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Data Transmission Service:Migrate data between PolarDB for MySQL clusters

Last Updated:Nov 21, 2023

This topic describes how to migrate data between PolarDB for MySQL clusters by using Data Transmission Service (DTS).

Note

A PolarDB for MySQL cluster of an earlier version cannot be updated to version 8.0. However, you can create a PolarDB for MySQL cluster of version 8.0, and then migrate data to this cluster. Before you migrate data across different versions of PolarDB for MySQL clusters, we recommend that you create a pay-as-you-go PolarDB for MySQL cluster to test the compatibility. After the test is complete, you can release the cluster.

Prerequisites

  • The source and destination PolarDB for MySQL clusters are created. For more information, see Purchase a pay-as-you-go cluster and Purchase a subscription cluster.

  • The available storage space of the destination PolarDB for MySQL cluster is larger than the total size of the data in the source PolarDB for MySQL cluster.

Limits

Note
  • During schema migration, DTS migrates foreign keys from the source database to the destination database.
  • During full data migration and incremental data migration, DTS temporarily disables the constraint check and cascade operations on foreign keys at the session level. If you perform the cascade and delete operations on the source database during data migration, data inconsistency may occur.
CategoryDescription
Limits on the source database
  • The server to which the source database belongs must have sufficient outbound bandwidth. Otherwise, the data migration speed decreases.
  • The tables to be migrated must have PRIMARY KEY or UNIQUE constraints, and all fields must be unique. Otherwise, the destination database may contain duplicate data records.
  • If you select tables as the objects to be migrated and you need to edit tables, such as renaming tables or columns in the destination database, up to 1,000 tables can be migrated in a single data migration task. If you run a task to migrate more than 1,000 tables, a request error occurs. In this case, we recommend that you configure multiple tasks to migrate the tables or configure a task to migrate the entire database.
  • If you need to migrate incremental data, the binary logging feature must be enabled and the loose_polar_log_bin parameter must be set to on. Otherwise, error messages are returned during precheck and the data migration task cannot be started. For more information about how to enable the binary logging feature and set the loose_polar_log_bin parameter, see Enable binary logging and Modify parameters.
    Note
    • If you enable the binary logging feature for a PolarDB for MySQL cluster, you are charged for the storage space that is occupied by binary logs.
    • For an incremental data migration task, the binary logs of the source database must be retained for more than 24 hours. For a full data and incremental data migration task, the binary logs of the source database must be retained for at least seven days. Otherwise, Data Transmission Service (DTS) may fail to obtain the binary logs and the task may fail. In exceptional circumstances, data inconsistency or loss may occur. After full data migration is complete, you can set the retention period to more than 24 hours. Make sure that you set the retention period of binary logs based on the preceding requirements. Otherwise, the Service Level Agreement (SLA) of DTS does not guarantee service reliability or performance.

  • Limits on operations to be performed on the source database:
    • During schema migration and full data migration, do not perform DDL operations to change the schemas of databases or tables. Otherwise, the data migration task fails.
    • If you perform only full data migration, do not write data to the source database during data migration. Otherwise, data inconsistency may occur between the source and destination databases. To ensure data consistency, we recommend that you select schema migration, full data migration, and incremental data migration as the migration types.
Other limits
  • To ensure compatibility, we recommend that you use the same MySQL version for the source and destination PolarDB for MySQL clusters.
  • Read-only nodes of the source PolarDB for MySQL cluster cannot be migrated.
  • Before you migrate data, evaluate the impact of data migration on the performance of the source and destination databases. We recommend that you migrate data during off-peak hours. During full data migration, DTS uses read and write resources of the source and destination databases. This may increase the loads on the database servers.
  • During full data migration, concurrent INSERT operations cause fragmentation in the tables of the destination database. After full data migration is complete, the tablespace of the destination database is larger than that of the source database.
  • You must make sure that the precision settings for columns of the FLOAT or DOUBLE data type meet your business requirements. DTS uses the ROUND(COLUMN,PRECISION) function to retrieve values from columns of the FLOAT or DOUBLE data type. If you do not specify a precision, DTS sets the precision for the FLOAT data type to 38 digits and the precision for the DOUBLE data type to 308 digits.
  • DTS attempts to resume data migration tasks that failed within the last seven days. Before you switch workloads to the destination database, stop or release the data migration task. You can also execute the REVOKE statement to revoke the write permissions from the accounts used by DTS to access the destination database. Otherwise, the data in the source database overwrites the data in the destination database after a failed task is resumed.
Usage notes

DTS executes the CREATE DATABASE IF NOT EXISTS `test` statement in the source database as scheduled to move forward the binary log file position.

