Data Transmission Service:Usage notes and limits for migrating data from a PolarDB for MySQL cluster

Category

Description

Limits on the source database

• The server on which the source database is deployed 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 the tables in the destination database, such as renaming tables or columns, you can migrate up to 1,000 tables 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 the precheck and the data migration task fails to be started. For more information, 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 stored for more than 24 hours. For a full data and incremental data migration task, the binary logs of the source database must be stored for at least seven days. Otherwise, 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 configure the retention period of binary logs based on the preceding requirements. Otherwise, the service reliability or performance stated in the Service Level Agreement (SLA) of DTS may not be guaranteed.

• Limits on operations to be performed on the source database:

  • During schema migration and full data migration, do not execute DDL statements 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 between the source and destination databases occurs. 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, the versions of the source and destination PolarDB for MySQL clusters must be the same.

• DTS does not migrate the read-only nodes of the source PolarDB for MySQL cluster.

• DTS does not migrate Object Storage Service (OSS) external tables from the source PolarDB for MySQL cluster.

• If the data to be migrated contains information such as rare characters or emojis that takes up four bytes, the destination databases and tables to receive the data must use UTF8mb4 character set.

  Note

  If you use the schema migration feature of DTS, set the instance parameter character_set_server in the destination database to UTF8mb4 character set.

• 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 the 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 complete, the size of used 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 value, 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, you must stop or release the failed tasks. You can also execute the REVOKE statement to revoke the write permissions from the accounts that are 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.

• If DDL statements fail to be executed in the destination database, the DTS task continues to run. You can view the DDL statements that fail to be executed in task logs. For more information about how to view task logs, see View task logs.

• If you want to migrate accounts from the source database to the destination database, you need to learn the prerequisites and precautions. For more information, see Migrate database accounts.

• If a DTS task fails to run, DTS technical support will try to restore the task within 8 hours. During the restoration, the task may be restarted, and the parameters of the task may be modified.

  Note

  Only the parameters of the task may be modified. The parameters of databases are not modified. The parameters that may be modified include but are not limited to the parameters in the "Modify instance parameters" section of the Modify the parameters of a DTS instance topic.

Special cases

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

Category

Description

Limits on the source database

• The server on which the source database is deployed 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 the tables in the destination database, such as renaming tables or columns, you can migrate up to 1,000 tables 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 the precheck and the data migration task fails to be started. For more information, 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 stored for more than 24 hours. For a full data and incremental data migration task, the binary logs of the source database must be stored for at least seven days. Otherwise, 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 configure the retention period of binary logs based on the preceding requirements. Otherwise, the service reliability or performance stated in the Service Level Agreement (SLA) of DTS may not be guaranteed.

• Limits on operations to be performed on the source database:

  • During schema migration and full data migration, do not execute DDL statements 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 between the source and destination databases occurs. To ensure data consistency, we recommend that you select schema migration, full data migration, and incremental data migration as the migration types.

Other limits

• DTS does not migrate the read-only nodes of the source PolarDB for MySQL cluster.

• DTS does not migrate Object Storage Service (OSS) external tables from the source PolarDB for MySQL cluster.

• If the data to be migrated contains information such as rare characters or emojis that takes up four bytes, the destination databases and tables to receive the data must use UTF8mb4 character set.

  Note

  If you use the schema migration feature of DTS, set the instance parameter character_set_server in the destination database to UTF8mb4 character set.

• 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 the 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 complete, the size of used 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 value, 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, you must stop or release the failed tasks. You can also execute the REVOKE statement to revoke the write permissions from the accounts that are 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.

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

• If DDL statements fail to be executed in the destination database, the DTS task continues to run. You can view the DDL statements that fail to be executed in task logs. For more information about how to view task logs, see View task logs.

• If you write column names that differ only in capitalization to the same table in the destination MySQL database, the data migration result may not meet your expectations because the column names in MySQL databases are not case-sensitive.

• After data migration is complete, we recommend that you run the analyze table <table name> command to check whether data is written to the destination table. For example, if a high-availability (HA) switchover is triggered in the source MySQL database, data may be written only to the memory. As a result, data loss occurs.

• If a DTS task fails to run, DTS technical support will try to restore the task within 8 hours. During the restoration, the task may be restarted, and the parameters of the task may be modified.

