ApsaraDB for OceanBase:Migrate data from an Oracle database to an Oracle tenant of OceanBase Database

Prerequisites

• The data transmission service has the privilege to access cloud resources. For more information, see Grant privileges to roles for data transmission.

• You have created dedicated database users for data migration in the source Oracle database and the destination Oracle tenant of OceanBase Database, and granted corresponding privileges to the users. For more information, see Create a database user.

• You have created a database for the destination Oracle tenant of OceanBase Database. OceanBase Database supports only the migration of tables and columns. You must create a destination database in advance.

• You have enabled ARCHIVELOG for the source Oracle instance and switched the log file before the data transmission service starts incremental data replication.

• You have installed LogMiner in the source Oracle instance, and LogMiner runs properly.

  LogMiner enables you to obtain data from the archive logs of the Oracle instance.

• You have made sure that the Oracle instance has enabled the database-level or table-level supplemental logging feature.

• If you enable supplemental logging of the primary key and unique key at the database level, when a large number of unnecessary logs are generated by tables that do not need to be synchronized, the pressure on LogMiner Reader to fetch logs and on the Oracle database increases. Therefore, the data transmission service allows you to enable only the table-level supplemental logging for primary keys and unique keys for Oracle databases. However, if you specify extract-transform-load (ETL) options to filter columns other than the primary key and unique key columns when you create a migration task, enable supplemental logging for the corresponding columns or all columns.

• Clock synchronization has been configured by using, for example, the network time protocol (NTP) service, between the source Oracle server and the server running the data transmission service, to avoid data risks. For an Oracle Real Application Cluster (RAC), clock synchronization is also required between Oracle instances.

• If the source is an Oracle database connected by using a database gateway, make sure that the database has been connected to Alibaba Cloud by using the database gateway. For more information, see Quick Start.

Limitations

• Limitations on the source database

  Do not perform DDL operations that modify database or table schemas during schema migration or full data migration. Otherwise, the data migration task may be interrupted.

• The data transmission service supports Oracle 10g, 11g, 12c, 18c, and 19c. Version 12c and later provide container databases (CDBs) and pluggable databases (PDBs).

• The data transmission service supports the migration of only regular tables and views.

• The data transmission service supports the migration of an object only when the following conditions are met: the database name, table name, and column name of the object are ASCII-encoded without special characters. The special characters are line breaks, spaces, and the following characters: . | " ' ` ( ) = ; / & \.

• The data transmission service does not support triggers in the destination database. If triggers exist in the destination database, the data migration may fail.

• To prevent data migration tasks from being interrupted, the data transmission service does not support the migration of index-organized tables (IOTs) from an Oracle database.

• Data type limitations

  • The data transmission service does not support incremental synchronization of a table in which all columns are of a LOB type, which can be BLOB, CLOB, or NCLOB.

  • If a table does not have a primary key but contains data of the large object (LOB) type, the reverse incremental migration of the table can suffer poor data quality.

• Data source identifiers and user accounts must be globally unique in the data transmission system.

• The data transmission service can parse up to 5 TB of incremental logs of Oracle databases per day.

• You cannot create a database object whose name exceeds 30 bytes in length in an Oracle database of version 11g or earlier. Note that you cannot create a database object that exceeds this limit in an Oracle tenant of OceanBase Database during reverse incremental migration.

• By default, the data transmission service does not support the migration of database objects, such as schemas, tables, and columns, with a name exceeding 30 bytes in length from an Oracle database of version 12c or later. To migrate database objects with a name exceeding 30 bytes in length, contact OceanBase Technical Support.

• The data transmission service does not support some UPDATE statements in the source database. For example, the following UPDATE statement is not supported:

   UPDATE TABLE_NAME SET KEY=KEY+1;

  In the preceding example, TABLE_NAME is the table name, and KEY is a primary key column of the NUMERIC type.

• The virtual private cloud (VPC) in which the self-managed database is located must be in the same region as the OceanBase database.

Considerations

• When you perform incremental synchronization for an Oracle database, we recommend that you make sure that each archive file is less than 2 GB in size in the Oracle database.

• Archive files must be stored for more than two days in the Oracle database. Otherwise, in the case of a sharp increase in the number of archive files, restore may fail due to the lack of required archive files.

