The basic

bsr synchronizes and replicates the volumes of hosts in a cluster in real time over the network.

Synchronization and Replication

To replicate, volume data on both hosts must first match. To achieve this, bsr performs a process of copying data from the source to the target using disk blocks as a unit, which is called synchronization.

Once synchronization is complete, both volumes will be in a completely identical state, and if data changes occur on the source side, only the changes will be reflected to the target side to maintain the consistency of both volumes.

Here, when data on the source side changes, the operation of reflecting the change in real time to the target side is called replication. Synchronization operates slowly in the background, while replication occurs quickly in the context of local I/O.

Replication works in the following way:

• The application writes data to the block device while replicating it in real time.

• Real-time replication does not affect other application services or system elements.

• Replicate synchronously or asynchronously

  • In the synchronous method, replication is considered complete when the replication data has been written to the local disk and the target host's disk.

  • The asynchronous method treats replication as complete when replication data is written to the local disk and transmitted to the target host.

Synchronization and replication operate separately within bsr, but can occur simultaneously at a single point in time. In other words, since replication can be processed simultaneously while synchronization is being performed (the operating node processes synchronization and simultaneously replicates write I/O that occurs during operation), the throughput between each node must be appropriately adjusted within the range of the maximum network bandwidth. . For information on setting the sync band, see Working | Adjusting the synchronization speed.

Kernel drivers

The core engine of BSR is implemented as a kernel driver.

The kernel driver sits at the disk volume layer and provides block-by-block control over write I/O from the filesystem. Because it sits at the lower layer of the filesystem, it provides a transparent replication environment that is independent of the filesystem and the application, making it ideal for building high availability. However, being at the lower layer of the filesystem means that it has no control over common operations on files. For example, it can't detect corruption in the filesystem or control the file data - it just replicates it block by block as it is written to disk.

BSR provides Active-Passive clustering by default, not Active-Active clustering.

Administration tools

BSR provides administrative tools for configuring and managing resources. It consists of bsradm, bsrsetup, bsrmeta, and bsrcon, which are described below. Administrator-level privileges are required to use the management commands.

bsradm

A utility that provides high-level commands that abstract from the detailed functionality of BSR. You can control most of the behaviour of BSR through bsradm.

bsradm gets all its configuration parameters from the configuration file etc\bsr.conf, and is responsible for passing commands to bsrsetup and bsrmeta with the appropriate options. This means that the actual behaviour is done by bsrsetup and bsrmeta.

bsradm can be run in dry-run mode with the -d option. This provides a way to see what combinations of options bsradm will run with, without actually invoking the bsrsetup and bsrmeta commands.

For more information about bsradm command options, see Appendix, bsradm in the Commands.

bsrsetup

Allows you to set the values required by the bsr kernel engine. All parameters to bsrsetup must be passed as text arguments.

The separation of bsradm and bsrsetup provides a flexible command scheme.

The parameters accepted by bsradm are replaced by more complex parameters to call bsrsetup.

bsradm prevents user mistakes by checking resource configuration files for grammatical errors, etc. bsrsetup does not check for these grammatical errors.

In most cases, you will not need to use bsrsetup directly, but use it when you need individual control between nodes or for special functions.

For more information about the bsrsetup command options, see Appendix, bsrsetup in the Commands.

bsrmeta

Provides the ability to create, dump, restore, and modify metadata for replication configurations. Like bsrsetup, most users do not need to use bsrmeta directly; they control metadata through commands provided by bsradm.

For more information about the bsrmeta command options, see Appendix, bsrmeta in the Commands.

bsrcon

View bsr-related information or adjust other necessary settings.

For more information about the bsrcon command options, see Appendix, bsrcon in the Commands.

Resource

A resource is an abstraction of everything you need to construct a replication dataset. You configure resources and control them to operate your replication environment.

To configure a resource, you must specify the following basic things: resource name, volume, and network connectivity.

Resource name

Specify a name in US-ASCII format without spaces.

Volume

• A resource is a replication group consisting of one or more volumes that share a common replication stream. bsr ensures the consistency of all volumes within a resource.

• A volume is described as a single device and is specified by a drive letter in Windows.

