requested to disk prior to requests subsequently submitted. This is implemented using 'tagged command queuing'
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of SCSI devices and 'native command queuing'
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of SATA devices. Only some devices and device stacks support this method. The device mapper (LVM) only supports barriers in some configurations.
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Using this option on systems
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that do not support disk
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-barrier can result in data loss or corruption.
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This option was supported by older Linux kernels, but kernels after linux-2.6.36 (or 2.6.32 RHEL6) can no longer
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detect if
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disk-barrier is supported. This option is off by default and
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must be
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explicitly
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enabled
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disk-drain Wait for the request queue to "drain" (that is,
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until the request is complete) before submitting a
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write request.
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To use this method, requests must be stable on disk
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disk-timeout
If the lower-level device on which a DRBD device stores its data does not finish an I/O request within the defined disk-timeout, DRBD treats this as a failure. The lower-level device is detached, and the device's disk state advances to Diskless. If DRBD is connected to one or more peers, the failed request is passed on to one of them. This option is dangerous and may lead to kernel panic! "Aborting" requests, or force-detaching the disk, is intended for completely blocked/hung local backing devices which do no longer complete requests at all, not even do error completions. In this situation, usually a hard-reset and failover is the only way out. By "aborting", basically faking a local error-completion, we allow for a more graceful swichover by cleanly migrating services. Still the affected node has to be rebooted "soon". By completing these requests, we allow the upper layers to re-use the associated data pages. If later the local backing device "recovers", and now DMAs some data from disk into the original request pages, in the best case it will just put random data into unused pages; but typically it will corrupt meanwhile completely unrelated data, causing all sorts of damage. Which means delayed successful completion, especially for READ requests, is a reason to panic(). We assume that a delayed *error* completion is OK, though we still will complain noisily about it. The default value of disk-timeout is 0, which stands for an infinite timeout. Timeouts are specified in units of 0.1 seconds. This option is available since DRBD 8.3.12.
md-flushes
Enable disk flushes and disk barriers on the meta-data device. This option is enabled by default. See the disk-flushes parameter.
on-io-error handler
Configure how DRBD reacts to I/O errors on a lower-level device. The following policies are defined:pass_onChange the disk status to Inconsistent, mark the failed block as inconsistent in the bitmap, and retry the I/O operation on a remote cluster node.call-local-io-errorCall the local-io-error handler (see the handlers section).detachDetach the lower-level device and continue in diskless mode.
read-balancing policy
Distribute read requests among cluster nodes as defined by policy. The supported policies are prefer-local (the default), prefer-remote, round-robin, least-pending, when-congested-remote, 32K-striping, 64K-striping, 128K-striping, 256K-striping, 512K-striping and 1M-striping. This option is available since DRBD 8.4.1.
resync-after res-name/volume
Define that a device should only resynchronize after the specified other device. By default, no order between devices is defined, and all devices will resynchronize in parallel. Depending on the configuration of the lower-level devices, and the available network and disk bandwidth, this can slow down the overall resync process. This option can be used to form a chain or tree of dependencies among devices.
rs-discard-granularity byte
When rs-discard-granularity is set to a non zero, positive value then DRBD tries to do a resync operation in requests of this size. In case such a block contains only zero bytes on the sync source node, the sync target node will issue a discard/trim/unmap command for the area. The value is constrained by the discard granularity of the backing block device. In case rs-discard-granularity is not a multiplier of the discard granularity of the backing block device DRBD rounds it up. The feature only gets active if the backing block device reads back zeroes after a discard command. The default value of is 0. This option is available since 8.4.7.
discard-zeroes-if-aligned {yes | no}
There are several aspects to discard/trim/unmap support on linux block devices. Even if discard is supported in general, it may fail silently, or may partially ignore discard requests. Devices also announce whether reading from unmapped blocks returns defined data (usually zeroes), or undefined data (possibly old data, possibly garbage). If on different nodes, DRBD is backed by devices with differing discard characteristics, discards may lead to data divergence (old data or garbage left over on one backend, zeroes due to unmapped areas on the other backend). Online verify would now potentially report tons of spurious differences. While probably harmless for most use cases (fstrim on a file system), DRBD cannot have that. To play safe, we have to disable discard support, if our local backend (on a Primary) does not support "discard_zeroes_data=true". We also have to translate discards to explicit zero-out on the receiving side, unless the receiving side (Secondary) supports "discard_zeroes_data=true", thereby allocating areas what were supposed to be unmapped. There are some devices (notably the LVM/DM thin provisioning) that are capable of discard, but announce discard_zeroes_data=false. In the case of DM-thin, discards aligned to the chunk size will be unmapped, and reading from unmapped sectors will return zeroes. However, unaligned partial head or tail areas of discard requests will be silently ignored. If we now add a helper to explicitly zero-out these unaligned partial areas, while passing on the discard of the aligned full chunks, we effectively achieve discard_zeroes_data=true on such devices. Setting discard-zeroes-if-aligned to yes will allow DRBD to use discards, and to announce discard_zeroes_data=true, even on backends that announce discard_zeroes_data=false. Setting discard-zeroes-if-aligned to no will cause DRBD to always fall-back to zero-out on the receiving side, and to not even announce discard capabilities on the Primary, if the respective backend announces discard_zeroes_data=false. We used to ignore the discard_zeroes_data setting completely. To not break established and expected behaviour, and suddenly cause fstrim on thin-provisioned LVs to run out-of-space instead of freeing up space, the default value is yes. This option is available since 8.4.7.