• If a DML operation is performed to exchange primary keys in the source Oracle database, errors occur when the data transmission service parses logs. This causes data loss when data is migrated to the destination database. Here is a sample DML statement that exchanges primary keys:

  update test set c1=(case when c1=1 then 2 when c1=2 then 1 end) where c1 in (1,2);

• The data transmission service allows you to migrate data from the source Oracle instance that uses character sets including AL32UTF8, AL16UTF16, ZHS16GBK, and GB18030.

  If the UTF-8 character set is used in the source database, we recommend that you use a compatible character set, such as UTF-8 or UTF-16, in the destination database to avoid garbled characters.

• When you migrate data from an Oracle database to an Oracle tenant of OceanBase Database, do not perform any operations on any table that may change the ROWID value, such as data import and export, ALTER TABLE, FLASHBACK TABLE, and partition splitting or compaction.

  If operations that change the ROWID value, such as updates of partitioning keys and partition compactions, are performed in the source Oracle database, hidden-column-dependent ROWIDs are generated in the destination Oracle tenant of OceanBase Database, which may lead to data loss.

• If the clocks between nodes or between the client and the server are out of synchronization, the latency may be inaccurate during incremental synchronization or reverse incremental migration.

  For example, if the clock is earlier than the standard time, the latency can be negative. If the clock is later than the standard time, the latency can be positive.

• Daylight Savings Time (DST) was once adopted in China, so a one-hour time difference between the source and the destination is expected for the data of the TIMESTAMP(6) WITH TIME ZONE type that was generated during the following periods of time: the DST periods from 1986 to 1991, and April 10 to 17, 1988.

• If you modify a unique index at the destination when DDL synchronization is disabled, you must restart the data migration task to avoid data inconsistency.

• If forward switchover is disabled for a data migration task, drop the unique indexes and pseudocolumns from the destination database. If you do not drop the unique indexes and pseudocolumns, data cannot be written, and pseudocolumns will be generated again when data is imported to the downstream system, causing conflicts with the pseudocolumns in the source database.

  If forward switchover is enabled for the data migration task, the data transmission service will automatically drop the hidden columns and unique indexes based on the type of the migration task. For more information, see Mechanisms for handling hidden columns.

• If a new table without the primary key is added in the source Oracle database during the incremental synchronization, the data transmission service does not automatically delete the hidden columns and the unique index added to the table in the destination Oracle tenant of OceanBase Database. You need to manually drop them before you start a reverse migration task.

  To confirm the tables without the primary key that are added during the incremental synchronization, view the manual_table.log file in the logs/msg/ directory.

• If the source and target databases use different character sets, a field length extension policy will be provided during schema migration. For example, the field length is extended by 1.5 times, and the length unit is changed from BYTE to CHAR.

  This ensures that data encoded by using different character sets in the source database can be migrated to the destination database. However, after cutover, data may fail to be written back to the source database during reverse incremental migration because of an extra long data length.

• If data types in the source database contain time zone information, such as TIMESTAMP WITH TIME ZONE, make sure that the destination database supports the time zone and contains the corresponding time zone information. Otherwise, data inconsistency may occur during data migration.

• Take note of the following considerations if you want to aggregate multiple tables:

  • We recommend that you configure the mappings between the source and destination databases by specifying matching rules.

  • We recommend that you manually create schemas at the destination. If you create schemas at the destination by using the data transmission service, skip the failed objects in the schema migration step.

• Check the objects in the recycle bin of the Oracle database. If the recycle bin contains more than 100 objects, internal table queries may time out. You must clear the objects in the recycle bin.

  • Query whether the recycle bin is enabled.

    SELECT Value FROM V$parameter WHERE Name = 'recyclebin';

  • Query the number of objects in the recycle bin.

    SELECT COUNT(*) FROM RECYCLEBIN;

• If you selected only Incremental Synchronization when you created the data migration task, the data transmission service requires that the archive logs of the source database be retained for at least 48 hours.

  If you selected Full Migration and Incremental Synchronization when you created the data migration task, the data transmission service requires that the archive logs of the source database be retained for at least 7 days. If the data transmission service cannot obtain incremental logs, the data migration task may fail or even the data between the source and destination databases may be inconsistent after migration.

