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

Prerequisites

  • The source and destination PolarDB for Oracle clusters are created. For more information, see Create a cluster.
  • The value of the wal_level parameter is set to logical for the source PolarDB for Oracle cluster. This setting ensures that logical decoding is supported in write-ahead logging (WAL). For more information, see Configure cluster parameters.

Limits

Category Description
Limits on the source database
  • Bandwidth requirements: The server to which the source database belongs must have sufficient egress bandwidth. Otherwise, the data migration speed is affected.
  • 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), 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 split the tables to be migrated, configure multiple tasks to migrate the tables, or configure a task to migrate the entire database.
  • If you need to migrate incremental data, you must make sure that the following requirements are met:
    • The write-ahead logging (WAL) feature must be enabled.

    • If you perform only incremental data migration, the WAL logs of the source database must be stored for more than 24 hours. If you perform both full data migration and incremental data migration, the WAL logs of the source database must be stored for at least seven days. After full data migration is completed, you can set the retention period to more than 24 hours. Otherwise, DTS may fail to obtain the WAL logs and the task may fail. In extreme cases, data may be inconsistent or lost. Make sure that you set the retention period of WAL logs based on the preceding requirements. Otherwise, the Service Level Agreement (SLA) of DTS does not guarantee service reliability and performance.

  • Limits on operations:
    • During schema migration and full data migration, do not perform data definition language (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 will be inconsistent 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
  • 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.
  • During incremental data migration, if you select a schema as the object to be migrated, take note of the following limits: If you 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
  • 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 of the database servers.
  • During full data migration, concurrent INSERT operations cause fragmentation in the tables of the destination database. After full data migration is completed, 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 meets 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 instance, stop or release the data migration task. You can also run the revoke command to revoke the write permissions from the accounts that are used by DTS to access the destination instance. Otherwise, the data in the source database will overwrite the data in the destination instance after the task is resumed.

Migration types

Migration type Description
Schema migration DTS migrates the schemas of the required objects from the source database to the destination database. DTS supports schema migration for the following types of objects: table, view, synonym, trigger, stored procedure, function, package, and user-defined type.
Note In this scenario, DTS is incompatible with triggers. We recommend that you delete the triggers of the source database to prevent data inconsistency caused by triggers. For more information, see Configure a data synchronization task for a source database that contains a trigger.
Full data migration DTS migrates historical data of the required objects from the source database to the destination database.
Note During schema migration and full data migration, we recommend that you do not perform data definition language (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 migrates incremental data from the source database to the destination database.

Incremental data migration allows you to ensure service continuity when you migrate data between PolarDB for Oracle clusters.

SQL operations that can be migrated

Operation type SQL statements
DML INSERT, UPDATE, and DELETE

Permissions required for database accounts

Database Required permissions
Source PolarDB for Oracle cluster A privileged account
Destination PolarDB for Oracle cluster The permissions of the database owner
Notice You can specify the database owner when you create a database.

For more information about how to create and authorize a database account, see the following topics:

PolarDB for Oracle cluster: Create an account

Procedure

  1. Go to the Data Migration page of the new DTS console.
    Note You can also log on to the Data Management (DMS) console. In the top navigation bar, click DTS.
  2. In the upper-left corner of the page, select the region in which the data migration instance resides.
    Region
  3. Click Create Task. On the page that appears, configure the source and destination databases.
    Warning After you select the source and destination instances, we recommend that you read the limits displayed at the top of the page. This helps you create and run the data migration task.
    Configure the source and destination databases
    Section Parameter Description
    N/A Task Name

    DTS automatically generates a task name. We recommend that you specify a descriptive name that makes it easy to identify. You do not need to use a unique task name.

    Source Database Database Type Select PolarDB for Oracle.
    Access Method Select Public IP Address.
    Instance Region Select the region where the source PolarDB for Oracle cluster resides.
    Data Sources
    • If you select Single Data Source, you must specify the Domain Name or IP Address and Port Number of the primary node in the source PolarDB for Oracle cluster.
    • If you select Multiple Data Sources, you must specify the IP addresses and port numbers of multiple nodes (including the primary node) in the source PolarDB for Oracle cluster in the IP:Port of Data Sources field.
      Note If you select Multiple Data Sources and specify the IP:Port of Data Sources parameter, DTS can automatically switch to a new primary node in the PolarDB for Oracle cluster during primary/secondary switchover. This is applicable to scenarios such as disaster recovery.
    In this example, select Single Data Source.
    Domain Name or IP Address Enter the endpoint of the primary node in the source PolarDB for Oracle cluster.
    Note You can run the ping command on your computer to obtain the IP address that is used to access the destination PolarDB O Edition cluster.
    Port Number Enter the service port number of the source database. The default port number is 1521.
    Note The service port of the source database must be accessible over the Internet.
    IP:Port of Data Sources Enter the IP addresses and port numbers of multiple nodes (including the primary node) in the source PolarDB O Edition cluster. Separate multiple IP:Port with commas (,).
    Oracle Type
    • If you select Non-RAC Instance, you must specify the SID parameter.
    • If you select RAC or PDB Instance, you must specify the Service Name parameter.
    In this example, select Non-RAC Instance.
    Database Name Enter the name of the source database in the PolarDB O Edition cluster.
    Database Account Enter the database account of the source PolarDB O Edition cluster. For information about the permissions that are required for the account, see Permissions required for database accounts.
    Database Password

    Enter the password of the database account.