Section peer-device-options Parameters
Please note that you open the section with the disk keyword.c-delay-target delay_target,
c-fill-target fill_target,
c-max-rate max_rate,
c-plan-ahead plan_time
Dynamically control the resync speed. This mechanism is enabled by setting the c-plan-ahead parameter to a positive value. The goal is to either fill the buffers along the data path with a defined amount of data if c-fill-target is defined, or to have a defined delay along the path if c-delay-target is defined. The maximum bandwidth is limited by the c-max-rate parameter. The c-plan-ahead parameter defines how fast drbd adapts to changes in the resync speed. It should be set to five times the network round-trip time or more. Common values for c-fill-target for "normal" data paths range from 4K to 100K. If drbd-proxy is used, it is advised to use c-delay-target instead of c-fill-target. The c-delay-target parameter is used if the c-fill-target parameter is undefined or set to 0. The c-delay-target parameter should be set to five times the network round-trip time or more. The c-max-rate option should be set to either the bandwidth available between the DRBD-hosts and the machines hosting DRBD-proxy, or to the available disk bandwidth. The default values of these parameters are: c-plan-ahead = 20 (in units of 0.1 seconds), c-fill-target = 0 (in units of sectors), c-delay-target = 1 (in units of 0.1 seconds), and c-max-rate = 102400 (in units of KiB/s). Dynamic resync speed control is available since DRBD 8.3.9.
c-min-rate min_rate
A node which is primary and sync-source has to schedule application I/O requests and resync I/O requests. The c-min-rate parameter limits how much bandwidth is available for resync I/O; the remaining bandwidth is used for application I/O. A c-min-rate value of 0 means that there is no limit on the resync I/O bandwidth. This can slow down application I/O significantly. Use a value of 1 (1 KiB/s) for the lowest possible resync rate. The default value of c-min-rate is 4096, in units of KiB/s.
resync-rate rate
Define how much bandwidth DRBD may use for resynchronizing. DRBD allows "normal" application I/O even during a resync. If the resync takes up too much bandwidth, application I/O can become very slow. This parameter allows to avoid that. Please note this is option only works when the dynamic resync controller is disabled.
Section global Parameters
dialog-refresh time
The DRBD init script can be used to configure and start DRBD devices, which can involve waiting for other cluster nodes. While waiting, the init script shows the remaining waiting time. The dialog-refresh defines the number of seconds between updates of that countdown. The default value is 1; a value of 0 turns off the countdown.
disable-ip-verification
Normally, DRBD verifies that the IP addresses in the configuration match the host names. Use the disable-ip-verification parameter to disable these checks.
usage-count {yes | no | ask}
A explained on DRBD's Online Usage Counter[2] web page, DRBD includes a mechanism for anonymously counting how many installations are using which versions of DRBD. The results are available on the web page for anyone to see. This parameter defines if a cluster node participates in the usage counter; the supported values are yes, no, and ask (ask the user, the default). We would like to ask users to participate in the online usage counter as this provides us valuable feedback for steering the development of DRBD.
udev-always-use-vnr
When udev asks drbdadm for a list of device related symlinks, drbdadm would suggest symlinks with differing naming conventions, depending on whether the resource has explicit volume VNR { } definitions, or only one single volume with the implicit volume number 0:
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# implicit single volume without "volume 0 {}" block
DEVICE=drbd<minor>
SYMLINK_BY_RES=drbd/by-res/<resource-name>
SYMLINK_BY_DISK=drbd/by-disk/<backing-disk-name>
# explicit volume definition: volume VNR { }
DEVICE=drbd<minor>
SYMLINK_BY_RES=drbd/by-res/<resource-name>/VNR
SYMLINK_BY_DISK=drbd/by-disk/<backing-disk-name>
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until the request is completed. Previously, this option was enabled by default, but is now disabled.
disk-timeout
If the I / O request fails to complete within the disk time defined for the child device that stores the data, bsr treats it as a failure. In this case, the child device is detached, and the disk status of the device is diskless. If bsr is connected to one or more peers, the failed request is forwarded to one of them. This option is dangerous and can lead to a kernel panic. Aborting the request and forcing the disk to be removed is an action for a completely blocked and stopped local backup device that no longer completes the request and returns no errors. In this situation, usually a hard reset and failover is the only way. The default value of disk-timeout is 0, which indicates an infinite timeout. Timeouts are specified in 0.1 second increments.
md-flushes
Enable disk flushes and disk barriers on the meta-data device. This option is enabled by default. See the disk-flushes parameter.
on-io-error handler
Configure how bsr responds to I / O errors on low-level devices. The following policies are defined.
passthrough If an error is returned from a lower device, the block layer is written to OOS and the error is passed to the upper layer. The error block is usually retried I / O by the upper layer, and if it succeeds at the time of retry, the OOS will be resolved naturally, otherwise the OOS will be recorded and left. This is the default for bsr.
call-local-io-error Call the local-io-error handler (see the handlers section).
detach Detach a low-level device and put it into diskless state. In diskless state, I / O cannot be performed and failover is required immediately.
resync-after res-name/volume
Define that a device should only resynchronize after the specified other device. By default, no order between devices is defined, and all devices will resynchronize in parallel. Depending on the configuration of the lower-level devices, and the available network and disk bandwidth, this can slow down the overall resync process. This option can be used to form a chain or tree of dependencies among devices.