Billing

Migration type

Instance configuration fee

Internet traffic fee

Schema migration and full data migration

Free of charge.

Charged only when data is migrated from Alibaba Cloud over the Internet. For more information, see Billing overview.

Incremental data migration

Charged. For more information, see Billing overview.

Migration types

  • Schema migration

    DTS migrates the schemas of objects from the source database to the destination database.

    Note
    • DTS supports schema migration for the following types of objects: tables, views, triggers, stored procedures, and stored functions.

    • During schema migration, DTS changes the value of the SECURITY attribute from DEFINER to INVOKER for views, stored procedures, and stored functions.

    • DTS does not migrate user information. To call a view, stored procedure, or stored function of the destination database, you must grant the read and write permissions to INVOKER.

  • Full data migration

    DTS migrates the existing data of objects from the source database to the destination database.

  • Incremental data migration

    After full data migration is complete, DTS migrates incremental data from the source database to the destination database. Incremental data migration allows data to be migrated smoothly without interrupting the services of self-managed applications during data migration.

SQL operations that can be incrementally migrated

Operation type

SQL statement

DML

INSERT, UPDATE, and DELETE

DDL

  • ALTER TABLE and ALTER VIEW

  • CREATE FUNCTION, CREATE INDEX, CREATE PROCEDURE, CREATE TABLE, and CREATE VIEW

  • DROP INDEX and DROP TABLE

  • RENAME TABLE

    Important

    RENAME TABLE operations may cause data inconsistency between the source and destination databases. For example, if you select a table as the object to be migrated and rename the table during data migration, the data of this table is not migrated to the destination database. To prevent this situation, you can select the database to which this table belongs as the object to be migrated when you configure the data migration task. Make sure that the databases to which the table belongs before and after the RENAME TABLE operation are added to the objects to be migrated.

  • TRUNCATE TABLE

Permissions required for database accounts

Database type

Required permissions

Source PolarDB for MySQL cluster

Read permissions on the objects to be migrated

Destination PolarDB for MySQL cluster

Read and write permissions on the destination database

For more information about how to create a database account for a PolarDB for MySQL cluster, see Create a database account.

Procedure

  1. Go to the Data Migration Tasks page.
    1. Log on to the Data Management (DMS) console.
    2. In the top navigation bar, click DTS.
    3. In the left-side navigation pane, choose DTS (DTS) > Data Migration.
    Note
  2. From the drop-down list next to Data Migration Tasks, select the region in which the data migration instance resides.
    Note If you use the new DTS console, you must select the region in which the data migration instance resides in the upper-left corner.
  3. Click Create Task. On the Create Task wizard page, configure the source and destination databases.

    Warning

    After you configure the source and destination databases, we recommend that you read the limits that are displayed in the upper part of the page. Otherwise, the task may fail or data inconsistency may occur.

    Section

    Parameter

    Description

    N/A

    Task Name

    The name of the task. DTS automatically assigns a name to the task. We recommend that you specify a descriptive name that makes it easy to identify the task. You do not need to specify a unique task name.

    Source Database

    Select an existing DMS database instance

    The database instance that you want to use. You can determine whether to select an existing instance based on your business requirements.

    • If you select an existing instance, DTS automatically populates the parameters for the instance.

    • If you do not select an existing instance, you must configure parameters for the source database.

    Database Type

    The type of the source database. Select PolarDB for MySQL.

    Access Method

    The access method of the source database. Select Alibaba Cloud Instance.

    Instance Region

    The region in which the source PolarDB for MySQL cluster resides.

    PolarDB Cluster ID

    The ID of the source PolarDB for MySQL cluster.