• A replica set requires one volume for data replication and a separate volume to store metadata associated with the volume. The meta volume is used to store and manage internal information for replication.

  • Metadata is divided into external and internal meta types based on where it is stored. For example, if the metadata is located on the disk of the volume being replicated, it is internal meta; if it is located on another device or another disk, it is external meta.

  • External meta types have an advantage over internal meta in terms of performance because replication I/O and meta data writing can be performed simultaneously during operation, and the I/O performance of the meta disk directly affects replication performance, so it is recommended to configure it with a high-performance disk as much as possible.

  • The volume for the meta should not be formatted with a filesystem like NTFS and should be configured as RAW.

Network Connections (Connection)

• A Connection is the communication link for a replica dataset between two hosts.

• Each resource is defined as a multi-host with a full-mesh connection setup between multiple hosts.

• The Connection Name is automatically assigned as the Resource Name at the bsradm level unless you specify otherwise.

Resource roles

A resource has a role of either Primary or Secondary.

• Primary can perform unlimited read and write operations on the resource.

• Secondary receives and records all changes to the disk from the other node and does not allow access to the volume. Therefore, applications cannot read or write to a Secondary volume.

The role of a resource can be changed through the bsr utility command. Changing the role of a resource from Secondary to Primary is called a promotion, and the opposite is called a demotion.

Main features

Replication clusters

BSR defines a set of nodes for replication as a replication cluster and supports single-primary mode by default, where only one node among the replication cluster members can act as a primary resource. It does not support multiple-primary mode. Single-primary mode, or the active-passive model, is the standard approach to handling data storage media in a highly available cluster for failover.

Replication methods

BSR supports three replication methods

Protocol A. Asynchronous

The asynchronous method considers replication complete when the primary node finishes writing to its local disk and simultaneously finishes writing to TCP's egress buffer. Therefore, in the event of a fail-over, data that has been written locally but is in the buffer may not fully pass to the standby node. After a failover, the data on the standby node is consistent, but some undelivered updates to writes that occurred during the failover may be lost. This method has good local I/O responsiveness and is suitable for long distant replication environments.

Protocol B. Semi-Synchronous

The semi-synchronous method considers replication to be complete when a local disk write occurs on the primary node and the replication packet is received by the other node.

While a forced fail-over typically does not result in data loss, the most recently written data on the Primary may be lost if both nodes lose power at the same time or if irreparable damage occurs on the Primary storage.

Protocol C. Synchronous

The synchronous method considers replication complete on the primary node when writes to both the local and remote disks are complete, thus ensuring that no data is lost in the event of a loss on either node.

Of course, if both nodes (or the nodes' storage subsystems) suffer irreversible damage at the same time, data loss is inevitable.

In general, BSR relies heavily on the Protocol C method.

The replication method should be determined by data consistency, local I/O latency performance, and throughput, which are factors that determine operational policy.

For an example of configuring replication mode, see Configuration examples.

Transport protocols

BSR's replication transport network supports the TCP/IP transport protocol.

TCP (IPv4/v6)

This is the default transport protocol for BSR and is a standard protocol that can be used on any system that supports IPv4/v6.

Efficient synchronization

As long as the replication connection between the primary and secondary is maintained, replication is performed continuously. However, if the replication connection is interrupted for any reason, such as a primary or secondary node failing, or the replication network being disconnected, synchronization between the primary and secondary is required.

When synchronizing, BSR does not synchronize blocks in the order in which the original I/O was written to the disk. It synchronizes only the unsynchronized areas sequentially, from sector 0 to the last sector, based on information in the metadata, and handles them efficiently as follows.

• Sync performs little disk traversal because it syncs on a block-by-block basis based on the block layout of the disk.

• Blocks with multiple consecutive write operations are synchronized only once, which is efficient.

During synchronization, the entire dataset on the Standby node is updated, some of it before past changes, and some of it up to date. The state of such data is called the Inconsistent state, and the state when all blocks have completed synchronization with the latest data is called the UpToDate state. A node in the Inconsistent state typically means that the volume is not available, so it is desirable to keep this state as short as possible.

Of course, application services on the Active node can continue to operate with little or no interruption while synchronization takes place in the background.