  Note

  Only the parameters of the task may be modified. The parameters of databases are not modified. The parameters that may be modified include but are not limited to the parameters in the "Modify instance parameters" section of the Modify the parameters of a DTS instance topic.

Special cases

If you migrate data to an ApsaraDB RDS for MySQL instance, DTS automatically creates a destination database in the ApsaraDB RDS for MySQL instance. However, If the name of the database to be migrated does not comply with the database naming conventions of ApsaraDB RDS for MySQL, you must manually create a database in the ApsaraDB RDS for MySQL instance before you configure the data migration task. For more information, see Manage databases.

Category

Description

Limits on the source database

• The server on which the source database is deployed 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 the tables in the destination database, such as renaming tables or columns, you can migrate up to 1,000 tables 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 the precheck and the data migration task fails to be started. For more information, 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 stored for more than 24 hours. For a full data and incremental data migration task, the binary logs of the source database must be stored for at least seven days. Otherwise, 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 configure the retention period of binary logs based on the preceding requirements. Otherwise, the service reliability or performance stated in the Service Level Agreement (SLA) of DTS may not be guaranteed.

• Limits on operations to be performed on the source database:

  • 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 inconsistency between the source and destination databases occurs. To ensure data consistency, we recommend that you select full data migration and incremental data migration as the migration types.

  • Incremental DDL operations cannot be migrated. If you perform DDL operations on the source database during incremental data migration, the data migration task fails. If you need to perform DDL operations, we recommend that you perform the operations in the destination database and then perform the operations in the source database.

Other limits

• DTS does not migrate the read-only nodes of the source PolarDB for MySQL cluster.

• DTS does not migrate Object Storage Service (OSS) external tables from the source PolarDB for MySQL cluster.

• 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 the 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 complete, the size of used 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 value, 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, you must stop or release the failed tasks. You can also execute the REVOKE statement to revoke the write permissions from the accounts that are 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.

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

• If a DTS task fails to run, DTS technical support will try to restore the task within 8 hours. During the restoration, the task may be restarted, and the parameters of the task may be modified.

  Note

  Only the parameters of the task may be modified. The parameters of databases are not modified. The parameters that may be modified include but are not limited to the parameters in the "Modify instance parameters" section of the Modify the parameters of a DTS instance topic.

Category

Description

Limits on the source database

• The server on which the source database is deployed 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 the tables in the destination database, such as renaming tables or columns, you can migrate up to 1,000 tables 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 the precheck and the data migration task fails to be started. For more information, 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 stored for more than 24 hours. For a full data and incremental data migration task, the binary logs of the source database must be stored for at least seven days. Otherwise, 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 configure the retention period of binary logs based on the preceding requirements. Otherwise, the service reliability or performance stated in the Service Level Agreement (SLA) of DTS may not be guaranteed.

• Limits on operations to be performed on the source database:

  • During schema migration and full data migration, do not execute DDL statements to change the schemas of databases or tables. Otherwise, the data migration task fails.

  • During data migration, do not execute DDL statements to add comments. Otherwise, the data migration task fails. For example, do not execute the ALTER TABLE table_name COMMENT='Table comment'; statement.

  • If you perform only full data migration, do not write data to the source database during data migration. Otherwise, data inconsistency between the source and destination databases occurs. To ensure data consistency, we recommend that you select schema migration, full data migration, and incremental data migration as the migration types.

Other limits

• Prefix indexes cannot be migrated. If the source database contains prefix indexes, data may fail to be migrated.

• DTS does not migrate the read-only nodes of the source PolarDB for MySQL cluster.

• DTS does not migrate Object Storage Service (OSS) external tables from the source PolarDB for MySQL cluster.

• Due to the limits of AnalyticDB for MySQL, if the disk space usage of the nodes in an AnalyticDB for MySQL cluster exceeds 80%, an exception occurs and the DTS task is delayed. We recommend that you estimate the disk space that is required based on the objects to be migrated. You must make sure that the destination cluster has sufficient storage space.

• If the destination AnalyticDB for MySQL V3.0 cluster is being backed up while the DTS task is running, the DTS task fails.

