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Data Migration Relay Log
Learn the directory structure, initial migration rules and data purge of DM relay logs.
/docs/tidb-data-migration/dev/relay-log/

Data Migration Relay Log

The Data Migration (DM) relay log consists of several sets of numbered files containing events that describe database changes, and an index file that contains the names of all used relay log files.

After DM-worker is started, it automatically migrates the upstream binlog to the local configuration directory (the default migration directory is <deploy_dir>/relay_log if DM is deployed using TiUP). When DM-worker is running, it migrates the upstream binlog to the local file in real time. The sync processing unit of DM-worker, reads the binlog events of the local relay log in real time, transforms these events to SQL statements, and then migrates these statements to the downstream database.

This document introduces the directory structure and initial migration rules DM relay logs, and how to pause, resume, and purge relay logs.

Note:

The relay log feature requires additional disk I/O operations, resulting in higher latency of data migration. If the disk I/O performance in the deployment environment is poor, the relay log feature may become a bottleneck of the migration task and thus slows the migration.

Directory structure

An example of the directory structure of the local storage for a relay log:

<deploy_dir>/relay_log/
|-- 7e427cc0-091c-11e9-9e45-72b7c59d52d7.000001
|   |-- mysql-bin.000001
|   |-- mysql-bin.000002
|   |-- mysql-bin.000003
|   |-- mysql-bin.000004
|   `-- relay.meta
|-- 842965eb-091c-11e9-9e45-9a3bff03fa39.000002
|   |-- mysql-bin.000001
|   `-- relay.meta
`-- server-uuid.index

  • subdir:

    • DM-worker stores the binlog migrated from the upstream database in the same directory. Each directory is a subdir.

    • subdir is named <Upstream database UUID>.<Local subdir serial number>.

    • After a switch between primary and secondary instances in the upstream, DM-worker generates a new subdir directory with an incremental serial number.

      • In the above example, for the 7e427cc0-091c-11e9-9e45-72b7c59d52d7.000001 directory, 7e427cc0-091c-11e9-9e45-72b7c59d52d7 is the upstream database UUID and 000001 is the local subdir serial number.

  • server-uuid.index: Records a list of names of currently available subdir directory.

  • relay.meta: Stores the information of the migrated binlog in each subdir. For example,

    $ cat c0149e17-dff1-11e8-b6a8-0242ac110004.000001/relay.meta
binlog-name = "mysql-bin.000010"                            # The name of the currently migrated binlog.
binlog-pos = 63083620                                       # The position of the currently migrated binlog.
binlog-gtid = "c0149e17-dff1-11e8-b6a8-0242ac110004:1-3328" # GTID of the currently migrated binlog.
                                                            # There might be multiple GTIDs.
$ cat 92acbd8a-c844-11e7-94a1-1866daf8accc.000001/relay.meta
binlog-name = "mysql-bin.018393"
binlog-pos = 277987307
binlog-gtid = "3ccc475b-2343-11e7-be21-6c0b84d59f30:1-14,406a3f61-690d-11e7-87c5-6c92bf46f384:1-94321383,53bfca22-690d-11e7-8a62-18ded7a37b78:1-495,686e1ab6-c47e-11e7-a42c-6c92bf46f384:1-34981190,03fc0263-28c7-11e7-a653-6c0b84d59f30:1-7041423,05474d3c-28c7-11e7-8352-203db246dd3d:1-170,10b039fc-c843-11e7-8f6a-1866daf8d810:1-308290454"

Initial migration rules

The starting position of the relay log migration is determined by the following rules:

  • From the checkpoint of the downstream sync unit, DM firstly gets the earliest position from which the migration tasks need to replicate from the data source. If the position is later than any of the following positions, DM-worker starts the migration from this position.

  • If a valid local relay log exists (a valid relay log is a relay log with valid server-uuid.index, subdir and relay.meta files), DM-worker resumes migration from a position recorded by relay.meta.