• If the source or destination database contains table objects that differ only in letter cases, the data migration results may not be as expected due to case insensitivity in the source or destination database.

Supported source and destination instance types

In the following table, OB_Oracle stands for an Oracle tenant of OceanBase Database.

Data type mappings

Check and modify the system configurations of the source Oracle database

Perform the following operations:

1. Enable ARCHIVELOG for the source Oracle database.

2. Enable supplemental logging in the source Oracle database.

3. (Optional) Set the system parameters of the source Oracle database.

Enable ARCHIVELOG for the source Oracle database

SELECT log_mode FROM v$database;

The value of the log_mode field must be archivelog. Otherwise, perform the following steps to modify the settings:

1. Run the following commands to enable ARCHIVELOG:

   SHUTDOWN IMMEDIATE;
   STARTUP MOUNT;
   ALTER DATABASE ARCHIVELOG;
   ALTER DATABASE OPEN;

2. Run the following command to view the path and quota of archive logs.

   View the path and quota of the recovery file. We recommend that you set the db_recovery_file_dest_size parameter to a relatively large value. After you enable ARCHIVELOG, you need to regularly clear the archive logs by using Recovery Manager (RMAN) or other methods.

   SHOW PARAMETER db_recovery_file_dest;

3. Modify the quota of archive logs based on business needs.

   ALTER SYSTEM SET db_recovery_file_dest_size =50G SCOPE = BOTH;

Enable supplemental logging in the source Oracle database

LogMiner Reader allows you to enable only table-level supplemental logging in an Oracle database. If you create a table in the source Oracle database during the migration and the table needs to be migrated, you must enable supplemental logging for the primary key and unique key before you perform DML operations. Otherwise, the data transmission service will return an exception indicating incomplete logs.

If the indexes are inconsistent between the source and destination databases, the ETL results are not as expected, or the migration performance of partitioned tables deteriorates, you need to perform the following operations:

• Add the database-level or table-level supplemental_log_data_pk and supplemental_log_data_ui parameters.

• Enable supplemental logging for columns.

  • Enable supplemental logging for all columns related to the primary keys or unique keys in the source and destination databases to resolve the problem of index inconsistency between the source and destination databases.

  • If ETL is involved, add ETL-related columns to resolve the problem that the ETL results are not as expected.

  • If the destination table is a partitioned table, add a partitioning column to resolve the problem of write performance deterioration caused because partition pruning cannot be performed.

  You can execute the following statement to check the result of the preceding operations:

  SELECT log_group_type FROM all_log_groups WHERE OWNER = '<schema_name>' AND table_name = '<table_name>';

  If the check result includes ALL COLUMN LOGGING, the check is passed. Otherwise, verify whether the ALL_LOG_GROUP_COLUMNS table contains all preceding columns.

  A sample statement for enabling supplemental logging for columns is as follows:

  ALTER TABLE <table_name> ADD SUPPLEMENTAL LOG GROUP <table_name_group> (c1, c2) ALWAYS;

The following table describes the possible risks when you perform DDL operations in a running data migration task and provides corresponding solutions as well.

LogMiner Reader uses the following methods to check whether supplemental logging is enabled. If supplemental logging is not enabled, LogMiner Reader exits.

• Enable supplemental_log_data_pk and supplemental_log_data_ui at the database level.

  Run the following command to check whether supplemental logging is enabled. If the return value is YES, supplemental logging is enabled.

  SELECT supplemental_log_data_pk, supplemental_log_data_ui FROM v$database;

  If supplemental logging is not enabled, perform the following steps:

  1. Execute the following statement to enable supplemental logging:

     ALTER DATABASE ADD supplemental log DATA(PRIMARY KEY, UNIQUE) columns;

  2. After you enable supplemental logging, perform switchover to the ARCHIVELOG mode twice and wait more than 5 minutes before you start a task. For an Oracle RAC, perform a switchover for the instances alternately.

     ALTER SYSTEM SWITCH LOGFILE;

     In an Oracle RAC, after you perform a switchover to the ARCHIVELOG mode multiple times on one instance, when you perform a switchover to the ARCHIVELOG mode on the next instance, the latter instance may pull the logs that are generated before supplemental logging is enabled.

