bsr (Block Sync & Replication) is a solution for real-time replication of block devices on the host in software over the network.

Based on Windows DRBD (wdrbd) forked from drbd (http://www.drbd.org), it is is an open source project built as a cross-platform common engine to support both Windows and Linux platforms. It inherits all the basic concepts and functions of Windows DRBD, and provides solutions to build a more stable and efficient replication environment by supplementing the problems and functions of DRBD9.

bsr is free to contribute and participate in development through the open source community (bsr) (For technical support or inquiries regarding bsr, please contact to github issues for bsr or Mantech's dev3@mantech.co.kr )

Basic

bsr replicates in the following way:

• The application writes data to the block device and replicates it in real time.

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

• Replicate synchronously or asynchronously

  • The synchronous protocol treats replication as complete when data is written to the local disk and the target host disk.

  • The asynchronous protocol treats replication as completed when data is written to the local disk and data is transferred to socket’s tx buffer.

Kernel Driver

The core engine of bsr was developed as a kernel driver.

The kernel driver is located at the disk volume layer, and controls write I/O from the file system in block units. And as it performs replication in the lower layer of the file system, it is suitable for high availability by providing a transparent replication environment independent of the file system and application. However, since it is located in the lower layer of the file system, it cannot control general operations related to files. For example, it is not possible to detect damage to the file system or control pure file data. It is only replicate in blocks written to disk.

bsr provides Active-Passive clustering by default, not support Active-Active clustering.

Management tools

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

bsradm

• It is a utility that provides high-level commands abstracting detailed functions of bsr. bsradm allows you to control most of the behavior of bsr.

• bsradm gets all configuration parameters from the configuration file etc/bsr.conf, and serves to pass commands by giving appropriate options to bsrsetup and bsrmeta. That is, the actual operation is performed in bsrsetup, bsrmeta.

• bsradm can be run in dry-run mode via the -d option. This provides a way to see in advance which bsradm will run with which combination of options without actually invoking the bsrsetup and bsrmeta commands.

• For more information about the bsradm command option, refer to bsradm in the Appendix B. System Manual.

bsrsetup

• bsrsetup can set option values for the bsr kernel engine. All parameters to bsrsetup must be passed as text arguments.

• The separation of bsradm and bsrsetup provides a flexible command system.

  • The parameters accepted by bsradm are replaced with more complex parameters, and then invoke bsrsetup.

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

• In most cases, it is not necessary to use bsrsetup directly, but it is used to perform individual functions or individual control between nodes.

• For details on the bsrsetup command options, refer to bsrsetup Appendix B. System Manual.

bsrmeta

• Create metadata files for replication configuration, or provide dump, restore, and modification capabilities for meta data. In most cases, like bsrsetup, you do not need to use bsrmeta directly, and control the metadata through the commands provided by bsradm.

• For details on the bsrmeta command options, refer to bsrmeta of Appendix B. System Manual.

bsrcon

• Check bsr related information or adjust other necessary settings.

• For more information about the bsrcon command option, refer to BSrcon in Appendix B. System Manual.

Resource

Resources are the abstraction of everything needed to construct a replicated data set. Users configure a resource and then control it to operate a replication environment.

In order to configure resources, the following basic options (resource name, volume, network connection) must be specified.

Resource Name

• Name it 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, ensuring write consistency of all volumes in the resource.

• The volume is described as a single device and is designated as a drive letter in Windows.

• One replication set requires one volume for data replication and a separate volume for storing metadata associated with the volume. Meta volumes are used to store and manage internal information for replication.

  • Metadata is divided into External Meta Type and Internal Meta Type according to the storage location. For example, if meta data is located on the disk of the replication volume, it is the internal meta, and if it is located on another device or another disk, it is the external meta.

  • In terms of performance, the external meta type is advantageous over the internal meta, because bsr can perform replication I/O and write metadata simultaneously during operation. And since the I/O performance of the meta disk directly affects the replication performance, it is recommended to configure the disk as good as possible.

  • Note that the volume for the meta should be configured in RAW without formatting to a file system(such as NTFS).

Network Connection

• Connection is a communication link for a replication data set between two hosts.