Section peer-device-options Parameters
Please note that you open the section with the disk keyword.c-delay-target delay_target,
c-fill-target fill_target,
c-max-rate max_rate,
c-plan-ahead plan_time
Dynamically control the speed of resynchronization. This mechanism can be used by setting the c-plan-ahead parameter to a positive value. The maximum bandwidth is limited by the c-max-rate parameter. The c-plan-ahead parameter defines how quickly bsr adapts to changes in the resynchronization rate. It should be set to at least 5 times the network round-trip time (RTT). When c-fill-target is defined, it tries to fill the buffer with a defined amount of data along the data path, and has a defined delay if c-delay-target is defined. The common value range for c-fill-target for "normal" data paths is 4K to 100K. If you use drx, we recommend using c-delay-target instead of c-fill-target. The c-delay-target parameter is used when the c-fill-target parameter is undefined or set to 0. The c-delay-target parameter should be set to at least 5 times the network round trip time. The c-max-rate option should be set to either the available bandwidth or the available disk bandwidth between the bsr host and the system hosting drx. The default values for these parameters are: c-plan-ahead = 20 (in 0.1 second increments), c-fill-target = 0 (in sector increments), c-delay-target = 1 (in 0.1 second increments) and c-max-rate = 102400 (KiB / s unit).
c-min-rate min_rate
Nodes that are primary and source of synchronization must schedule application I / O requests and synchronization requests. The c-min-rate parameter limits the amount of bandwidth available for resynchronization I / O. The rest of the bandwidth is used for replication of application I / O. If the c-min-rate value is 0, it means there is no limit to the resynchronization I / O bandwidth. This can significantly slow down application I / O. Use the value of 1 (1 KiB / s) for the lowest resynchronization rate. The default value of c-min-rate is 250 in KiB / s.
resync-rate rate
Defines the bandwidth available for resynchronization. bsr allows general application I / O even during resynchronization. If resynchronization takes up too much bandwidth, application I / O can be very slow and this parameter can be avoided. This option only works if the dynamic resync controller is disabled.
Section global Parameters
dialog-refresh time
You can configure and start the device using the bsr initialization script. This may involve waiting for other cluster nodes. While waiting, the init script shows the remaining wait time. Refresh dialog defines the number of seconds between updates to that countdown and defaults to 1. A value of 0 turns countdown off.
disable-ip-verification
Normally, bsr checks if the IP address in the configuration matches the host name. You can disable these checks using the disable-ip-verification parameter.
usage-count {yes | no | ask}
Ability to aggregate usage statistics, but not used by bsr.
Section handlers Parameters
after-resync-target cmd
Called on a resync target when a node state changes from Inconsistent to Consistent when a resync finishes. This handler can be used for removing the snapshot created in the before-resync-target handler.
before-resync-target cmd
Called on a resync target before a resync begins. This handler can be used for creating a snapshot of the lower-level device for the duration of the resync: if the resync source becomes unavailable during a resync, reverting to the snapshot can restore a consistent state.
before-resync-source cmd
Called on a resync source before a resync begins.
out-of-sync cmd
Called on all nodes after a verify finishes and out-of-sync blocks were found. This handler is mainly used for monitoring purposes. An example would be to call a script that sends an alert SMS.
quorum-lost cmd
Called on a Primary that lost quorum. This handler is usually used to reboot the node if it is not possible to restart the application that uses the storage on top of DRBD.
fence-peer cmd
Called when a node should fence a resource on a particular peer. The handler should not use the same communication path that DRBD uses for talking to the peer.
unfence-peer cmd
Called when a node should remove fencing constraints from other nodes.
initial-split-brain cmd
Called when DRBD connects to a peer and detects that the peer is in a split-brain state with the local node. This handler is also called for split-brain scenarios which will be resolved automatically.
local-io-error cmd
Called when an I/O error occurs on a lower-level device.
pri-lost cmd
The local node is currently primary, but DRBD believes that it should become a sync target. The node should give up its primary role.
pri-lost-after-sb cmd
The local node is currently primary, but it has lost the after-split-brain auto recovery procedure. The node should be abandoned.
pri-on-incon-degr cmd
The local node is primary, and neither the local lower-level device nor a lower-level device on a peer is up to date. (The primary has no device to read from or to write to.)
split-brain cmd
DRBD has detected a split-brain situation which could not be resolved automatically. Manual recovery is necessary. This handler can be used to call for administrator attention.
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Define how to react if a split-brain scenario is detected and both nodes are in primary role. (We detect split-brain scenarios when two nodes connect, so split-brain decisions are always among two nodes.) The defined policies are:disconnectNo automatic resynchronization, simply disconnect.violently-as0pSee the violently-as0p policy for after-sb-1pri.call-pri-lost-after-sbCall the pri-lost-after-sb helper program on one of the machines unless that machine can demote to secondary. The helper program is expected to reboot the machine, which brings the node into a secondary role. Which machine runs the helper program is determined by the after-sb-0pri strategy.
allow-two-primaries
The most common way to configure DRBD devices is to allow only one node to be primary (and thus writable) at a time. In some scenarios it is preferable to allow two nodes to be primary at once; a mechanism outside of DRBD then must make sure that writes to the shared, replicated device happen in a coordinated way. This can be done with a shared-storage cluster file system like OCFS2 and GFS, or with virtual machine images and a virtual machine manager that can migrate virtual machines between physical machines. The allow-two-primaries parameter tells DRBD to allow two nodes to be primary at the same time. Never enable this option when using a non-distributed file system; otherwise, data corruption and node crashes will result!