• Enable supplemental_log_data_pk and supplemental_log_data_ui at the table level.

  1. Execute the following statement to check whether supplemental_log_data_min is enabled at the database level:

     SELECT supplemental_log_data_min FROM v$database;

     If the return value is YES or IMPLICIT, supplemental logging is enabled.

  2. Execute the following statement to check whether table-level supplemental logging is enabled for the tables to be synchronized:

     SELECT log_group_type FROM all_log_groups WHERE OWNER = '<schema_name>' AND table_name = '<table_name>';

     One row is returned for each type of supplemental logging. The results must contain ALL COLUMN LOGGING or both PRIMARY KEY LOGGING and UNIQUE KEY LOGGING.

     If table-level supplemental logging is not enabled, execute the following statement:

     ALTER TABLE table_name ADD SUPPLEMENTAL LOG DATA (PRIMARY KEY, UNIQUE) COLUMNS;

  3. After you enable supplemental logging, perform switchover to the ARCHIVELOG mode twice and wait more than 5 minutes before you start a task. For an Oracle RAC, perform a switchover for the instances alternately.

     ALTER SYSTEM SWITCH LOGFILE;

(Optional) Set the system parameters of the source Oracle database

We recommend that you set _log_parallelism_max to 1. The default value is usually 2.

1. Query the value of the _log_parallelism_max parameter by using either of the following methods:

   • Method 1

     SELECT NAM.KSPPINM,VAL.KSPPSTVL,NAM.KSPPDESC FROM SYS.X$KSPPI NAM,SYS.X$KSPPSV VAL WHERE NAM.INDX= VAL.INDX AND NAM.KSPPINM LIKE '_%' AND UPPER(NAM.KSPPINM) LIKE '%LOG_PARALLEL%';

   • Method 2

     SELECT VALUE FROM v$parameter WHERE name = '_log_parallelism_max';

2. Modify the value of the _log_parallelism_max parameter. The statement for modifying the parameter varies with the database environment.

   • For an Oracle RAC database:

     ALTER SYSTEM SET "_log_parallelism_max" = 1 SID = '*' SCOPE = spfile;

   • For a non-Oracle RAC database

     ALTER SYSTEM SET "_log_parallelism_max" = 1 SCOPE = spfile;

   When you modify the value of the _log_parallelism_max parameter in Oracle 10g, if the error message write to SPFILE requested but no SPFILE specified at startup is returned, perform the following operations:

   CREATE SPFILE FROM PFILE;
   SHUTDOWN IMMEDIATE;
   STARTUP;
   SHOW PARAMETER SPFILE;

3. After you modify the value of _log_parallelism_max, restart the instance, perform a switchover to the ARCHIVELOG mode twice, and wait at least 5 minutes before you start a task.

Procedure

1. Log on to the ApsaraDB for OceanBase console and purchase a data migration task.

   For more information, see Purchase a data migration task.

2. Choose Data Transmission > Data Migration. On the page that appears, click Configuration for the data migration task.

   

   If you want to reference the configurations of an existing task, click Reference Configuration. For more information, see Reference the configuration of a data migration task.

3. On the Select Source and Destination page, configure the parameters.

   Parameter	Description
   Migration Task Name	We recommend that you set it to a combination of digits and letters. It must not contain any spaces and cannot exceed 64 characters in length.
   Tag (Optional)	Select a target tag from the drop-down list. You can also click Manage Tags to create, modify, and delete tags. For more information, see Use tags to manage data migration tasks.
   Source	If you have created an Oracle data source, select it from the drop-down list. Otherwise, click New Data Source in the drop-down list and create one in the dialog box that appears on the right. For more information about the parameters, see Create an Oracle data source.
   Destination	If you have created an Oracle tenant in OceanBase Database as a data source, select it from the drop-down list. Otherwise, click New Data Source in the drop-down list and create one in the dialog box that appears on the right. For more information about the parameters, see Create an OceanBase data source. Important The destination can only be an Oracle tenant of OceanBase Database, and Instance Type must be OceanBase Cluster Instance.

4. Click Next. On the Select Migration Type page, specify migration types for the current data migration task.

   Options for Migration Type are Schema Migration, Full Data Migration, Incremental Synchronization, Full Verification, and Reverse Increment.

