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. DTS does not support schema migration of tables that have only UNIQUE constraints. Therefore, we recommend that you create PRIMARY KEY constraints for the tables to be migrated. If you select tables as the objects to be migrated and you need to edit tables in the destination database, 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 configure multiple tasks to migrate the tables or configure a task to migrate the entire database. The ApsaraDB RDS for MySQL instances attached to the PolarDB-X 1.0 instance must meet the following requirements for binary logs: The binary logging feature is enabled. The binlog_row_image parameter is set to full. For more information, see View the parameters of an ApsaraDB RDS for MySQL instance. Otherwise, error messages are returned during precheck and the data migration task cannot be started. 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 the full data migration is complete, you can set the retention period to more than 24 hours. Make sure that you set the retention period of binary logs based on the preceding requirements. Otherwise, the Service Level Agreement (SLA) of DTS does not guarantee service reliability or performance.
Limits on operations to be performed on the source database: If you switch the network type of the PolarDB-X 1.0 instance during data migration, adjust the network connection information about the migration after the switch succeeds. When the data migration task is running, do not scale the capacity of the source instance, migrate frequently accessed tables, change shards, or perform DDL operations. Otherwise, the data migration task fails or data inconsistency occurs. 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.
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Other limits | The storage type of the PolarDB-X 1.0 instance must be ApsaraDB RDS for MySQL, including custom instances and purchased instances. PolarDB for MySQL cannot be used as the storage type. When DTS migrates data from a PolarDB-X 1.0 instance, data is distributed across the attached ApsaraDB RDS for MySQL instances. DTS runs a subtask for each ApsaraDB RDS for MySQL instance. You can view the state of subtasks on the Task Topology page. 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 on 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 the 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, 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, stop or release the data migration task. 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 the data migration task is resumed.
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