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Normally the automatic after-split-brain policies are only used if current states of the UUIDs do not indicate the presence of a third node. With this option you request that the automatic after-split-brain policies are used as long as the data sets of the nodes are somehow related. This might cause a full sync, if the UUIDs indicate the presence of a third node. (Or double faults led to strange UUID sets.)
connect-int time
As soon as a connection between two nodes is configured with drbdsetup connect, DRBD immediately tries to establish the connection. If this fails, DRBD waits for connect-int seconds and then repeats. The default value of connect-int is 10 seconds.
cram-hmac-alg hash-algorithm
Configure the hash-based message authentication code (HMAC) or secure hash algorithm to use for peer authentication. The kernel supports a number of different algorithms, some of which may be loadable as kernel modules. See the shash algorithms listed in /proc/crypto. By default, cram-hmac-alg is unset. Peer authentication also requires a shared-secret to be configured.
csums-alg hash-algorithm
Normally, when two nodes resynchronize, the sync target requests a piece of out-of-sync data from the sync source, and the sync source sends the data. With many usage patterns, a significant number of those blocks will actually be identical. When a csums-alg algorithm is specified, when requesting a piece of out-of-sync data, the sync target also sends along a hash of the data it currently has. The sync source compares this hash with its own version of the data. It sends the sync target the new data if the hashes differ, and tells it that the data are the same otherwise. This reduces the network bandwidth required, at the cost of higher cpu utilization and possibly increased I/O on the sync target. The csums-alg can be set to one of the secure hash algorithms supported by the kernel; see the shash algorithms listed in /proc/crypto. By default, csums-alg is unset.
csums-after-crash-only
Enabling this option (and csums-alg, above) makes it possible to use the checksum based resync only for the first resync after primary crash, but not for later "network hickups". In most cases, block that are marked as need-to-be-resynced are in fact changed, so calculating checksums, and both reading and writing the blocks on the resync target is all effective overhead. The advantage of checksum based resync is mostly after primary crash recovery, where the recovery marked larger areas (those covered by the activity log) as need-to-be-resynced, just in case. Introduced in 8.4.5.
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DRBD normally relies on the data integrity checks built into the TCP/IP protocol, but if a data integrity algorithm is configured, it will additionally use this algorithm to make sure that the data received over the network match what the sender has sent. If a data integrity error is detected, DRBD will close the network connection and reconnect, which will trigger a resync. The data-integrity-alg can be set to one of the secure hash algorithms supported by the kernel; see the shash algorithms listed in /proc/crypto. By default, this mechanism is turned off. Because of the CPU overhead involved, we recommend not to use this option in production environments. Also see the notes on data integrity below.
fencing fencing_policy
Fencing is a preventive measure to avoid situations where both nodes are primary and disconnected. This is also known as a split-brain situation. DRBD supports the following fencing policies:dont-careNo fencing actions are taken. This is the default policy.resource-onlyIf a node becomes a disconnected primary, it tries to fence the peer. This is done by calling the fence-peer handler. The handler is supposed to reach the peer over an alternative communication path and call ' drbdadm outdate minor' there.resource-and-stonithIf a node becomes a disconnected primary, it freezes all its IO operations and calls its fence-peer handler. The fence-peer handler is supposed to reach the peer over an alternative communication path and call ' drbdadm outdate minor' there. In case it cannot do that, it should stonith the peer. IO is resumed as soon as the situation is resolved. In case the fence-peer handler fails, I/O can be resumed manually with ' drbdadm resume-io'.
ko-count number
If a secondary node fails to complete a write request in ko-count times the timeout parameter, it is excluded from the cluster. The primary node then sets the connection to this secondary node to Standalone. To disable this feature, you should explicitly set it to 0; defaults may change between versions.
max-buffers number
Limits the memory usage per DRBD minor device on the receiving side, or for internal buffers during resync or online-verify. Unit is PAGE_SIZE, which is 4 KiB on most systems. The minimum possible setting is hard coded to 32 (=128 KiB). These buffers are used to hold data blocks while they are written to/read from disk. To avoid possible distributed deadlocks on congestion, this setting is used as a throttle threshold rather than a hard limit. Once more than max-buffers pages are in use, further allocation from this pool is throttled. You want to increase max-buffers if you cannot saturate the IO backend on the receiving side.
max-epoch-size number
Define the maximum number of write requests DRBD may issue before issuing a write barrier. The default value is 2048, with a minimum of 1 and a maximum of 20000. Setting this parameter to a value below 10 is likely to decrease performance.
on-congestion policy,
congestion-fill threshold,
congestion-extents threshold
By default, DRBD blocks when the TCP send queue is full. This prevents applications from generating further write requests until more buffer space becomes available again. When DRBD is used together with DRBD-proxy, it can be better to use the pull-ahead on-congestion policy, which can switch DRBD into ahead/behind mode before the send queue is full. DRBD then records the differences between itself and the peer in its bitmap, but it no longer replicates them to the peer. When enough buffer space becomes available again, the node resynchronizes with the peer and switches back to normal replication. This has the advantage of not blocking application I/O even when the queues fill up, and the disadvantage that peer nodes can fall behind much further. Also, while resynchronizing, peer nodes will become inconsistent. The available congestion policies are block (the default) and pull-ahead. The congestion-fill parameter defines how much data is allowed to be "in flight" in this connection. The default value is 0, which disables this mechanism of congestion control, with a maximum of 10 GiBytes. The congestion-extents parameter defines how many bitmap extents may be active before switching into ahead/behind mode, with the same default and limits as the al-extents parameter. The congestion-extents parameter is effective only when set to a value smaller than al-extents. Ahead/behind mode is available since DRBD 8.3.10.