   

   Migration type	Description
   Schema migration	After a schema migration task is started, the data transmission service migrates the definitions of database objects (such as tables, indexes, constraints, comments, and views) from the source database to the destination database and automatically filters out temporary tables.
   Full migration	After a full migration task is started, the data transmission service migrates existing data of tables in the source database to corresponding tables in the destination database. If you select Full Migration, we recommend that you use the GATHER_SCHEMA_STATS or GATHER_TABLE_STATS statement to collect the statistics of the Oracle database before data migration.
   Incremental synchronization	After an incremental synchronization task is started, the data transmission service synchronizes changed data (data that is added, modified, or removed) from the source database to corresponding tables in the destination database. Options for Incremental Synchronization are DML Synchronization and DDL Synchronization. You can select operations as needed. For more information, see Configure DDL/DML synchronization. Incremental Synchronization has the following limitations: DDL synchronization is not supported when multiple tables are merged into a single table. If you select DDL Synchronization, when you perform a DDL operation that is not supported by the data transmission service in the source database, the data migration may be interrupted. If the DDL operation is ADD COLUMN, we recommend that you set the column to a NULL column. Otherwise, the data migration may be interrupted.
   Full verification	After the full migration and incremental synchronization tasks are completed, the data transmission service automatically initiates a full verification task to verify the tables in the source and destination databases. If you select Full Verification, we recommend that you collect the statistics of the Oracle database and the Oracle tenant of OceanBase Database before full verification. If you have selected Incremental Synchronization but did not select all DML operations in the DML Synchronization section, you cannot select Full Verification.
   Reverse incremental migration	Data changes made in the destination database after the business database switchover are synchronized to the source database in real time through reverse incremental migration. Generally, incremental synchronization configurations are reused for reverse incremental migration. You can also customize the configurations for reverse incremental migration as needed.

5. Click Next. On the Select Migration Objects page, specify the migration objects for the migration task.

   You can select Specify Objects or Match Rules to specify the migration objects.

   Important

   • The names of tables to be migrated, as well as the names of columns in the tables, must not contain Chinese characters.

   • If a database or table name contains double dollar signs ($$), you cannot create the migration task.

   • If you selected DDL Synchronization in the Select Synchronization Type step, we recommend that you select migration objects by using the Match Rules option. This ensures that all new objects meeting the matching rules are synchronized. If you select migration objects by using the Specify Objects option, new or renamed objects will not be synchronized.

   • If you select Specify Objects, select the objects to be migrated on the left and click > to add them to the list on the right. You can select tables and views of one or more databases as the migration objects.

     The data transmission service allows you to import objects from text files, rename destination objects, set row filters, view column information, and remove a single or all migration objects.

     Note

     When you select Match Rules to specify migration objects, object renaming is implemented based on the syntax of the specified matching rules. In the operation area, you can only set filter conditions. For more information, see Configure matching rules.

     

     Operation	Description
     Import objects	In the list on the right, click Import Objects in the upper-right corner. In the dialog box that appears, click OK. Important This operation will overwrite previous selections. Proceed with caution. In the Import Objects dialog box, import the objects to be migrated. You can import CSV files to rename databases or tables and set row filtering conditions. For more information, see Download and import the settings of migration objects. Click Validate. After you import the migration objects, check their validity. Column field mapping is not supported at present. After the validation succeeds, click OK.
     Rename objects	The data transmission service allows you to rename migration objects. For more information, see Rename a database table.
     Configure settings	The data transmission service allows you to filter rows by using WHERE conditions. For more information, see Use SQL conditions to filter data. You can also view column information of the migration objects in the View Columns section.
     Remove one or all objects	The data transmission service allows you to remove a single object or all migration objects that are added to the right-side list during data mapping. Remove a single migration object In the list on the right, move the pointer over the object that you want to remove, and click Remove to remove the migration object. Remove all migration objects In the list on the right, click Remove All in the upper-right corner. In the dialog box that appears, click OK to remove all migration objects.

   • Select Match Rules. For more information, see Configure matching rules.

