PolarDB is a next-generation relational database service that is developed by Alibaba Cloud. It is compatible with MySQL, PostgreSQL, and Oracle database engines. PolarDB provides superior performance in storage and computing to meet diverse requirements of enterprises. This topic describes how to migrate data between PolarDB-O clusters by using Data Transmission Service (DTS).

Prerequisites

  • The source and destination PolarDB-O clusters reside in the China (Shanghai) region. This is because data migration between PolarDB-O clusters is available only in the China (Shanghai) region.
  • The tables to be migrated from the source PolarDB-O cluster contain primary keys or UNIQUE NOT NULL indexes.
  • The value of the wal_level parameter is set to logical for the source PolarDB-O cluster. This setting ensures that logical decoding is supported in write-ahead logging (WAL). For more information, see Configure cluster parameters.

Precautions

  • DTS uses read and write resources of the source and destination databases during full data migration. This may increase the loads of the database servers. If the database performance is unfavorable, the specification is low, or the data volume is large, database services may become unavailable. For example, DTS occupies a large amount of read and write resources in the following cases: a large number of slow SQL queries are performed on the source database, the tables have no primary keys, or a deadlock occurs in the destination database. 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. For example, you can migrate data when the CPU utilization of the source and destination databases is less than 30%.
  • A single data migration task can migrate data from only one database. To migrate data from multiple databases, you must create a data migration task for each database.
  • If you select a schema as the object to be migrated and create a table in the schema or run the RENAME command to rename the table, you must run the ALTER TABLE schema.table REPLICA IDENTITY FULL; command before you write data to the table.
    Note Replace the schema and table in the preceding sample command with the actual schema name and table name.
  • To ensure that the delay time of incremental data migration is accurate, DTS adds a heartbeat table named dts_postgres_heartbeat to the source database. The following figure shows the schema of the heartbeat table. Schema of a heartbeat table
  • If a data migration task fails, DTS automatically resumes the task. Before you switch your workloads to the destination database, stop or release the data migration task. Otherwise, the data in the source database will overwrite the data in the destination database after the task is resumed.

Billing

Migration type Task 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 Pricing.
Incremental data migration Charged. For more information, see Pricing.

Migration types

Migration type Description
Schema migration DTS migrates the schemas of the required objects from the source database to the destination PolarDB-O cluster. DTS supports schema migration for the following types of objects: table, view, synonym, trigger, stored procedure, function, package, and user-defined type.
Notice In this scenario, DTS is incompatible with triggers. If an object contains triggers, data will become inconsistent between the source and destination databases.
Full data migration DTS migrates historical data of the required objects from the source database to the destination PolarDB-O cluster.
Notice During schema migration and full data migration, we recommend that you do not perform DDL operations on the required objects. Otherwise, the objects may fail to be migrated.
Incremental data migration DTS retrieves redo log files from the source database. Then, DTS synchronizes incremental data from the source database to the destination PolarDB-O cluster. DTS can synchronize DML operations, such as INSERT, UPDATE, and DELETE. DTS cannot synchronize DDL operations.

Incremental data migration allows you to ensure service continuity when you migrate data between PolarDB-O clusters.