ping-int interval
When the TCP/IP connection to a peer is idle for more than ping-int seconds, DRBD will send a keep-alive packet to make sure that a failed peer or network connection is detected reasonably soon. The default value is 10 seconds, with a minimum of 1 and a maximum of 120 seconds. The unit is seconds.
ping-timeout timeout
Define the timeout for replies to keep-alive packets. If the peer does not reply within ping-timeout, DRBD will close and try to reestablish the connection. The default value is 0.5 seconds, with a minimum of 0.1 seconds and a maximum of 3 seconds. The unit is tenths of a second.
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Use the specified protocol on this connection. The supported protocols are:AWrites to the DRBD device complete as soon as they have reached the local disk and the TCP/IP send buffer.BWrites to the DRBD device complete as soon as they have reached the local disk, and all peers have acknowledged the receipt of the write requests.CWrites to the DRBD device complete as soon as they have reached the local and all remote disks.
rcvbuf-size size
Configure the size of the TCP/IP receive buffer. A value of 0 (the default) causes the buffer size to adjust dynamically. This parameter usually does not need to be set, but it can be set to a value up to 10 MiB. The default unit is bytes.
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This option helps to solve the cases when the outcome of the resync decision is incompatible with the current role assignment in the cluster. The defined policies are:disconnectNo automatic resynchronization, simply disconnect.violentlyResync to the primary node is allowed, violating the assumption that data on a block device are stable for one of the nodes. Do not use this option, it is dangerous.call-pri-lostCall the pri-lost handler on one of the machines. The handler is expected to reboot the machine, which puts it into secondary role.
shared-secret secret
Configure the shared secret used for peer authentication. The secret is a string of up to 64 characters. Peer authentication also requires the cram-hmac-alg parameter to be set.
sndbuf-size size
Configure the size of the TCP/IP send buffer. Since DRBD 8.0.13 / 8.2.7, a value of 0 (the default) causes the buffer size to adjust dynamically. Values below 32 KiB are harmful to the throughput on this connection. Large buffer sizes can be useful especially when protocol A is used over high-latency networks; the maximum value supported is 10 MiB.
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By default, DRBD uses the TCP_CORK socket option to prevent the kernel from sending partial messages; this results in fewer and bigger packets on the network. Some network stacks can perform worse with this optimization. On these, the tcp-cork parameter can be used to turn this optimization off.
timeout time
Define the timeout for replies over the network: if a peer node does not send an expected reply within the specified timeout, it is considered dead and the TCP/IP connection is closed. The timeout value must be lower than connect-int and lower than ping-int. The default is 6 seconds; the value is specified in tenths of a second.
use-rle
Each replicated device on a cluster node has a separate bitmap for each of its peer devices. The bitmaps are used for tracking the differences between the local and peer device: depending on the cluster state, a disk range can be marked as different from the peer in the device's bitmap, in the peer device's bitmap, or in both bitmaps. When two cluster nodes connect, they exchange each other's bitmaps, and they each compute the union of the local and peer bitmap to determine the overall differences. Bitmaps of very large devices are also relatively large, but they usually compress very well using run-length encoding. This can save time and bandwidth for the bitmap transfers. The use-rle parameter determines if run-length encoding should be used. It is on by default since DRBD 8.4.0.
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address [address-family] address: port
Defines the address family, address, and port of a connection endpoint. The address families ipv4, ipv6, ssocks (Dolphin Interconnect Solutions' "super sockets"), sdp (Infiniband Sockets Direct Protocol), and sci are supported ( sci is an alias for ssocks). If no address family is specified, ipv4 is assumed. For all address families except ipv6, the address is specified in IPV4 address notation (for example, 1.2.3.4). For ipv6, the address is enclosed in brackets and uses IPv6 address notation (for example, [fd01:2345:6789:abcd::1]). The port is always specified as a decimal number from 1 to 65535. On each host, the port numbers must be unique for each address; ports cannot be shared.
node-id value
Defines the unique node identifier for a node in the cluster. Node identifiers are used to identify individual nodes in the network protocol, and to assign bitmap slots to nodes in the metadata. Node identifiers can only be reasssigned in a cluster when the cluster is down. It is essential that the node identifiers in the configuration and in the device metadata are changed consistently on all hosts. To change the metadata, dump the current state with drbdmeta dump-md, adjust the bitmap slot assignment, and update the metadata with drbdmeta restore-md. The node-id parameter exists since DRBD 9. Its value ranges from 0 to 16; there is no default.