• Each resource is defined as multiple hosts, including full-mesh connections between multiple hosts.

• The connection name is automatically assigned as the resource name at the bsradm level, unless otherwise specified.

Resource Role

Resources have the role of a Primary or Secondary.

• Primary can read and write without limitation on resources.

• Secondary receives and records any changes made to the disk from the other node and does not allow access to the volume. Therefore, the application cannot read or write to the secondary volume.

The role of a resource can be changed through the bsr utility command. When changing the resource role from Secondary to Primary, it is called Promotion, and the opposite is Demotion.

Main Features

Replication Cluster

bsr defines a set of nodes for replication as a replication cluster, and basically supports a single primary mode that can act as a primary resource on only one node of the replication cluster member. Dual or multiple primary modes are not supported. The single primary mode, the Active-Passive model, is the standard approach to handling data storage media in a highly available cluster for failover.

Replication Protocol

bsr supports three replication methods.

Protocol A. Asynchronous

The asynchronous method considers replication complete when the primary node finishes writing to the local disk and simultaneously writes to the send buffer of TCP. Therefore, this method is used locally when fail-over, but the data in the buffer may not be able to completely pass to the standby node. The data on the standby node after transfer is consistent, but some unsuccessful updates to the writes that occurred during transfer may be lost. This method has good local I / O responsiveness and is suitable for WAN remote replication environments.

Protocol B. Semi Synchronous

In the case of a semi-synchronous method, when a local disk write occurs on the primary node, replication is considered complete when the replication packet is received from the other node.

Normally, data loss does not occur during a fail-over, but if both nodes are powered off simultaneously or irreparable damage occurs in the primary storage, the most recently recorded data in the primary may be lost.

Protocol C. Synchronous

The synchronous method is considered complete when the primary node has completed writing to both the local and remote disks. This ensures that no data is lost in the event of loss on either node.

Of course, loss of data is inevitable if both nodes (or a node's storage subsystem) suffer irreparable damage at the same time.

In general, bsr uses the Protocol C method a lot.

The replication method should be determined by data consistency, local I / O latency performance, and throughput.

For an example of configuring the replication mode, refer to create resources.

Replication Transport Protocol

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

TCP(IPv4/v6)

It is the basic transport protocol of bsr and is a standard protocol that can be used on all systems that support IPv4/v6.

Efficient synchronization

In bsr, replication and (re)synchronization are separate concepts. Replication is a process that reflects all disk write operations of the resource of the primary role in real time to a secondary node, and synchronization is a process of copying block data from the perspective of all block devices excluding real-time write I/O. Replication and synchronization work individually, but they can be processed simultaneously.

If the connection between the primary and secondary is maintained, replication continues. However, if the replication connection is interrupted due to a failure of the primary or secondary node, or the replication network is 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 disk. Synchronization sequentially synchronizes only the areas that are not synchronized from sector 0 to the last sector based on the information in the metadata and efficiently processes as follows.

• Synchronization is performed block by block according to the block layout of the disk, so disk search is rarely performed.

• It is efficient because it synchronizes only once for blocks in which multiple writes have been made in succession.

During synchronization, some of the Standby node's entire dataset is past and some are updated to the latest. The status of this data is called Inconsistent, and the status that all blocks are synchronized with the latest data is called UpToDate. Nodes in an inconsistent state are generally in a state where the volume is not available, so it is desirable to keep this state as short as possible.

Of course, even if synchronization is performed in the background, the application service of the Active node can be operated continuously with or without interruption.

Fixed-rate synchronization

In fixed-rate synchronization, the data rate synchronized to the peer node can be adjusted within the upper limit in seconds (this is called the synchronization rate), and can be specified as the minimum (c-min-rate) and maximum (c-max-rate).

Variable-rate synchronization

In Variable-rate synchronization bsr detects the available network bandwidth and compares it to I/O received from the application, automatically calculates the appropriate synchronization speed. bsr defaults to variable-band synchronization.