    Database Account

    The database account of the source PolarDB for MySQL cluster. For more information about the permissions that are required for the account, see the Permissions required for database accounts section of this topic.

    Database Password

    The password of the database account.

    Encryption

    Specifies whether to encrypt the connection to the source PolarDB for MySQL cluster. You can set this parameter based on your business requirements. For more information about the SSL encryption feature, see Configure SSL encryption.

    Destination Database

    Select an existing DMS database instance

    The database instance that you want to use. You can determine whether to select an existing instance based on your business requirements.

    • If you select an existing instance, DTS automatically populates the parameters for the instance.

    • If you do not select an existing instance, you must configure parameters for the destination database.

    Database Type

    The type of the destination database. Select PolarDB for MySQL.

    Access Method

    The access method of the destination database. Select Alibaba Cloud Instance.

    Instance Region

    The region in which the destination PolarDB for MySQL cluster resides.

    PolarDB Cluster ID

    The ID of the destination PolarDB for MySQL cluster.

    Database Account

    The database account of the destination PolarDB for MySQL cluster. For more information about the permissions that are required for the account, see the Permissions required for database accounts section of this topic.

    Important

    We recommend that you use a privileged account.

    Database Password

    The password of the database account.

    Encryption

    Specifies whether to encrypt the connection to the destination PolarDB for MySQL cluster. You can set this parameter based on your business requirements. For more information about the SSL encryption feature, see Configure SSL encryption.

  4. In the lower part of the page, click Test Connectivity and Proceed.

    If the source or destination database is an Alibaba Cloud database instance, such as an ApsaraDB RDS for MySQL or ApsaraDB for MongoDB instance, DTS automatically adds the CIDR blocks of DTS servers to the IP address whitelist of the instance. If the source or destination database is a self-managed database hosted on an Elastic Compute Service (ECS) instance, DTS automatically adds the CIDR blocks of DTS servers to the security group rules of the ECS instance, and you must make sure that the ECS instance can access the database. If the self-managed database is hosted on multiple ECS instances, you must manually add the CIDR blocks of DTS servers to the security group rules of each ECS instance. If the source or destination database is a self-managed database that is deployed in a data center or provided by a third-party cloud service provider, you must manually add the CIDR blocks of DTS servers to the IP address whitelist of the database to allow DTS to access the database. For more information, see the "CIDR blocks of DTS servers" section of the Add the CIDR blocks of DTS servers to the security settings of on-premises databases topic.

    Warning

    If the public CIDR blocks of DTS servers are automatically or manually added to the IP address whitelist of a database instance or to the security group rules of an ECS instance, security risks may arise. Therefore, before you use DTS to migrate data, you must understand and acknowledge the potential risks and take preventive measures, including but not limited to the following measures: enhancing the security of your account and password, limiting the ports that are exposed, authenticating API calls, regularly checking the IP address whitelist or ECS security group rules and forbidding unauthorized CIDR blocks, and connecting the database to DTS by using Express Connect, VPN Gateway, or Smart Access Gateway.

  5. Configure the objects to be migrated and advanced settings.

    Parameter

    Description

    Migration Types

    • To perform only full data migration, select Schema Migration and Full Data Migration.

    • To ensure service continuity during data migration, select Schema Migration, Full Data Migration, and Incremental Data Migration.

    Note

    If you do not select Incremental Data Migration, we recommend that you do not write data to the source database during data migration. This ensures data consistency between the source and destination databases.

    Method to Migrate Triggers in Source Database

    The method used to migrate triggers from the source database. You can select a migration method based on your business requirements. If no triggers are to be migrated, you do not need to set this parameter. For more information, see Synchronize or migrate triggers from the source database.

    Note

    This parameter is displayed only if Schema Migration is selected for the Migration Types parameter.

    Processing Mode of Conflicting Tables

    • Precheck and Report Errors: checks whether the destination database contains tables that have the same names as tables in the source database. If the source and destination databases do not contain tables that have identical table names, the precheck is passed. Otherwise, an error is returned during the precheck and the data migration task cannot be started.

      Note

      You can use the object name mapping feature to rename the tables that are migrated to the destination database. You can use this feature if the source and destination databases contain tables that have identical table names and the tables in the destination database cannot be deleted or renamed. For more information, see Map object names.