Partial synchronization

Once a full sync has been performed, it always operates as a partial sync. It is efficient by synchronizing only for out-of-sync areas (OOS).

Fast synchronization (FastSync)

bsr implements FastSync, which synchronizes only the parts of the volume that are in filesystem use. Without FastSync, you would have to synchronize over the entire volume, which can take a lot of synchronization time if the volume is large. FastSync is a powerful feature of bsr that can significantly reduce sync time.

Checksum-based synchronization

The efficiency of the synchronization algorithm can be further improved by using a summary of the checksum data. Checksum-based sync reads a block before syncing, obtains a hash summary of what is currently on the disk, and then compares it to the hash summary obtained by reading the same sector from the other node. If the hashes match, it skips the sync rewrite for that block. This can have a performance advantage over simply overwriting the block that needs to be synchronized, and if the file system rewrote the same content to a sector while disconnected (disconnect state), it will skip the re-sync for that sector, which can reduce the overall sync time.

Specify synchronization bandwidth

If you specify a synchronization band within the replication network band, the remaining bands are used as replication bands. If there is no synchronization behavior, all bands will be used as replication. You can specify the minimum value (c-min-rate) and maximum value (c-max-rate).

Fixed-rate synchronization

The data rate synchronized to the counterpart node is fixed to the resync-rate value.

Variable-rate synchronization

Variable-band synchronization handles synchronization between c-min-rate and c-max-rate by detecting available network bandwidth and arbitrating with replication throughput. In variable band synchronization, resync-rate only has the meaning of the initial synchronization band value.

bsr defaults to variable band synchronization.

Fixed-rate synchronization

In fixed-rate synchronization, the data rate of synchronization to the relative node per second can be adjusted within upper bounds (this is called the synchronization rate) and can be specified as a minimum (c-min-rate) and maximum (c-max-rate).

Variable-rate synchronization

Variable-rate sync detects the available network bandwidth and compares it to the I/O received from the application, and automatically calculates the appropriate sync rate. BSR uses variable-rate sync as the default setting.

Congestion mode

BSR provides a congestion mode feature that allows asynchronous replication to detect and proactively deal with congestion on the replication network. Congestion Mode provides three modes of operation: Blocking, Disconnect, and Ahead.

• If no settings are made, it defaults to Blocking mode. Blocking mode waits until there is free space in the TX transmit buffer to send replication data.

• You can set it to disconnect mode to temporarily relieve local I/O load by disconnecting the replication connection.

• It can be set to Ahead mode, which maintains the replication connection but writes the primary node's I/O to local disk first and writes those areas as out-of-sync, automatically resyncing when congestion is released. Once in the Ahead state, the primary node is in the Ahead data state relative to the secondary node, at which point the secondary is in the Behind data state, but the data on the standby node is consistent and available. When the congestion state is lifted, replication to the Secondary automatically resumes and background synchronization is automatically performed for any out-of-sync blocks that could not be replicated in the Ahead state. Congestion mode is typically useful in environments with variable bandwidth network links, such as wide area replication environments over shared connections between data centers or cloud instances.

Online data integrity checks

Online integrity verification is a feature that verifies the integrity of block-by-block data between nodes during device operation. Integrity checks make efficient use of network bandwidth and avoid redundant checks.

Online integrity verification sequentially cryptographically digests all data blocks on a specific resource storage on one node (verification source) and sends the digested contents to the other node (verification target) for summary comparison of the contents of the same block locations. If the summaries do not match, the block is marked as out-of-sync and will be subject to synchronization later. This is an efficient use of network bandwidth because we're not sending the entire contents of the block, just a minimal summary.

Because the work of verifying the integrity of the resource is done online, there may be a slight degradation in replication performance if online checks and replication are performed at the same time. However, it has the advantage of not requiring service interruption and no system downtime during the inspection or post-inspection synchronization process. And because bsr performs FastSync as its underlying logic, it is more efficient by performing online inspection only on the disk area that is being used by the filesystem.

A common usage for online integrity checks is to register them as scheduled tasks at the OS level and perform them periodically during times of low operational I/O load. For more information on how to configure online integrity checks, see Using on-line device verification.