Checksum-based synchronization

Checksum data summarization can further improve the efficiency of the synchronization algorithm. Checksum-based synchronization reads blocks before synchronization, obtains a hash summary of what is currently on disk, and then compares the obtained hash summary by reading the same sector from the other node. If the hash values match, the re-write of the block is omitted. This method can be advantageous in performance compared to simply overwriting a block that needs to be synchronized, and if the file system rewrites the same data to a sector while disconnected (disconnected), resynchronization is skipped for that sector, so you can shorten the synchronization time in overall.

Congestion Mode

bsr provides a congestion mode function that can actively operate by detecting the congestion of the replication network during asynchronous replication. The congestion mode provides three operation modes: Blocking, Disconnect, and Ahead.

• If you do not set anything, it is basically a blocking mode. In Blocking mode, it waits until there is enough space in the TX transmission buffer to transmit duplicate data.

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

• Ahead mode responds to congestion by automatically re-synchronizing when congestion is released by first writing the primary node's I / O to the local disk while maintaining the replication connection and recording the area as out-of-sync. The primary node that is in the Ahead state becomes the Ahead data state compared to the Secondary node. And at this point, the secondary becomes the behind data state, but the data on the standby node is available in a consistent state. When the congestion state is released, replication to the secondary automatically resumes and background synchronization is performed automatically for out-of-sync blocks that were not replicated in the Ahead state. The congestion mode is generally useful in a network link environment with variable bandwidth, such as a wide area replication environment through a shared connection between data centers or cloud instances.

Online Verification

Online Verification is a function that checks the integrity of block-specific data between nodes during device operation. Integrity checks efficiently use network bandwidth and do not duplicate checks.

Online Verification is a cryptographic digest of all data blocks on a specific resource storage in one node (verification source), and the summary is compared to the contents of the same block location by transmitting the summarized content to a verification target. To do. If the summarized ㅗhash does not match, the block is marked out-of-sync and is later synchronized. Here, network bandwidth is effectively used because only the smallest summary is transmitted, not the entire block.

Since the operation of verifying the integrity of the resource is checked online, there may be a slight decrease in replication performance when online checking and replication are performed simultaneously. However, there is an advantage that there is no need to stop the service, and there is no downtime of the system during the scan or synchronization process after the scan. And since bsr executes FastSync as the basic logic, it is more efficient by performing an online scan only on the disk area used by the file system.

It is common practice to perform tasks according to Online Verification as scheduled tasks at the OS level and periodically perform them during times of low operational I / O load. For more information on how to configure online integrity checking, see Using on-line device verification.

Replication traffic integrity checking

bsr can use cryptographic message digest algorithms to verify the integrity of replication traffic between nodes in real time.

When you use this feature, Primary verifies the integrity of the replication traffic by generating a message digest of all data blocks and passing it to the Secondary node. If the summarized blocks do not match, request retransmission. bsr protects the source data against the following error conditions through this integrity of replication traffic. If you don't respond in advance to these situations, potential data corruption during duplication can occur.

• Bit errors (bit flips) occurring in data transferred between the main memory and the network interface of the transmitting node (If the TCP checksum offload function provided by the latest LAN card is activated, these hardware bit flips may not be detected by software).

• Bit errors that occur on data transferred from the network interface to the receiving node's main memory (the same consideration applies to TCP checksum offloading).

• Bugs or race conditions in the network interface firmware and drivers.

• Bit flips or random corruption injected by linked network components between nodes (if direct connections or back-to-back connections are not used).

Split-Brain notification and recovery

Split brain refers to a situation in which two or more nodes have a primary role due to manual intervention of the cluster management software or administrator in a temporary failure situation in which all networks are disconnected between cluster nodes. This is a situation that suggests that modifications to the data have been made on each node rather than being replicated to the other side, which can lead to potential problems. Because of this, the data may not be merged and two data sets may be created.

bsr provides the functions to automatically detect and recover split brains. For more information on this, see the split brain topic in Troubleshooting.

Disk I/O Error Policy

디스크 장비에서 장애가 발생할 경우 bsr은 디스크 장애 정책의 사전 설정을 통해 해당 I/O 에러를 상위 계층(대부분 파일시스템)으로 단순히 전달해서 처리하거나 복제 디스크를 detach 하여 복제를 중단하도록 합니다. 전자는 패스스루 정책, 후자는 분리 정책입니다.

