Migrate from Databases that Use GH-ost/PT-osc

This document introduces the online-ddl feature of DM when DM is used to migrate data from MySQL to TiDB and how online DDL tools perform during the data migration process.

Overview

DDL statements are always used in database applications. MySQL 5.6 and later versions support the online-ddl feature, but there are limitations. For example, DDL execution sometimes acquires an MDL lock, resulting in a table lock that can block reads or writes to and from a database to a certain extent in production scenarios. In other cases, some DDLs copy the whole table and this affects overall database performance.

Therefore, online DDL tools are often used to execute DDLs to reduce the impact on reads and writes. Common DDL tools are gh-ost and pt-osc.

Generally, these tools work by the following steps:

  1. Create a new ghost table according to the table schema of the DDL real table;
  2. Apply DDLs on the ghost table;
  3. Replicate the data of the DDL real table to the ghost table;
  4. After the data are consistent between the two tables, use the rename statement to replace the real table with the ghost table.

DM online-ddl

When you migrate data from MySQL to TiDB using DM, online DDL tools can identify the DDLs in the above step 2 and apply them downstream in step 4, which can reduce the replication workload for the ghost table.

online-ddl Configuration

Generally, it is recommended to enbale the online-ddl configuration and you can see the following effects:

DM online-ddl

  • The downstream TiDB does not need to create and replicate the ghost table, saving the storage space and network transmission overhead;
  • When you merge and migrate data from sharded tables, the RENAME operation is ignored for each sharded ghost tables to ensure the correctness of the replication;
  • Currently, one limitation for DM is that DMLs in this task are blocked until DDL operation is finished when you apply DDL operation to the downstream TiDB. This limitation will be removed later.
Note

If you need to disable the online-ddl configuration, pay attention to the following effects:

  • The downstream TiDB replicates the behaviors of online DDL tools like gh-ost/pt-osc;
  • You need to manually add various temporary tables and ghost tables generated by the online DDL tools to the task configuration white list;
  • You cannot merge and migrate data from sharded tables.

Configuration

In the task configuration file, online-ddl is at the same level of name. For example:

# ----------- Global configuration -----------
## ********* Basic configuration *********
name: test                      # The name of the task. Should be globally unique.
task-mode: all                  # The task mode. Can be set to `full`/`incremental`/`all`.
shard-mode: "pessimistic"       # The shard merge mode. Optional modes are ""/"pessimistic"/"optimistic". The "" mode is used by default which means sharding DDL merge is disabled. If the task is a shard merge task, set it to the "pessimistic" mode. After understanding the principles and restrictions of the "optimistic" mode, you can set it to the "optimistic" mode.
meta-schema: "dm_meta"          # The downstream database that stores the `meta` information.
online-ddl: true                # Supports automatic processing of "gh-ost" and "pt" for the upstream database.
online-ddl-scheme: "gh-ost"     # `online-ddl-scheme` will be deprecated in the future, so it is recommended to use `online-ddl`.
target-database:                # Configuration of the downstream database instance.
  host: "192.168.0.1"
  port: 4000
  user: "root"
  password: ""                  # It is recommended to use password encrypted with dmctl if the password is not empty.

For the advanced configuration and the description of each configuration parameter, refer to DM advanced task configuration file template.

When you merge and migrate data from sharded tables, you need to coordinate the DDL of each sharded table, and the DML before and after the DDL. DM supports two different modes: pessimistic mode and optimistic mode. For the differences and scenarios between the two modes, refer to Merge and Migrate Data from Sharded Tables.

Working details for DM with online DDL tools

This section describes the working details for DM with the online DDL tools gh-ost and pt-osc when implementing online-schema-change.

online-schema-change: gh-ost

When gh-ost implements online-schema-change, 3 types of tables are created:

  • gho: used to apply DDLs. When the data is fully replicated and the gho table is consistent with the origin table, the origin table is replaced by renaming.
  • ghc: used to store information that is related to online-schema-change.
  • del: created by renaming the origin table.

In the process of migration, DM divides the above tables into 3 categories:

  • ghostTable: \_\*\_gho
  • trashTable: \_\*\_ghc, \_\*\_del
  • realTable: the origin table that executes online-ddl.