  • If a valid local relay log does not exist, but relay-binlog-name or relay-binlog-gtid is specified in the source configuration file:

    • In the non-GTID mode, if relay-binlog-name is specified, DM-worker starts migration from the specified binlog file.
    • In the GTID mode, if relay-binlog-gtid is specified, DM-worker starts migration from the specified GTID.

  • If a valid local relay log does not exist, and relay-binlog-name or relay-binlog-gtid is not specified in the DM configuration file:

    • In the non-GTID mode, DM-worker starts migration from the initial upstream binlog and migrates all the upstream binlog files to the latest successively.

    • In the GTID mode, DM-worker starts migration from the initial upstream GTID.

      Note:

      If the upstream relay log is purged, an error occurs. In this case, set relay-binlog-gtid to specify the starting position of migration.

Start and stop the relay log feature

Note:

Since DM v2.0.2, the configuration item enable-relay in the source configuration file is no longer valid. If DM finds that enable-relay is set to true when loading the data source configuration, it outputs the following message:

Please use `start-relay` to specify which workers should pull relay log of relay-enabled sources.

In DM v2.0.2 and later versions, you can use the command start-relay to start pulling relay logs and use stop-relay to stop the process. See the following examples:

{{< copyable "" >}}

» start-relay -s mysql-replica-01 worker1 worker2

In the command start-relay, you can configure one or more DM-workers to migrate relay logs for the specified data source, but the DM-workers specified in the parameter must be free or have been bound to the upstream data source.

{
    "result": true,
    "msg": ""
}

{{< copyable "" >}}

» stop-relay -s mysql-replica-01 worker1 worker2
{
    "result": true,
    "msg": ""
}

In DM versions earlier than v2.0.2 (not including v2.0.2), DM checks the configuration item enable-relay in the source configuration file when binding a DM-worker to an upstream data source. If enable-relay is set to true, DM enables the relay log feature for the data source.

See Upstream Database Configuration File for how to set the configuration item enable-relay.

Query relay logs

You can use the command query-status -s to query the status of the relay log pulling process of an upstream data source. See the following example:

{{< copyable "" >}}

» query-status -s mysql-replica-01
{
    "result": true,
    "msg": "",
    "sources": [
        {
            "result": true,
            "msg": "no sub task started",
            "sourceStatus": {
                "source": "mysql-replica-01",
                "worker": "worker2",
                "result": null,
                "relayStatus": {
                    "masterBinlog": "(mysql-bin.000005, 916)",
                    "masterBinlogGtid": "09bec856-ba95-11ea-850a-58f2b4af5188:1-28",
                    "relaySubDir": "09bec856-ba95-11ea-850a-58f2b4af5188.000001",
                    "relayBinlog": "(mysql-bin.000005, 4)",
                    "relayBinlogGtid": "09bec856-ba95-11ea-850a-58f2b4af5188:1-28",
                    "relayCatchUpMaster": false,
                    "stage": "Running",
                    "result": null
                }
            },
            "subTaskStatus": [
            ]
        },
        {
            "result": true,
            "msg": "no sub task started",
            "sourceStatus": {
                "source": "mysql-replica-01",
                "worker": "worker1",
                "result": null,
                "relayStatus": {
                    "masterBinlog": "(mysql-bin.000005, 916)",
                    "masterBinlogGtid": "09bec856-ba95-11ea-850a-58f2b4af5188:1-28",
                    "relaySubDir": "09bec856-ba95-11ea-850a-58f2b4af5188.000001",
                    "relayBinlog": "(mysql-bin.000005, 916)",
                    "relayBinlogGtid": "",
                    "relayCatchUpMaster": true,
                    "stage": "Running",
                    "result": null
                }
            },
            "subTaskStatus": [
            ]
        }
    ]
}

Pause and resume the relay log feature

You can use the command pause-relay to pause the pulling process of relay logs and use the command resume-relay to resume the process. You need to specify the source-id of the upstream data source when executing these two commands. See the following examples:

{{< copyable "" >}}

» pause-relay -s mysql-replica-01 -s mysql-replica-02
{
    "op": "PauseRelay",
    "result": true,
    "msg": "",
    "sources": [
        {
            "result": true,
            "msg": "",
            "source": "mysql-replica-01",
            "worker": "worker1"
        },
        {
            "result": true,
            "msg": "",
            "source": "mysql-replica-02",
            "worker": "worker2"
        }
    ]
}

{{< copyable "" >}}

» resume-relay -s mysql-replica-01
{
    "op": "ResumeRelay",
    "result": true,
    "msg": "",
    "sources": [
        {
            "result": true,
            "msg": "",
            "source": "mysql-replica-01",
            "worker": "worker1"
        }
    ]
}

Purge relay logs

Through the detection mechanism of reading and writing files, DM-worker does not purge the relay log that is being used or will be used later by the currently running data migration task.

The data purge methods for the relay log include automatic purge and manual purge.

Automatic data purge

You can enable automatic data purge and configure its strategy in the source configuration file. See the following example:

# relay log purge strategy
purge:
    interval: 3600
    expires: 24
    remain-space: 15

  • purge.interval

    • The interval of automatic purge in the background, in seconds.
    • "3600" by default, indicating a background purge task is performed every 3600 seconds.

  • purge.expires

    • The number of hours for which the relay log (that has been previously written to the relay processing unit, and that is not being used or will not be read later by the currently running data migration task) can be retained before being purged in the automatic background purge.
    • "0" by default, indicating data purge is not performed according to the update time of the relay log.

  • purge.remain-space

    • The amount of remaining disk space in GB less than which the specified DM-worker machine tries to purge the relay log that can be purged securely in the automatic background purge. If it is set to 0, data purge is not performed according to the remaining disk space.
    • "15" by default, indicating when the available disk space is less than 15GB, DM-master tries to purge the relay log securely.

Manual data purge

Manual data purge means using the purge-relay command provided by dmctl to specify subdir and the binlog name thus to purge all the relay logs before the specified binlog. If the -subdir option in the command is not specified, all relay logs before the current relay log sub-directory are purged.

Assuming that the directory structure of the current relay log is as follows:

$ tree .
.
|-- deb76a2b-09cc-11e9-9129-5242cf3bb246.000001
|   |-- mysql-bin.000001
|   |-- mysql-bin.000002
|   |-- mysql-bin.000003
|   `-- relay.meta
|-- deb76a2b-09cc-11e9-9129-5242cf3bb246.000003
|   |-- mysql-bin.000001
|   `-- relay.meta
|-- e4e0e8ab-09cc-11e9-9220-82cc35207219.000002
|   |-- mysql-bin.000001
|   `-- relay.meta
`-- server-uuid.index

$ cat server-uuid.index
deb76a2b-09cc-11e9-9129-5242cf3bb246.000001
e4e0e8ab-09cc-11e9-9220-82cc35207219.000002
deb76a2b-09cc-11e9-9129-5242cf3bb246.000003

  • Executing the following purge-relay command in dmctl purges all relay log files before e4e0e8ab-09cc-11e9-9220-82cc35207219.000002/mysql-bin.000001, which is all relay log files in deb76a2b-09cc-11e9-9129-5242cf3bb246.000001.

    {{< copyable "" >}}

    » purge-relay -s mysql-replica-01 --filename mysql-bin.000001 --sub-dir e4e0e8ab-09cc-11e9-9220-82cc35207219.000002

  • Executing the following purge-relay command in dmctl purges all relay log file before the current (deb76a2b-09cc-11e9-9129-5242cf3bb246.000003) directory's mysql-bin.000001, which is all relay log files in deb76a2b-09cc-11e9-9129-5242cf3bb246.000001 and e4e0e8ab-09cc-11e9-9220-82cc35207219.000002.

    {{< copyable "" >}}

    » purge-relay -s mysql-replica-01 --filename mysql-bin.000001