6. Click Next. On the Migration Options page, configure the parameters.

   • Full migration

     The following table describes the parameters for full migration, which are displayed only if you have selected Full Data Migration on the Select Migration Type page.

     

     Parameter	Description
     Read Concurrency Configuration	The concurrency for reading data from the source during full migration. The maximum value is 512. A high read concurrency may incur excessive stress on the source, affecting the business.
     Write Concurrency Configuration	The concurrency for writing data to the destination during full migration. The maximum value is 512. A high write concurrency may incur excessive stress on the destination, affecting the business.
     Full Data Migration Rate Limit	You can choose whether to limit the full migration rate as needed. If you choose to limit the full migration rate, you must specify the records per second (RPS) and bytes per second (BPS). The RPS specifies the maximum number of data rows migrated to the destination per second during full migration, and the BPS specifies the maximum amount of data in bytes migrated to the destination per second during full migration. Note The RPS and BPS values specified here are only for throttling. The actual full migration performance is subject to factors such as the settings of the source and destination and the instance specifications.
     Processing Strategy When Destination Table Has Records	If you select Ignore, when the data to be inserted conflicts with existing data of a destination table, the data transmission service logs the conflicting data while retaining the existing data. Important If you select Ignore, data is pulled in IN mode during full verification. In this case, verification is inapplicable if the destination contains data that does not exist in the source, and the verification performance is downgraded. If you select Stop Migration and a destination table contains records, an error prompting migration unsupported is reported during full migration. In this case, you must process the data in the destination table before continuing with the migration. Important If you click Resume in the dialog box prompting the error, the data transmission service ignores this error and continues to migrate data. Proceed with caution.
     Whether to Allow Post-indexing	Specifies whether to create indexes after the full data migration is completed. Post-indexing can shorten the time required for full data migration. For more information about the considerations on post-indexing, see the description below. Important This parameter is displayed only if you have selected both Schema Migration and Full Data Migration on the Select Migration Type page. Only non-unique key indexes can be created after the migration is completed. If the name is already used by an existing object error occurs in the destination Oracle tenant of OceanBase Database during indexing, the data transmission service ignores the error and determines that the index is created, without creating an index again. If you select Allow, set the following parameters: Single Index DDL Concurrency Configuration: the maximum number of concurrent DDL operations allowed for a single index. A larger value indicates higher resource consumption and faster data migration. Maximum concurrent index DDL quantity configuration: the maximum number of post-indexing DDL operations that the system can call at a time.

     If post-indexing is allowed, we recommend that you adjust the parameter settings based on the hardware conditions of OceanBase Database and the business traffic.

     • If you use OceanBase Database V4.x, adjust the settings of the following parameters of the sys tenant and business tenants by using a command-line interface (CLI) client.

       • Adjust the parameter settings of the sys tenant

         // parallel_servers_target specifies the queue condition for parallel queries on each server. 
         // To maximize performance, we recommend that you set this parameter to a value greater than, for example, 1.5 times, the number of physical CPU cores. In addition, make sure that the value does not exceed 64, to prevent database kernels from contending for locks. 
         set global parallel_servers_target = 64; 

       • Adjust the parameter settings of a business tenant

         // Specify the limit on the file memory buffer size.
         alter system set _temporary_file_io_area_size = '10' tenant = 'xxx'; 
         // Disable throttling in V4.x.
         alter system set sys_bkgd_net_percentage = 100;

     • If you use OceanBase Database V3.x, adjust the settings of the following parameters of the sys tenant by using a CLI client.

       // parallel_servers_target specifies the queue condition for parallel queries on each server. 
       // To maximize performance, we recommend that you set this parameter to a value greater than, for example, 1.5 times, the number of physical CPU cores. In addition, make sure that the value does not exceed 64, to prevent database kernels from contending for locks. 
       set global parallel_servers_target = 64; 
       // data_copy_concurrency specifies the maximum number of concurrent data migration and replication tasks allowed in the system. 
       alter system set data_copy_concurrency = 200;

   • Incremental synchronization

     The following table describes the parameters for incremental synchronization, which are displayed only if you have selected Incremental Synchronization on the Select Migration Type page.