The SQL statements mostly used by gh-ost and the corresponding operation of DM are as follows:

  1. Create the _ghc table:

    Create /* gh-ost */ table `test`.`_test4_ghc` (
                            id bigint auto_increment,
                            last_update timestamp not null DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
                            hint varchar(64) charset ascii not null,
                            value varchar(4096) charset ascii not null,
                            primary key(id),
                            unique key hint_uidx(hint)
                    ) auto_increment=256 ;
    

    DM does not create the _test4_ghc table.

  2. Create the _gho table:

    Create /* gh-ost */ table `test`.`_test4_gho` like `test`.`test4` ;
    

    DM does not create the _test4_gho table. DM deletes the dm_meta.{task_name}\_onlineddl record in the downstream according to ghost_schema, ghost_table, and the server_id of dm_worker, and clears the related information in memory.

    DELETE FROM dm_meta.{task_name}_onlineddl WHERE id = {server_id} and ghost_schema = {ghost_schema} and ghost_table = {ghost_table};
    
  3. Apply the DDL that needs to be executed in the _gho table:

    Alter /* gh-ost */ table `test`.`_test4_gho` add column cl1 varchar(20) not null ;
    

    DM does not perform the DDL operation of _test4_gho. It records this DDL in dm_meta.{task_name}\_onlineddl and memory.

    REPLACE INTO dm_meta.{task_name}_onlineddl (id, ghost_schema , ghost_table , ddls) VALUES (......);
    
  4. Write data to the _ghc table, and replicate the origin table data to the _gho table:

    Insert /* gh-ost */ into `test`.`_test4_ghc` values (......);
    Insert /* gh-ost `test`.`test4` */ ignore into `test`.`_test4_gho` (`id`, `date`, `account_id`, `conversion_price`, `ocpc_matched_conversions`, `ad_cost`, `cl2`)
      (select `id`, `date`, `account_id`, `conversion_price`, `ocpc_matched_conversions`, `ad_cost`, `cl2` from `test`.`test4` force index (`PRIMARY`)
        where (((`id` > _binary'1') or ((`id` = _binary'1'))) and ((`id` < _binary'2') or ((`id` = _binary'2')))) lock in share mode
      )   ;
    

    DM does not execute DML statements that are not for realtable.

  5. After the migration is completed, both the origin table and _gho table are renamed, and the online DDL operation is completed:

    Rename /* gh-ost */ table `test`.`test4` to `test`.`_test4_del`, `test`.`_test4_gho` to `test`.`test4`;
    

    DM performs the following two operations:

    • DM splits the above rename operation into two SQL statements.

      rename test.test4 to test._test4_del;
      rename test._test4_gho to test.test4;
      
    • DM does not execute rename to _test4_del. When executing rename ghost_table to origin table, DM takes the following steps:

      • Read the DDL recorded in memory in Step 3

      • Replace ghost_table and ghost_schema with origin_table and its corresponding schema

      • Execute the DDL that has been replaced

        alter table test._test4_gho add column cl1 varchar(20) not null;
        -- Replaced with:
        alter table test.test4 add column cl1 varchar(20) not null;
        
Note

The specific SQL statements of gh-ost vary with the parameters used in the execution. This document only lists the major SQL statements. For more details, refer to the gh-ost documentation.

online-schema-change: pt

When pt-osc implements online-schema-change, 2 types of tables are created:

  • new: used to apply DDL. When the data is fully replicated and the new table is consistent with the origin table, the origin table is replaced by renaming.
  • old: created by renaming the origin table.
  • 3 kinds of Trigger: pt_osc\_\*\_ins, pt_osc\_\*\_upd, pt_osc\_\*\_del. In the process of pt_osc, the new data generated by the origin table is replicated to new by the Trigger.

In the process of migration, DM divides the above tables into 3 categories:

  • ghostTable: \_\*\_new
  • trashTable: \_\*\_old
  • realTable: the origin table that executes online-ddl.

