This topic describes how to migrate data from a self-managed PostgreSQL database to a PolarDB for Oracle cluster by using Data Transmission Service (DTS).

Prerequisites

  • The destination PolarDB for Oracle cluster is created. For more information, see Create a cluster.
  • The available storage space of the destination PolarDB for Oracle cluster is larger than the total size of the data in the self-managed PostgreSQL database.

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.

    The name of the source database cannot contain hyphens (-), for example, dts-testdata.

  • 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. The value of the wal_level parameter must be set to logical.

    • 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:
    • If you perform a primary/secondary switchover on the self-managed PostgreSQL database, the data migration task fails.
    • During 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 will be inconsistent between the source and destination databases. To ensure data consistency, we recommend that you select Full Data Migration and Incremental Data Migration as the migration types.
Other limits
  • Schema migration is not supported in this scenario. Before you configure a data migration task, you must create databases and tables in the destination instance.
  • 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.
  • DTS creates the following temporary tables in the source database to obtain the DDL statements of incremental data, the schemas of incremental tables, and the heartbeat information. During data migration, do not delete temporary tables in the source database. Otherwise, the data migration task may fail. After the DTS instance is released, temporary tables are automatically deleted.

    public.DTS_PG_CLASS, public.DTS_PG_ATTRIBUTE, public.DTS_PG_TYPE, public.DTS_PG_ENUM, public.DTS_POSTGRES_HEARTBEAT, public.DTS_DDL_COMMAND, and public.DTS_ARGS_SESSION.

  • 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.
  • During incremental data migration, DTS creates a replication slot for the source database. The replication slot is prefixed with dts_sync_. DTS automatically clears historical replication slots every 90 minutes to reduce storage usage.
    Note If the data migration task is released or fails, DTS automatically clears the replication slot. If a primary/secondary switchover is performed on the source ApsaraDB RDS for PostgreSQL instance, you must log on to the secondary database to clear the replication slot.
    Replication slot information
  • 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.
  • 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.

Billing

Migration type Task configuration fee Internet traffic fee
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

  • Full data migration

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

  • Incremental data migration

    After full data migration is completed, DTS migrates incremental data from the source database to the destination database. Incremental data migration ensures service continuity when you migrate data between self-managed databases.

SQL operations that can be migrated

Operation type SQL statements
DML INSERT, UPDATE, and DELETE

Permissions required for database accounts

Database Schema migration Full data migration Incremental data migration
Self-managed PostgreSQL database The USAGE permission on pg_catalog The SELECT permission on the objects to be migrated The permissions of the superuser role
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:

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 PostgreSQL.
    Access Method Select Express Connect, VPN Gateway, or Smart Access Gateway.
    Note If your source database is a self-managed database, you must deploy the network environment for the database. For more information, see Preparation overview.
    Instance Region Select the region where the self-managed PostgreSQL database resides.
    Replicate Data Across Alibaba Cloud Accounts In this example, select No because data is migrated within the same Alibaba Cloud account.
    Connected VPC Select the ID of the virtual private cloud (VPC) that is connected to the self-managed PostgreSQL database.
    IP Address Enter the server IP address of the self-managed PostgreSQL database.
    Port Number Enter the service port number of the self-managed PostgreSQL database. The default port number is 5432.
    Database Name Enter the name of the self-managed PostgreSQL database.
    Database Account Enter the account of the self-managed PostgreSQL database. 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 for Oracle.
    Access Method Select Express Connect, VPN Gateway, or Smart Access Gateway.
    Instance Region Select the region where the destination PolarDB for Oracle cluster resides.
    Connected VPC Select the ID of the VPC that is connected to the destination PolarDB for Oracle cluster. You can view the VPC ID on the Overview page of the destination cluster.
    Domain Name or IP Address Enter the endpoint of the primary node in the destination 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 for Oracle cluster.
    Port Number Enter the service port number of the destination database. The default port number is 1521.
    Database Name Enter the name of the destination database in the PolarDB for Oracle cluster.
    Database Account Enter the database account of the destination PolarDB for Oracle 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. If a whitelist is configured for your self-managed database, you must add the CIDR blocks of DTS servers to the whitelist. Then, 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 Settings基础配置
      Parameter Description
      Task Stages Processing Mode of Conflicting Tables
      • To perform only full data migration, select only Full Data Migration.
      • To ensure service continuity during data migration, select 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.

      • 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 Settings高级配置
      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.