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Data Migration Relay Log

The Data Migration (DM) relay log consists of a set 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 replicates the upstream binlog to the local configuration directory (the default migration directory is <deploy_dir>/relay_log if DM is deployed using DM-Ansible). 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, initial migration rules and data purge of DM relay logs.

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 replicated binlog.
binlog-pos = 63083620                                       # The position of the currently replicated binlog.
binlog-gtid = "c0149e17-dff1-11e8-b6a8-0242ac110004:1-3328" # GTID of the currently replicated 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

For each start of DM-worker (or the relay log resuming migration after a pause), the starting position of migration includes the following conditions:

If a valid local relay log (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, 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.
If a valid local relay log does not exist:
- 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.

Data purge

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 existing data migration tasks.

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

Automatic data purge

You can configure the automatic data purge strategy of DM-worker using the following two methods:

Method 1: Use the command-line options.

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 that a relay log that is not written by the relay processing unit, or that does not need to be read by the existing data migration task, can be retained for 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.

Method 2: Add the [purge] section in the configuration file of DM-worker.

# relay log purge strategy
[purge]
interval = 3600
expires = 24
remain-space = 15

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.
```
» purge-relay -w 10.128.16.223:10081 --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.
```
» purge-relay -w 10.128.16.223:10081 --filename mysql-bin.000001
```