• 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 the 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 complete, the size of used 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 value, 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, you must stop or release the failed tasks. You can also execute the REVOKE statement to revoke the write permissions from the accounts that are 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.

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

• If DDL statements fail to be executed in the destination database, the DTS task continues to run. You can view the DDL statements that fail to be executed in task logs. For more information about how to view task logs, see View task logs.

• If a DTS task fails to run, DTS technical support will try to restore the task within 8 hours. During the restoration, the task may be restarted, and the parameters of the task may be modified.

  Note

  Only the parameters of the task may be modified. The parameters of databases are not modified. The parameters that may be modified include but are not limited to the parameters in the "Modify instance parameters" section of the Modify the parameters of a DTS instance topic.

Category

Description

Limits on the source database

• The server on which the source database is deployed 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 the tables in the destination database, such as renaming tables or columns, you can migrate up to 1,000 tables 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 the precheck and the data migration task fails to be started. For more information, 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 stored for more than 24 hours. For a full data and incremental data migration task, the binary logs of the source database must be stored for at least seven days. Otherwise, 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 configure the retention period of binary logs based on the preceding requirements. Otherwise, the service reliability or performance stated in the Service Level Agreement (SLA) of DTS may not be guaranteed.

• Limits on operations to be performed on the source database:

  • During schema migration and full data migration, do not execute DDL statements 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 between the source and destination databases occurs. To ensure data consistency, we recommend that you select schema migration, full data migration, and incremental data migration as the migration types.

Other limits

• DTS does not migrate the read-only nodes of the source PolarDB for MySQL cluster.

• DTS does not migrate Object Storage Service (OSS) external tables from the source PolarDB for MySQL cluster.

• 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 the 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 complete, the size of used 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 value, 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, you must stop or release the failed tasks. You can also execute the REVOKE statement to revoke the write permissions from the accounts that are 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.

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

• If a DTS task fails to run, DTS technical support will try to restore the task within 8 hours. During the restoration, the task may be restarted, and the parameters of the task may be modified.

  Note

  Only the parameters of the task may be modified. The parameters of databases are not modified. The parameters that may be modified include but are not limited to the parameters in the "Modify instance parameters" section of the Modify the parameters of a DTS instance topic.

Special cases

If the self-managed Oracle database is deployed in a Real Application Cluster (RAC) architecture and is connected to DTS over an Alibaba Cloud virtual private cloud (VPC), you must connect the Single Client Access Name (SCAN) IP address of the Oracle RAC and the virtual IP address (VIP) of each node to the VPC and configure routes. The settings ensure that your DTS task can run as expected. For more information, see Connect an on-premises database to Alibaba Cloud and Connect a data center to DTS by using VPN Gateway.

Category

Description

Limits on the source database

• 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 the tables in the destination database, such as renaming tables or columns, you can migrate up to 1,000 tables 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 the precheck and the data migration task fails to be started. For more information, 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 stored for more than 24 hours. For a full data and incremental data migration task, the binary logs of the source database must be stored for at least seven days. Otherwise, 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 configure the retention period of binary logs based on the preceding requirements. Otherwise, the service reliability or performance stated in the Service Level Agreement (SLA) of DTS may not be guaranteed.

• Limits on operations to be performed on the source database: During schema migration, do not execute DDL statements to change the schemas of databases or tables. Otherwise, the data migration task fails.

Other limits

• Initial full data migration is not supported. DTS does not migrate the historical data of objects from the source PolarDB for MySQL cluster to the destination DataHub project.

• Only tables can be selected as the objects to be migrated.

• DTS does not migrate the read-only nodes of the source PolarDB for MySQL cluster.

• DTS does not migrate Object Storage Service (OSS) external tables from the source PolarDB for MySQL cluster.

• 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 value, 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, you must stop or release the failed tasks. You can also execute the REVOKE statement to revoke the write permissions from the accounts that are 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.

• If a DTS task fails to run, DTS technical support will try to restore the task within 8 hours. During the restoration, the task may be restarted, and the parameters of the task may be modified.

  Note

  Only the parameters of the task may be modified. The parameters of databases are not modified. The parameters that may be modified include but are not limited to the parameters in the "Modify instance parameters" section of the Modify the parameters of a DTS instance topic.

Special cases

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