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A resource must be promoted to primary role before any of its devices can be mounted or opened for writing. Before DRBD 9, this could only be done explicitly ("drbdadm primary"). Since DRBD 9, the auto-promote parameter allows to automatically promote a resource to primary role when one of its devices is mounted or opened for writing. As soon as all devices are unmounted or closed with no more remaining users, the role of the resource changes back to secondary. Automatic promotion only succeeds if the cluster state allows it (that is, if an explicit drbdadm primary command would succeed). Otherwise, mounting or opening the device fails as it already did before DRBD 9: the mount(2) system call fails with errno set to EROFS (Read-only file system); the open(2) system call fails with errno set to EMEDIUMTYPE (wrong medium type). Irrespective of the auto-promote parameter, if a device is promoted explicitly ( drbdadm primary), it also needs to be demoted explicitly (drbdadm secondary). The auto-promote parameter is available since DRBD 9.0.0, and defaults to yes.
cpu-mask cpu-mask
Set the cpu affinity mask for DRBD kernel threads. The cpu mask is specified as a hexadecimal number. The default value is 0, which lets the scheduler decide which kernel threads run on which CPUs. CPU numbers in cpu-mask which do not exist in the system are ignored.
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Determine how to deal with I/O requests when the requested data is not available locally or remotely (for example, when all disks have failed). The defined policies are:io-errorSystem calls fail with errno set to EIO.suspend-ioThe resource suspends I/O. I/O can be resumed by (re)attaching the lower-level device, by connecting to a peer which has access to the data, or by forcing DRBD to resume I/O with drbdadm resume-io res. When no data is available, forcing I/O to resume will result in the same behavior as the io-error policy. This setting is available since DRBD 8.3.9; the default policy is io-error.
peer-ack-window value
On each node and for each device, DRBD maintains a bitmap of the differences between the local and remote data for each peer device. For example, in a three-node setup (nodes A, B, C) each with a single device, every node maintains one bitmap for each of its peers. When nodes receive write requests, they know how to update the bitmaps for the writing node, but not how to update the bitmaps between themselves. In this example, when a write request propagates from node A to B and C, nodes B and C know that they have the same data as node A, but not whether or not they both have the same data. As a remedy, the writing node occasionally sends peer-ack packets to its peers which tell them which state they are in relative to each other. The peer-ack-window parameter specifies how much data a primary node may send before sending a peer-ack packet. A low value causes increased network traffic; a high value causes less network traffic but higher memory consumption on secondary nodes and higher resync times between the secondary nodes after primary node failures. (Note: peer-ack packets may be sent due to other reasons as well, e.g. membership changes or expiry of the peer-ack-delay timer.) The default value for peer-ack-window is 2 MiB, the default unit is sectors. This option is available since 9.0.0.
peer-ack-delay expiry-time
If after the last finished write request no new write request gets issued for expiry-time, then a peer-ack packet is sent. If a new write request is issued before the timer expires, the timer gets reset to expiry-timea write request propagates from node A to B and C, nodes B and C know that they have the same data as node A, but not whether or not they both have the same data. As a remedy, the writing node occasionally sends peer-ack packets to its peers which tell them which state they are in relative to each other. The peer-ack-window parameter specifies how much data a primary node may send before sending a peer-ack packet. A low value causes increased network traffic; a high value causes less network traffic but higher memory consumption on secondary nodes and higher resync times between the secondary nodes after primary node failures. (Note: peer-ack packets may be sent due to other reasons as well, e.g. membership changes or expiry of the peer-ack-window option.) This parameter may influence resync behavior on remote nodes. Peer nodes need to wait until they receive an peer-ack for releasing a lock on an AL-extent. Resync operations between peers may need to wait for for these locks.delay timer.) The default value for peer-ack-delay is 100 milliseconds, the default unit is milliseconds. This option is available since 9.0.0.
quorum value
When activated, a cluster partition requires quorum in order to modify the replicated data set. That means a node in the cluster partition can only be promoted to primary if the cluster partition has quorum. Every node with a disk directly connected to the node that should be promoted counts. If a primary node should execute a write request, but the cluster partition has lost quorum, it will freeze IO or reject the write request with an error (depending on the on-no-quorum setting). Upon loosing quorum a primary always invokes the quorum-lost handler. The handler is intended for notification purposes, its return code is ignored. The option's value might be set to off, majority, all or a numeric value. If you set it to a numeric value, make sure that the value is greater than half of your number of nodes. Quorum is a mechanism to avoid data divergence, it might be used instead of fencing when there are more than two repicas. It defaults to off If all missing nodes are marked as outdated, a partition always has quorum, no matter how small it is. I.e. If you disconnect all secondary nodes gracefully a single primary continues to operate. In the moment a single secondary is lost, it has to be assumed that it forms a partition with all the missing outdated nodes. In case my partition might be smaller than the other, quorum is lost in this moment. In case you want to allow permanently diskless nodes to gain quorum it is recommendet to not use majority or all. It is recommended to specify an absolute number, since DBRD's heuristic to determine the complete number of diskfull nodes in the cluster is unreliable. The quorum implementation is available starting with the DRBD kernel driver version 9.0.7.
quorum-minimum-redundancy value
This option sets the minimal required number of nodes with an UpToDate disk to allow the partition to gain quorum. This is a different requirement than the plain quorum option expresseswindow is 2 MiB, the default unit is sectors. This option is available since 9.0.0.
peer-ack-delay expiry-time
If after the last finished write request no new write request gets issued for expiry-time, then a peer-ack packet is sent. If a new write request is issued before the timer expires, the timer gets reset to expiry-time. (Note: peer-ack packets may be sent due to other reasons as well, e.g. membership changes or the peer-ack-window option.) This parameter may influence resync behavior on remote nodes. Peer nodes need to wait until they receive an peer-ack for releasing a lock on an AL-extent. Resync operations between peers may need to wait for for these locks. The default value for peer-ack-delay is 100 milliseconds, the default unit is milliseconds. This option is available since 9.0.0.
