This topic describes the precautions and limits when you migrate data from an SQL Server database, such as a self-managed SQL Server database and an database. To ensure that your data migration task runs as expected, read the precautions and limits before you configure the task.

Scenarios of migrating data from an SQL Server database

You can view the precautions and limits based on the following migration scenarios:

Migrate data between SQL Server databases

The following table describes the precautions and limits.
Type 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 source database must have PRIMARY KEY or UNIQUE constraints and all fields must be unique. Otherwise, the destination database may contain duplicate data records.
  • If you want to migrate incremental data, you must make sure that the following requirements are met:
    • The data logging feature must be enabled. The backup mode must be set to Full, and full logical backup must be performed.
    • Data logs are retained for at least 7 days during full data migration. You can wait until full data migration is complete, and then clear the data logs generated in the source database after the DTS task is run.
      Note DTS stores only 50 GB of data logs or the data logs for the last 24 hours. If the limit is exceeded, DTS automatically clears the logs that exceed the limit.
      Warning If you clear the data logs of the source database during full data migration, the task may fail to migrate incremental data. For example, full data migration takes more than 24 hours due to the large data volume in the source database and abnormal writing in the destination database. In this case, if the data logs of the source database are cleared during full data migration, DTS cannot obtain the data logs generated 24 hours ago. Therefore, the task may fail to migrate incremental data.
  • 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
  • DTS does not migrate data of the following types: CURSOR, ROWVERSION, SQL_VARIANT, HIERACHYID, and GEOMETRY.
  • DTS does not migrate heap tables, tables that contain non-clustered indexes, or tables that contain computed columns.
    Note If you set SQL Server Incremental Synchronization Mode to CDC-based Incremental Synchronization in the Configure Objects and Advanced Settings step, heap tables and tables that contain non-clustered indexes can be migrated.
  • If you migrate data between different versions of databases, make sure that the database versions are compatible.
  • To ensure that the delay time of incremental data migration is accurate, DTS adds a heartbeat table to the self-managed SQL Server database. The name of the heartbeat table is Source table name_dts_mysql_heartbeat.
  • 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 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 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.
Special cases
  • In the Configure Objects and Advanced Settings step, if you set SQL Server Incremental Synchronization Mode to
    • CDC-based Incremental Synchronization, primary/secondary switchover cannot be performed on the source SQL Server database.
    • Incremental Synchronization Based on Logs of Source Database, primary/secondary switchover can be performed on the source SQL Server database.
  • If the destination instance is an ApsaraDB RDS for SQL Server instance, take note of the following limits:

    DTS automatically creates a destination database in the ApsaraDB RDS for SQL Server instance. However, if the name of the source database is invalid, you must create a database in the ApsaraDB RDS for SQL Server instance before you configure the data migration task. For more information, see Create a database on an ApsaraDB RDS for SQL Server instance.

Migrate data from an SQL Server database to an AnalyticDB for MySQL cluster V3.0

The following table describes the precautions and limits.
Type 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 source database must have PRIMARY KEY or UNIQUE constraints and all fields must be unique. Otherwise, the destination database may contain duplicate data records.
  • If you want to migrate incremental data, you must make sure that the following requirements are met:
    • The data logging feature must be enabled. The backup mode must be set to Full, and full logical backup must be performed.
    • Data logs are retained for at least 7 days during full data migration. You can wait until full data migration is complete, and then clear the data logs generated in the source database after the DTS task is run.
      Note DTS stores only 50 GB of data logs or the data logs for the last 24 hours. If the limit is exceeded, DTS automatically clears the logs that exceed the limit.
      Warning If you clear the data logs of the source database during full data migration, the task may fail to migrate incremental data. For example, full data migration takes more than 24 hours due to the large data volume in the source database and abnormal writing in the destination database. In this case, if the data logs of the source database are cleared during full data migration, DTS cannot obtain the data logs generated 24 hours ago. Therefore, the task may fail to migrate incremental data.
  • 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
  • DTS does not synchronize data of the following types: TIMESTAMP, CURSOR, ROWVERSION, HIERACHYID, SQL_VARIANT, SPATIAL GEOMETRY, SPATIAL GEOGRAPHY, and TABLE.
  • DTS does not migrate heap tables, tables that contain non-clustered indexes, or tables that contain computed columns.
    Note If you set SQL Server Incremental Synchronization Mode to CDC-based Incremental Synchronization in the Configure Objects and Advanced Settings step, heap tables and tables that contain non-clustered indexes can be migrated.
  • To ensure that the delay time of incremental data migration is accurate, DTS adds a heartbeat table to the self-managed SQL Server database. The name of the heartbeat table is Source table name_dts_mysql_heartbeat.
  • V3.0 has limits on the usage of disk space. If the disk space usage of the nodes in an cluster V3.0 reaches 80%, the performance of data writing to the destination database is compromised and the DTS task is delayed. If the usage reaches 90%, data cannot be written to the destination database and error messages are returned. We recommend that you estimate the required disk space based on the objects that you want to migrate. You must ensure that the destination cluster has sufficient storage space.
  • 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 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 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.
Special cases
In the Configure Objects and Advanced Settings step, if you set SQL Server Incremental Synchronization Mode to
  • CDC-based Incremental Synchronization, primary/secondary switchover cannot be performed on the source SQL Server database.
  • Incremental Synchronization Based on Logs of Source Database, primary/secondary switchover can be performed on the source SQL Server database.