Procedure

  1. Log on to the DTS console.
  2. In the left-side navigation pane, click Data Migration.
  3. At the top of the Migration Tasks page, select the region where the destination cluster resides.
    Select a region
  4. In the upper-right corner of the page, click Create Migration Task.
  5. Configure the source and destination databases.
    Migrate data between PolarDB-O clusters
    Section Parameter Description
    N/A Task Name DTS automatically generates a task name. We recommend that you specify an informative name for easy identification. You do not need to use a unique task name.
    Source Database Instance Type Select PolarDB.
    Instance Region Select the region where the source PolarDB-O cluster resides.
    PolarDB Instance ID Select the ID of the source PolarDB-O cluster.
    Database Name Enter the name of the source database.
    Database Account Enter the privileged account of the source PolarDB-O cluster. For more information, see Create database accounts.
    Database Password Enter the password of the database account.
    Note After you specify the source database parameters, click Test Connectivity next to Database Password to verify whether the specified parameters are valid. If the specified parameters are valid, the Passed message appears. If the Failed message appears, click Check next to Failed. Modify the source database parameters based on the check results.
    Destination Database Instance Type Select PolarDB.
    Instance Region Select the region where the destination PolarDB cluster resides.
    PolarDB Instance ID Select the ID of the destination PolarDB cluster.
    Database Name Enter the name of the destination database.
    Database Account Enter the database account of the destination PolarDB-O cluster. The account must have the permissions of the database owner.
    Notice You can specify the database owner when you create a database.
    Database Password Enter the password of the database account.
    Note After you specify the destination database parameters, click Test Connectivity next to Database Password to verify whether the parameters are valid. If the specified parameters are valid, the Passed message appears. If the Failed message appears, click Check next to Failed. Modify the destination database parameters based on the check results.
  6. In the lower-right corner of the page, click Set Whitelist and Next.
    DTS adds the CIDR blocks of DTS servers to the whitelists of the source and destination PolarDB-O clusters. This ensures that DTS servers can connect to the source and destination PolarDB-O clusters.
  7. Select the migration types and the objects to be migrated.
    Select the migration types and the objects to be migrated
    Setting Description
    Select the 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.
    Notice
    • If Incremental Data Migration is not selected, we recommend that you do not write data to the source database during full data migration. This ensures data consistency between the source and destination databases.
    • During schema migration and full data migration, we recommend that you do not perform DDL operations on the required objects. Otherwise, the objects may fail to be migrated.
    Select the objects to be migrated Select one or more objects from the Available section and click the Rightwards arrow icon to move the objects to the Selected section. You can select columns, tables, or schemas as the objects to be migrated.
    Notice
    • By default, after an object is migrated to the destination database, the name of the object remains unchanged. You can use the object name mapping feature to rename the objects that are migrated to the destination PolarDB-O cluster. For more information, see Object name mapping.
    • If you use the object name mapping feature to rename an object, other objects that are dependent on the object may fail to be migrated.
    Specify whether to rename objects You can use the object name mapping feature to rename the objects that are migrated to the destination instance. For more information, see Object name mapping.
    Specify the retry time for failed connections to the source or destination database By default, if DTS fails to connect to the source or destination database, DTS retries within the next 720 minutes (12 hours). You can specify the retry time based on your needs. If DTS reconnects to the source and destination databases within the specified time, DTS resumes the data migration task. Otherwise, the data migration task fails.
    Note When DTS retries a connection, you are charged for the DTS instance. We recommend that you specify the retry time based on your business needs. You can also release the DTS instance at your earliest opportunity after the source and destination instances are released.
  8. In the lower-right corner of the page, click Precheck.
    Note
    • Before you can start the data migration task, a precheck is performed. You can start the data migration task only after the task passes the precheck.
    • If the task fails to pass the precheck, you can click the Info icon icon next to each failed item to view details.
      • You can troubleshoot the issues based on the causes and run a precheck again.
      • If you do not need to troubleshoot the issues, you can ignore failed items and run a precheck again.
  9. After the task passes the precheck, click Next.
  10. In the Confirm Settings dialog box, specify the Channel Specification parameter and select Data Transmission Service (Pay-As-You-Go) Service Terms.
  11. Click Buy and Start to start the data migration task.
    • Schema migration and full data migration

      We recommend that you do not manually stop the task during full data migration. Otherwise, the data migrated to the destination database will be incomplete. You can wait until the data migration task automatically stops.

    • Schema migration, full data migration, and incremental data migration

      The task does not automatically stop during incremental data migration. You must manually stop the task.

      Notice We recommend that you select an appropriate time to manually stop the data migration task. For example, you can stop the task during off-peak hours or before you switch your workloads to the destination cluster.
      1. Wait until Incremental Data Migration and The migration task is not delayed appear in the progress bar of the migration task. Then, stop writing data to the source database for a few minutes. The delay time of incremental data migration may be displayed in the progress bar.
      2. Wait until the status of incremental data migration changes to The migration task is not delayed again. Then, manually stop the migration task. Stop a task during incremental data migration

References

Switch workloads to the destination database