    Destination Database Database Type Select PolarDB O Edition.
    Access Method Select Public IP Address.
    Instance Region Select the region where the destination PolarDB O Edition cluster resides.
    Data Sources
    • If you select Single Data Source, you must specify the Domain Name or IP Address and Port Number of the primary node in the destination PolarDB O Edition cluster.
    • If you select Multiple Data Sources, you must specify the IP addresses and port numbers of multiple nodes (including the primary node) in the destination PolarDB O Edition cluster in the IP:Port of Data Sources field.
      Note If you select Multiple Data Sources and specify the IP:Port of Data Sources parameter, DTS can automatically switch to a new primary node in the PolarDB O Edition cluster during primary/secondary switchover. This is applicable to scenarios such as disaster recovery.
    In this example, select Single Data Source.
    Domain Name or IP Address Enter the endpoint of the primary node in the destination PolarDB O Edition cluster.
    Note You can run the ping command on your computer to obtain the IP address that is used to access the destination PolarDB O Edition cluster.
    Port Number Enter the service port number of the destination database. The default port number is 1521.
    IP:Port of Data Sources Enter the IP addresses and port numbers of multiple nodes (including the primary node) in the destination PolarDB O Edition cluster. Separate multiple IP:Port with commas (,).
    Database Name Enter the name of the destination database in the PolarDB O Edition cluster.
    Database Account Enter the database account of the destination PolarDB O Edition cluster. For information about the permissions that are required for the account, see Permissions required for database accounts.
    Database Password

    Enter the password of the database account.

  4. In the lower part of the page, click Test Connectivity and Proceed.
    Warning
    • If the source or destination database instance is an Alibaba Cloud database instance, such as an ApsaraDB RDS for MySQL or ApsaraDB for MongoDB instance, or is a self-managed database hosted on Elastic Compute Service (ECS), DTS automatically adds the CIDR blocks of DTS servers to the whitelist of the database instance or ECS security group rules. For more information, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases. If the source or destination database is a self-managed database on data centers or is from other cloud service providers, you must manually add the CIDR blocks of DTS servers to allow DTS to access the database.
    • If the CIDR blocks of DTS servers are automatically or manually added to the whitelist of the database instance or ECS security group rules, 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: enhance the security of your account and password, limit the ports that are exposed, authenticate API calls, regularly check the whitelist or ECS security group rules and forbid unauthorized CIDR blocks, or connect the database to DTS by using Express Connect, VPN Gateway, or Smart Access Gateway.
    • After the DTS task is completed or released, we recommend that you manually detect and remove the added CIDR blocks from the whitelist of the database instance or ECS security group rules.
  5. Configure objects to migrate and advanced settings.
    • Basic SettingsBasic Settings
      Parameter Description
      Task Stages
      • 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 Incremental Data Migration is not selected, we recommend that you do not write data to the source instance during data migration. This ensures data consistency between the source and destination instances.
      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 identical table names, the precheck is passed. Otherwise, an error is returned during 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 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 potential risks.
        • If the source and destination databases have the same schema, DTS does not migrate data records that have the same primary keys 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.
      Select Objects

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

      Rename Databases and Tables
      • 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 are dependent on the object may fail to be migrated.
      Filter data

      You can specify WHERE conditions to filter data. For more information, see Use SQL conditions to filter data.

      In the Selected Objects section, right-click an object. In the dialog box that appears, select the DML operations that you want to migrate. For more information, see SQL operations that can be migrated.
    • Advanced SettingsAdvanced Settings (Not Replicate Temporary Tables When DMS Performs DDL Operations)
      Parameter Description
      Set Alerts
      Specify whether to set alerts for the data migration task. If the task fails or the migration latency exceeds the threshold, the alert contacts will receive notifications.
      • Select No if you do not want to set alerts.
      • Select Yes to set alerts. In this case, you must also set the alert threshold and alert contacts.
      Retry Time for Failed Connections
      Specify the retry time range for failed connections. Valid values: 10 to 720. Unit: minutes. Default value: 720. We recommend that you set the retry time range to more than 30 minutes. If DTS reconnects to the source and destination databases within the specified time range, DTS resumes the data synchronization task. Otherwise, the data synchronization task fails.
      Note
      • If an instance serves as the source or destination database of multiple data synchronization tasks, the value that is set later 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 needs. You can also release the DTS instance at your earliest opportunity after the source and destination instances are released.
  6. Click Next: Save Task Settings and Precheck in the lower part of the page.
    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, 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.
  7. Wait until the Success Rate becomes 100%. Then, click Next: Purchase Instance.
  8. On the Purchase Instance page, specify the Instance Class parameter for the data migration instance. The following table describes the parameter.
    Section Parameter Description
    Parameters Instance Class

    DTS provides various migration specifications. The migration speed varies based on the migration specifications that you select based on your business requirements. For more information, see Specifications of data migration instances.

  9. Read and select Data Transmission Service (Pay-as-you-go) Service Terms.
  10. Click Buy and Start to start the data migration task. You can view the progress of the task in the task list.