quorum value
When activated, a cluster partition requires quorum in order to modify the replicated data set. That means a node in the cluster partition can only be promoted to primary if the cluster partition has quorum. Every node with a disk directly connected to the node that should be promoted counts. If a primary node should execute a write request, but the cluster partition has lost quorum, it will freeze IO or reject the write request with an error (depending on the on-no-quorum setting). Upon loosing quorum a primary always invokes the quorum-lost handler. The handler is intended for notification purposes, its return code is ignored. The option's value might be set to off, majority, all or a numeric value. If you set it to a numeric value, make sure that the value is greater than half of your number of nodes. In case you want to allow permanently diskless nodes to gain quorum it is recommendet to not use majority or all. It is recommended to specify an absolute number, since DBRD's heuristic to determine the complete number of diskfull nodes in the cluster is unreliable. This option is available starting with the DRBD kernel driver version 9.0.10.
on-no-quorum {io-error | suspend-io}
By default DRBD freezes IO on a device, that lost quorum. By setting the on-no-quorum to io-error it completes all IO operations with an error if quorum ist lost. The on-no-quorum options is available starting with the DRBD kernel driver version 9.0.8.
Section startup Parameters
The parameters in this section define the behavior of DRBD at system startup time, in the DRBD init script. They have no effect once the system is up and running.degr-wfc-timeout timeout
Define how long to wait until all peers are connected in case the cluster consisted of a single node only when the system went down. This parameter is usually set to a value smaller than wfc-timeout. The assumption here is that peers which were unreachable before a reboot are less likely to be reachable after the reboot, so waiting is less likely to help. The timeout is specified in seconds. The default value is 0, which stands for an infinite timeout. Also see the wfc-timeout parameter.
outdated-wfc-timeout timeout
Define how long to wait until all peers are connected if all peers were outdated when the system went down. This parameter is usually set to a value smaller than wfc-timeout. The assumption here is that an outdated peer cannot have become primary in the meantime, so we don't need to wait for it as long as for a node which was alive before. The timeout is specified in seconds. The default value is 0, which stands for an infinite timeout. Also see the wfc-timeout parameter.
stacked-timeouts
On stacked devices, the wfc-timeout and degr-wfc-timeout parameters in the configuration are usually ignored, and both timeouts are set to twice the connect-int timeout. The stacked-timeouts parameter tells DRBD to use the wfc-timeout and degr-wfc-timeout parameters as defined in the configuration, even on stacked devices. Only use this parameter if the peer of the stacked resource is usually not available, or will not become primary. Incorrect use of this parameter can lead to unexpected split-brain scenarios.
wait-after-sb
This parameter causes DRBD to continue waiting in the init script even when a split-brain situation has been detected, and the nodes therefore refuse to connect to each other.
wfc-timeout timeout
Define how long the init script waits until all peers are connected. This can be useful in combination with a cluster manager which cannot manage DRBD resources: when the cluster manager starts, the DRBD resources will already be up and running. With a more capable cluster manager such as Pacemaker, it makes more sense to let the cluster manager control DRBD resources. The Quorum is a mechanism to avoid data divergence, it might be used instead of fencing when there are more than two repicas. It defaults to off If all missing nodes are marked as outdated, a partition always has quorum, no matter how small it is. I.e. If you disconnect all secondary nodes gracefully a single primary continues to operate. In the moment a single secondary is lost, it has to be assumed that it forms a partition with all the missing outdated nodes. In case my partition might be smaller than the other, quorum is lost in this moment. In case you want to allow permanently diskless nodes to gain quorum it is recommendet to not use majority or all. It is recommended to specify an absolute number, since DBRD's heuristic to determine the complete number of diskfull nodes in the cluster is unreliable. The quorum implementation is available starting with the DRBD kernel driver version 9.0.7.
quorum-minimum-redundancy value
This option sets the minimal required number of nodes with an UpToDate disk to allow the partition to gain quorum. This is a different requirement than the plain quorum option expresses. The option's value might be set to off, majority, all or a numeric value. If you set it to a numeric value, make sure that the value is greater than half of your number of nodes. In case you want to allow permanently diskless nodes to gain quorum it is recommendet to not use majority or all. It is recommended to specify an absolute number, since DBRD's heuristic to determine the complete number of diskfull nodes in the cluster is unreliable. This option is available starting with the DRBD kernel driver version 9.0.10.
on-no-quorum {io-error | suspend-io}
By default DRBD freezes IO on a device, that lost quorum. By setting the on-no-quorum to io-error it completes all IO operations with an error if quorum ist lost. The on-no-quorum options is available starting with the DRBD kernel driver version 9.0.8.
Section startup Parameters
The parameters in this section define the behavior of DRBD at system startup time, in the DRBD init script. They have no effect once the system is up and running.degr-wfc-timeout timeout
Define how long to wait until all peers are connected in case the cluster consisted of a single node only when the system went down. This parameter is usually set to a value smaller than wfc-timeout. The assumption here is that peers which were unreachable before a reboot are less likely to be reachable after the reboot, so waiting is less likely to help. The timeout is specified in seconds. The default value is 0, which stands for an infinite timeout. Also see the degr-wfc-timeout parameter.