     

     Parameter	Description
     Write Concurrency Configuration	The concurrency for writing data to the destination during incremental synchronization. The maximum value is 512. A high write concurrency may incur excessive stress on the destination, affecting the business.
     Incremental Synchronization Rate Limit	You can choose whether to limit the incremental synchronization rate as needed. If you choose to limit the incremental synchronization rate, you must specify the the records per second (RPS) and bytes per second (BPS). The RPS specifies the maximum number of data rows synchronized to the destination per second during incremental synchronization, and the BPS specifies the maximum amount of data in bytes synchronized to the destination per second during incremental synchronization. Note The RPS and BPS values specified here are only for throttling. The actual incremental synchronization performance is subject to factors such as the settings of the source and destination and the instance specifications.
     Incremental Synchronization Start Timestamp	If you have set the migration type to Full Data Migration, this parameter is not displayed. If you have selected Incremental Synchronization but not Full Data Migration, specify a point in time after which the data is to be synchronized. The default value is the current system time. For more information, see Set an incremental synchronization timestamp.

   • Reverse incremental migration

     The following table describes the parameters for reverse incremental migration, which are displayed only if you have selected Reverse Increment on the Select Migration Type page. By default, incremental synchronization configurations are reused for reverse incremental migration.

     

     You can choose not to reuse the incremental synchronization configurations and configure reverse incremental migration as needed.

     Parameter	Description
     Write Concurrency Configuration	The concurrency for writing data to the source during reverse incremental migration. The maximum value is 512. A high concurrency may incur excessive stress on the source, thereby affecting the business.
     Incremental Synchronization Rate Limit	You can choose whether to limit the incremental synchronization rate as needed. If you choose to limit the reverse incremental migration rate, you must specify the the RPS and BPS. The RPS specifies the maximum number of data rows synchronized to the source per second during reverse incremental migration, and the BPS specifies the maximum amount of data in bytes synchronized to the source per second during reverse incremental migration. Note The RPS and BPS values specified here are only for throttling. The actual reverse incremental migration performance is subject to factors such as the settings of the source and destination and the instance specifications.
     Incremental Synchronization Start Timestamp	By default, the forward switchover start timestamp (if any) prevails. This parameter cannot be modified.

   • Advanced migration configuration

     

     Parameter	Description
     Encoding and Length Options	Advanced migration parameters are displayed only when you have selected Schema Migration on the Select Migration Type page and the source and destination databases use different character sets. Note If the character set of the source database is different from that of the destination database, for example, the character set of the source database is GBK while that of the destination database is UTF-8, fields may be truncated, which results in data inconsistency. If you select Automatically Extend Fields at Destination, Namely from N Bytes to 1.5N Bytes, the data after conversion is truncated to the maximum length limit if it exceeds the limit.
     Target table object storage type	This section is displayed only if the destination is an Oracle tenant of OceanBase Database V4.3.0 or later and you have selected Schema Migration or Incremental Synchronization > DDL Synchronization on the Select Migration Type page. This parameter specifies the storage type for destination table objects during schema migration or incremental synchronization. The storage types supported for destination table objects are Default, Row storage, Column storage, and Hybrid columnar storage. For more information, see default_table_store_format. Note The value Default means that other parameters are automatically set based on the parameter configurations of the destination. For table objects in schema migration or new table objects in incremental DDL synchronization, the schemas are subject to the specified storage type.

7. Click Precheck to start a precheck on the data migration task.

   During the precheck, the data transmission service checks the read and write privileges of the database users and the network connections of the databases. A data migration task can be started only after it passes all check items. If an error is returned during the precheck, you can perform the following operations:

   • Identify and troubleshoot the problem and then perform the precheck again.

   • Click Skip in the Actions column of the failed precheck item. In the dialog box that prompts the consequences of the operation, click OK.

8. After the precheck succeeds, click Start Task.

   If you do not need to start the task now, click Save. You can start the task later on the Migration Tasks page or by performing batch operations. For more information about batch operations, see Perform batch operations on data migration tasks.

   The data transmission service allows you to modify the migration objects when a migration task is running. For more information, see View and modify migration objects and their filter conditions. After the data migration task is started, it is executed based on the selected migration types. For more information, see View migration details.

References

• Create an Oracle data source

• Create an OceanBase data source

• Configure matching rules

• View details of a data migration task