    • Ignore Errors and Proceed: skips the precheck for identical table names in the source and destination databases.

      Warning

      If you select Ignore Errors and Proceed, data inconsistency may occur and your business may be exposed to the following potential risks:

      • If the source and destination databases have the same schemas, DTS does not migrate data records that have the same primary key values as data records in the destination database.

      • If the source and destination databases have different schemas, only specific columns are migrated or the data migration task fails. Proceed with caution.

    Source Objects

    Select one or more objects from the Source Objects section. Click the Rightwards arrow icon and add the objects to the Selected Objects section.

    Note

    You can select columns, tables, or schemas as the objects to be migrated. If you select tables or columns as the objects to be migrated, DTS does not migrate other objects, such as views, triggers, or stored procedures, to the destination database.

    Selected Objects

    • To rename an object that you want to migrate to the destination instance, right-click the object in the Selected Objects section. For more information, see Map the name of a single object.
    • To rename multiple objects at a time, click Batch Edit in the upper-right corner of the Selected Objects section. For more information, see Map multiple object names at a time.
    Note
    • If you use the object name mapping feature to rename an object, other objects that depend on the object may fail to be migrated.

    • To specify WHERE conditions to filter data, right-click an object in the Selected Objects section. In the dialog box that appears, specify the conditions. For more information, see Use SQL conditions to filter data.

    • To select the SQL operations performed on a specific database or table, right-click an object in the Selected Objects section. In the dialog box that appears, select the SQL operations that you want to migrate. For more information, see the SQL operations that can be incrementally migrated section of this topic.

  6. Click Next: Advanced Settings to configure advanced settings.

    • Data Verification Settings

      For more information about how to use the data verification feature, see Enable data verification.

    • Advanced Settings

      Parameter

      Description

      Select the dedicated cluster used to schedule the task

      By default, DTS schedules the data migration task to the shared cluster if you do not specify a dedicated cluster. You can purchase a dedicated cluster of the specified specifications to run data migration tasks. For more information, see What is a DTS dedicated cluster?

      Set Alerts

      Specifies whether to configure alerting for the data migration task. If the task fails or the migration latency exceeds the specified threshold, the alert contacts will receive notifications. Valid values:

      • No: does not configure alerting.

      • Yes: configures alerting. If you select Yes, you must also specify the alert threshold and alert contacts. For more information, see Configure monitoring and alerting.

      Select the engine type of the destination database

      The engine type of the destination PolarDB for MySQL cluster. Valid values:

      • InnoDB: the default storage engine.

      • X-Engine: an online transaction processing (OLTP) database storage engine.

      Copy the temporary table of the Online DDL tool that is generated in the source table to the destination database.

      If you use DMS or the gh-ost tool to perform online DDL operations on the source database, you can specify whether to migrate the data of temporary tables generated by online DDL operations. Valid values:

      Important

      You cannot use tools such as pt-online-schema-change to perform online DDL operations on the source database. Otherwise, the DTS task fails.

      • Yes: DTS migrates the data of temporary tables generated by online DDL operations.

        Note

        If online DDL operations generate a large amount of data, the data migration task may take an extended period of time to complete.

      • No, Adapt to DMS Online DDL: DTS does not migrate the data of temporary tables generated by online DDL operations. Only the original DDL operations that are performed by using DMS are migrated.

        Note

        If you select No, Adapt to DMS Online DDL, the tables in the destination database may be locked.

      • No, Adapt to gh-ost: DTS does not migrate the data of temporary tables generated by online DDL operations. Only the original DDL operations that are performed by using the gh-ost tool are migrated. You can use the default or custom regular expressions to filter out the shadow tables of the gh-ost tool and tables that are not required.

        Note

        If you select No, Adapt to gh-ost, the tables in the destination database may be locked.