Section volume Parameters
device /dev/drbdminor-number
Define the device name and minor number of a replicated block device. This is the device that applications are supposed to access; in most cases, the device is not used directly, but as a file system. This parameter is required and the standard device naming convention is assumed. In addition to this device, udev will create /dev/drbd/by-res/resource /volume and /dev/drbd/by-disk/lower-level-device symlinks to the device.
disk {[disk] | none}
Define the lower-level block device that DRBD will use for storing the actual data. While the replicated drbd device is configured, the lower-level device must not be used directly. Even read-only access with tools like dumpe2fs(8) and similar is not allowed. The keyword none specifies that no lower-level block device is configured; this also overrides inheritance of the lower-level device.
meta-disk internal,
meta-disk device,
meta-disk device [index]
Define where the metadata of a replicated block device resides: it can be internal, meaning that the lower-level device contains both the data and the metadata, or on a separate device. When the index form of this parameter is used, multiple replicated devices can share the same metadata device, each using a separate index. Each index occupies 128 MiB of data, which corresponds to a replicated device size of at most 4 TiB with two cluster nodes. We recommend not to share metadata devices anymore, and to instead use the lvm volume manager for creating metadata devices as needed. When the index form of this parameter is not used, the size of the lower-level device determines the size of the metadata. The size needed is 36 KiB + (size of lower-level device) / 32K * (number of nodes - 1). If the metadata device is bigger than that, the extra space is not used. This parameter is required if a disk other than none is specified, and ignored if disk is set to none. A meta-disk parameter without a disk parameter is not allowed.
NOTES ON DATA INTEGRITY
DRBD supports two different mechanisms for data integrity checking: first, the data-integrity-alg network parameter allows to add a checksum to the data sent over the network. Second, the online verification mechanism ( drbdadm verify and the verify-alg parameter) allows to check for differences in the on-disk data.Both mechanisms can produce false positives if the data is modified during I/O (i.e., while it is being sent over the network or written to disk). This does not always indicate a problem: for example, some file systems and applications do modify data under I/O for certain operations. Swap space can also undergo changes while under I/O.Network data integrity checking tries to identify data modification during I/O by verifying the checksums on the sender side after sending the data. If it detects a mismatch, it logs an error. The receiver also logs an error when it detects a mismatch. Thus, an error logged only on the receiver side indicates an error on the network, and an error logged on both sides indicates data modification under I/O.The most recent example of systematic data corruption was identified as a bug in the TCP offloading engine and driver of a certain type of GBit NIC in 2007: the data corruption happened on the DMA transfer from core memory to the card. Because the TCP checksum were calculated on the card, the TCP/IP protocol checksums did not reveal this problemoutdated-wfc-timeout timeout
Define how long to wait until all peers are connected if all peers were outdated when the system went down. This parameter is usually set to a value smaller than wfc-timeout. The assumption here is that an outdated peer cannot have become primary in the meantime, so we don't need to wait for it as long as for a node which was alive before. The timeout is specified in seconds. The default value is 0, which stands for an infinite timeout. Also see the wfc-timeout parameter.
stacked-timeouts
On stacked devices, the wfc-timeout and degr-wfc-timeout parameters in the configuration are usually ignored, and both timeouts are set to twice the connect-int timeout. The stacked-timeouts parameter tells DRBD to use the wfc-timeout and degr-wfc-timeout parameters as defined in the configuration, even on stacked devices. Only use this parameter if the peer of the stacked resource is usually not available, or will not become primary. Incorrect use of this parameter can lead to unexpected split-brain scenarios.
wait-after-sb
This parameter causes DRBD to continue waiting in the init script even when a split-brain situation has been detected, and the nodes therefore refuse to connect to each other.
wfc-timeout timeout
Define how long the init script waits until all peers are connected. This can be useful in combination with a cluster manager which cannot manage DRBD resources: when the cluster manager starts, the DRBD resources will already be up and running. With a more capable cluster manager such as Pacemaker, it makes more sense to let the cluster manager control DRBD resources. The timeout is specified in seconds. The default value is 0, which stands for an infinite timeout. Also see the degr-wfc-timeout parameter.
Section volume Parameters
device /dev/drbdminor-number
Define the device name and minor number of a replicated block device. This is the device that applications are supposed to access; in most cases, the device is not used directly, but as a file system. This parameter is required and the standard device naming convention is assumed. In addition to this device, udev will create /dev/drbd/by-res/resource /volume and /dev/drbd/by-disk/lower-level-device symlinks to the device.
disk {[disk] | none}
Define the lower-level block device that DRBD will use for storing the actual data. While the replicated drbd device is configured, the lower-level device must not be used directly. Even read-only access with tools like dumpe2fs(8) and similar is not allowed. The keyword none specifies that no lower-level block device is configured; this also overrides inheritance of the lower-level device.
meta-disk internal,
meta-disk device,
meta-disk device [index]
Define where the metadata of a replicated block device resides: it can be internal, meaning that the lower-level device contains both the data and the metadata, or on a separate device. When the index form of this parameter is used, multiple replicated devices can share the same metadata device, each using a separate index. Each index occupies 128 MiB of data, which corresponds to a replicated device size of at most 4 TiB with two cluster nodes. We recommend not to share metadata devices anymore, and to instead use the lvm volume manager for creating metadata devices as needed. When the index form of this parameter is not used, the size of the lower-level device determines the size of the metadata. The size needed is 36 KiB + (size of lower-level device) / 32K * (number of nodes - 1). If the metadata device is bigger than that, the extra space is not used. This parameter is required if a disk other than none is specified, and ignored if disk is set to none. A meta-disk parameter without a disk parameter is not allowed.