패스스루(passthrough)

하위 디스크 계층에서 에러 발생 시 별도 처리없이 상위(파일시스템) 계층으로 에러 내용을 전달합니다. 에러에 따른 적절한 처리는 상위 계층에게 맡깁니다. 예를 들어, 파일 시스템이 에러 내용을 보고 디스크 쓰기 재시도를 하거나 read-only 방식으로 다시 마운트를 시도할 수 있습니다. 이렇게 오류를 상위 계층으로 전달하는 방식을 통해 파일시스템 스스로가 에러를 인지할 수 있도록 하여 스스로 에러에 대처할 수 있는 기회를 부여합니다.

분리(detach)

에러 정책을 detach 방식으로 구성하였다면 하위 계층에서 에러 발생 시 bsr이 자동으로 디스크를 분리(detach)하는 방식으로 처리합니다. 디스크가 detach 되면 diskless 상태가 되고 디스크로의 I/O 가 차단되며, 이에 따라 디스크 장애가 인지되고 장애 후속조치가 취해져야 합니다. bsr에서 diskless 상태는 디스크에 I/O 가 유입되지 못하도록 차단된 상태로 정의합니다. 보다 자세한 내용은 문제해결의 디스크 장애에서 설명합니다.

Outdated 데이터 정책

bsr은 Inconsistent 데이터와 Outdated 데이터를 구분합니다. Inconsistent 데이터란 어떤 방식으로든 접근이 불가능하거나 사용할 수  없는 데이터를 말합니다. 대표적인 예로 동기화 진행 중인 타겟 쪽 데이터가 Inconsistent 상태 입니다. 동기화가 진행 중인 타깃 데이터는 일부는 최신 이지만 일부는 지난 시점의 데이터 이므로 이를 한 시점의 데이터로 간주할 수 없습니다. 또한 이 때에는 장치에 적재 되었을 파일시스템이 마운트(mount)될 수 없거나 파일시스템 자동 체크 조차도 할 수 없는 상태 일 수 있습니다.

Outdated 디스크 상태는 데이터의 일관성은 보장되지만 Primary 노드와 최신의 데이터로 동기화되지 않았거나 이를 암시하는 데이터 입니다. 이런 경우는 임시적이든 영구적이든 복제 링크가 중단할 경우 발생합니다. 연결이 끊어진 Oudated 데이터는 결국 지난 시점의 데이터 이기 때문에 이러한 상태의 데이터에서 서비스가 되는 것을 막기 위해 bsr은 Outdated 데이터를 가진 노드에 대해 승격(promoting a resource)하는 것을 기본적으로 허용하지 않습니다. 그러나 필요하다면(긴급한 상황에서) Outdated 데이터를 강제로 승격할 수는 있습니다. 이와 관련하여 bsr은 네트워크 단절이 발생하자마자 응용프로그램이 측에서 즉시 Secondary노드를 Outdated 상태가 되도록 만들 수 있는 인터페이스를 갖추고 있습니다. Outdated 상태가 된 리소스에서 해당 복제링크가 다시 연결된다면 Outdated 상태 플래그는 자동으로 지워지며 백그라운드로 동기화(background synchronization)가 완료되어 최종 최신 데이터(UpToDate)로 갱신됩니다.

Primary 가 Crash 되었거나 연결이 단절된 Secondary 노드는 디스크 상태가 Outdated 일 수 있습니다.

운송 동기화

디스크를 직접 가져와서 구성하는 운송 동기화(Truck based replication)는 아래와 같은 상황에 적합합니다.

• 초기 동기화 할 데이터의 량이 매우 많은 경우(수십 테라바이트 이상)

• 거대한 데이터 사이즈에 비해 복제할 데이터의 변화율이 적을 것으로 예상되는 경우

• 사이트간 가용 네트워크 대역폭이 제한적인 경우

위의 경우 직접 디스크를 가져다가 동기화 하지 않고 bsr의 일반적인 초기 동기화를 진행한다면 매우 오랜 시간이 걸릴 것입니다. 디스크 크기가 크고 물리적으로 직접 복사하여 초기화를 시킬 수 있다면 이 방법을 권장합니다.  운송 동기화 사용을 참고 하세요.