The SQL statements mostly used by pt-osc and the corresponding operation of DM are as follows:

  1. Create the _new table:

    CREATE TABLE `test`.`_test4_new` ( id int(11) NOT NULL AUTO_INCREMENT,
    date date DEFAULT NULL, account_id bigint(20) DEFAULT NULL, conversion_price decimal(20,3) DEFAULT NULL,  ocpc_matched_conversions bigint(20) DEFAULT NULL, ad_cost decimal(20,3) DEFAULT NULL,cl2 varchar(20) COLLATE utf8mb4_bin NOT NULL,cl1 varchar(20) COLLATE utf8mb4_bin NOT NULL,PRIMARY KEY (id) ) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin ;
    

    DM does not create the _test4_new table. DM deletes the dm_meta.{task_name}\_onlineddl record in the downstream according to ghost_schema, ghost_table, and the server_id of dm_worker, and clears the related information in memory.

    DELETE FROM dm_meta.{task_name}_onlineddl WHERE id = {server_id} and ghost_schema = {ghost_schema} and ghost_table = {ghost_table};
    
  2. Execute DDL in the _new table:

    ALTER TABLE `test`.`_test4_new` add column c3 int;
    

    DM does not perform the DDL operation of _test4_new. Instead, it records this DDL in dm_meta.{task_name}\_onlineddl and memory.

    REPLACE INTO dm_meta.{task_name}_onlineddl (id, ghost_schema , ghost_table , ddls) VALUES (......);
    
  3. Create 3 Triggers used for data migration:

    CREATE TRIGGER `pt_osc_test_test4_del` AFTER DELETE ON `test`.`test4` ...... ;
    CREATE TRIGGER `pt_osc_test_test4_upd` AFTER UPDATE ON `test`.`test4` ...... ;
    CREATE TRIGGER `pt_osc_test_test4_ins` AFTER INSERT ON `test`.`test4` ...... ;
    

    DM does not execute Trigger operations that are not supported in TiDB.

  4. Replicate the origin table data to the _new table:

    INSERT LOW_PRIORITY IGNORE INTO `test`.`_test4_new` (`id`, `date`, `account_id`, `conversion_price`, `ocpc_matched_conversions`, `ad_cost`, `cl2`, `cl1`) SELECT `id`, `date`, `account_id`, `conversion_price`, `ocpc_matched_conversions`, `ad_cost`, `cl2`, `cl1` FROM `test`.`test4` LOCK IN SHARE MODE /*pt-online-schema-change 3227 copy table*/
    

    DM does not execute the DML statements that are not for realtable.

  5. After the data migration is completed, the origin table and _new table are renamed, and the online DDL operation is completed:

    RENAME TABLE `test`.`test4` TO `test`.`_test4_old`, `test`.`_test4_new` TO `test`.`test4`
    

    DM performs the following two operations:

    • DM splits the above rename operation into two SQL statements:

       rename test.test4 to test._test4_old;
       rename test._test4_new to test.test4;
      
    • DM does not execute rename to _test4_old. When executing rename ghost_table to origin table, DM takes the following steps:

      • Read the DDL recorded in memory in Step 2

      • Replace ghost_table and ghost_schema with origin_table and its corresponding schema

      • Execute the DDL that has been replaced

        ALTER TABLE `test`.`_test4_new` add column c3 int;
        -- Replaced with:
        ALTER TABLE `test`.`test4` add column c3 int;
        
  6. Delete the _old table and 3 Triggers of the online DDL operation:

    DROP TABLE IF EXISTS `test`.`_test4_old`;
    DROP TRIGGER IF EXISTS `pt_osc_test_test4_del` AFTER DELETE ON `test`.`test4` ...... ;
    DROP TRIGGER IF EXISTS `pt_osc_test_test4_upd` AFTER UPDATE ON `test`.`test4` ...... ;
    DROP TRIGGER IF EXISTS `pt_osc_test_test4_ins` AFTER INSERT ON `test`.`test4` ...... ;
    

    DM does not delete _test4_old and Triggers.

Note

The specific SQL statements of pt-osc vary with the parameters used in the execution. This document only lists the major SQL statements. For more details, refer to the pt-osc documentation.

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