      Retry Time for Failed Connections

      The retry time range for failed connections. If the source or destination database fails to be connected after the data migration task is started, DTS immediately retries a connection within the time range. Valid values: 10 to 1440. Unit: minutes. Default value: 720. We recommend that you set the parameter to a value greater than 30. If DTS reconnects to the source and destination databases within the specified time range, DTS resumes the data migration task. Otherwise, the data migration task fails.
      Note
      • If you set different retry time ranges for multiple data migration tasks that have the same source or destination database, the shortest retry time range that is set takes precedence.
      • When DTS retries a connection, you are charged for the DTS instance. We recommend that you specify the retry time range based on your business requirements. You can also release the DTS instance at your earliest opportunity after the source and destination instances are released.

      The wait time before a retry when other issues occur in the source and destination databases.

      The retry time range for other issues. For example, if DDL or DML operations fail to be performed after the data migration task is started, DTS immediately retries the operations within the retry time range. Valid values: 1 to 1440. Unit: minutes. Default value: 10. We recommend that you set the parameter to a value greater than 10. If the failed operations are successfully performed within the specified retry time range, DTS resumes the data migration task. Otherwise, the data migration task fails.

      Important

      The value of the The wait time before a retry when other issues occur in the source and destination databases parameter must be smaller than the value of the Retry Time for Failed Connections parameter.

      Enable Throttling for Full Data Migration

      Specifies whether to enable throttling for full data migration. During full data migration, DTS uses the read and write resources of the source and destination databases. This may increase the loads of the database servers. You can enable throttling for full data migration based on your business requirements. To configure throttling, you must configure the Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s) parameters. This reduces the loads of the destination database server.

      Note

      This parameter is displayed only when Full Data Migration is selected as Migration Types.

      Enable Throttling for Incremental Data Migration

      Specifies whether to enable throttling for incremental data migration. To configure throttling, you must configure the RPS of Incremental Data Migration and Data migration speed for incremental migration (MB/s) parameters. This reduces the loads of the destination database server.

      Note

      This parameter is displayed only when Incremental Data Migration is selected as Migration Types.

      Environment Tag

      The environment tag that is used to identify the DTS instance. You can select an environment tag based on your business requirements. In this example, no environment tag is selected.

      Configure ETL

      Specifies whether to configure the extract, transform, and load (ETL) feature. For more information, see What is ETL?. Valid values:

      Whether to delete SQL operations on heartbeat tables of forward and reverse tasks

      Specifies whether to write SQL operations on heartbeat tables to the source database while the DTS instance is running.

      • Yes: does not write SQL operations on heartbeat tables. In this case, a latency of the DTS instance may be displayed.

      • No: writes SQL operations on heartbeat tables. In this case, specific features such as physical backup and cloning of the source database may be affected.

  7. In the lower part of the page, click Next: Save Task Settings and Precheck.

    You can move the pointer over Next: Save Task Settings and Precheck and click Preview OpenAPI parameters to view the parameters to be specified when you call the relevant API operation to configure the DTS task.

    Note
    • Before you can start the data migration task, DTS performs a precheck. You can start the data migration task only after the task passes the precheck.

    • If the task fails to pass the precheck, click View Details next to each failed item. After you troubleshoot the issues based on the error message, you can run a precheck again.

    • If an alert is triggered for an item during the precheck:

      • If the alert item cannot be ignored, click View Details next to the failed item and troubleshoot the issues. Then, run a precheck again.

      • If the alert item can be ignored, click Confirm Alert Details. In the View Details dialog box, click Ignore. In the message that appears, click OK. Then, click Precheck Again to run a precheck again. If you ignore the alert item, data inconsistency may occur and your business may be exposed to potential risks.

  8. Wait until the success rate becomes 100%. Then, click Next: Purchase Instance.

  9. On the Purchase Instance page, configure the Instance Class parameter for the data migration instance. The following table describes the parameters.

    Section

    Parameter

    Description

    New Instance Class

    Resource Group

    The resource group to which the data migration instance belongs. Default value: default resource group. For more information, see What is Resource Management?

    Instance Class

    DTS provides instance classes that vary in the migration speed. You can select an instance class based on your business scenario. For more information, see Specifications of data migration instances.

  10. Read and agree to Data Transmission Service (Pay-as-you-go) Service Terms by selecting the check box.

  11. Click Buy and Start to start the data migration task. You can view the progress of the task in the task list.