Replication traffic integrity checks

BSR can perform real-time integrity verification of replication traffic between two nodes using a cryptographic message summarization algorithm.

When this feature is enabled, the primary generates a message summary of all data blocks and forwards it to the secondary node to verify the integrity of the replication traffic. If the summarized blocks do not match, it requests a retransmission. BSR uses these replication traffic integrity checks to protect source data against the following error situations. If these situations are not addressed proactively, they can lead to potential data corruption during replication.

• Bit errors (bit flips) that occur in the data passed between main memory and the network interface of the sending node (these hardware bit flips may go undetected by software if the TCP checksum offload feature offered by recent rancards is enabled).

• Bit errors occurring in the data being transferred from the network interface to the receiving node's main memory (the same considerations apply to TCP checksum offloading).

• Corruption caused by bugs or race conditions within the network interface firmware and drivers.

• Bit flips or random corruption injected by recombinant network components between nodes (unless direct, back-to-back connections are used).

Split-brain

A split-brain is a situation where two or more nodes have had a primary role due to manual intervention by the cluster management software or administrator in a temporary failure situation where all networks are disconnected between the cluster nodes. This is a potentially problematic situation because it implies that modifications to the data were made on each node rather than replicated to the other side. This can result in data not being merged and creating two data sets.

BSR provides the ability to automatically detect split brains and repair them. For more information about this, see the Split brain topic in Troubleshooting.

Disk status

The disk status in BSR is represented by one of the following states, depending on the situation.

• Diskless This is the state before the backing device is attached as a replica disk (Attach), or the disk is detached due to an I/O failure (Detach).

• UpToDate The disk data is up to date. If the target's disk is UpToDate, it means it is in a failover-able state.

• Outdated The data is consistent at a point in time, but may not be up to date. If the mirror connection is explicitly disconnected, the target's disk state defaults to Outdated.

• Inconsistent Refers to broken data where data consistency is not guaranteed. If the target's disk is Inconsistent, it is in an incorrigible state by default.

BSR distinguishes between inconsistent and outdated data. Inconsistent data is data that is inaccessible or unusable in some way. Typically, data on the target side of a synchronization is in an inconsistent state. The target data being synchronized is partly current and partly out of date, so it can't be considered data from a single point in time. Also, the filesystems that would have been loaded on the device may not be mountable at this time, or the filesystems may not even be automatically checked.

The Outdated disk state is data that is consistent but not synchronized with the primary node to the most recent data, or data that suggests it is. This happens when a replication link goes down, whether temporarily or permanently. Since disconnected Oudated data is, after all, data from a past point in time, to prevent data in this state from becoming a service, BSR disallows promoting a resource to a node with outdated data by default. However, it can force promotion of outdated data if necessary (in an emergency situation). In this regard, BSR provides an interface that allows applications to immediately cause a secondary node to become Outdated on their side as soon as a network disconnect occurs. Once the replication link is reconnected from the Outdated resource, the Outdated status flag is automatically cleared and a background synchronization is completed to update it to the latest and greatest data (UpToDate). A secondary node with a crashed primary or a disconnected secondary may have an Outdated disk status.

Handling disk I/O errors

When a disk device fails, BSR uses presets in the disk failure policy to either simply pass the I/O error to a higher tier (most likely the filesystem) to handle it, or to detach the replication disk to stop replication. The former is a pass-through policy, the latter a detach policy.

Pass-through

When an error occurs at the lower disk tier, it is passed to the upper (filesystem) tier without further processing. The corresponding handling of the error is left to the higher tier. For example, the filesystem might see the error and attempt to retry writing to the disk or remount in a read-only fashion. This way of passing errors to higher layers allows the filesystem to recognize errors on its own, giving it a chance to react on its own.

Detach

If you configure your error policy to DETACH, BSR will automatically detach the disk when an error occurs at a lower tier. When a disk is detached, it becomes diskless and I/O to the disk is blocked, which means that a disk failure is recognized and failure follow-up should be taken. BSR defines a diskless state as a state in which I/O to the disk is blocked. This is discussed in more detail in Disk failures in Troubleshooting.