If you want to migrate data from one Tair (Redis OSS-compatible) instance to another Tair (Redis OSS-compatible) instance but you have not created an instance as the destination instance, we recommend that you use a backup set or the data flashback feature to clone data to a new instance. If you have created an instance and want to migrate data to the instance, we recommend that you use Data Transmission Service (DTS) to implement one-way data migration between the instances. The instances can be self-managed Redis databases or Tair (Redis OSS-compatible) instances. DTS supports full and incremental data migration. When you configure a data migration task, you can select both types to ensure service continuity. This topic compares the three data migration methods and details how to use DTS to migrate Redis data.
Comparison of methods for data migration between Tair (Redis OSS-compatible) instances
DTS overview
Full data migration
DTS allows you to migrate all existing data from a source database to a destination database free of charge.
Incremental data migration
After you perform full data migration, DTS can synchronize incremental data from the source database to the destination database in real time. To perform incremental data migration, you must run the
PSYNCorSYNCcommand in the source database. Otherwise, you can perform only full data migration. You are charged for incremental data migration based on the duration of the migration instead of the amount of data that is transferred. For more information, see Billable items.NoteTo ensure that incremental data migration tasks run as expected, we recommend that you remove the limit on the replication output buffer for the source database. To remove the limit, connect to the source database and run the following command:
CONFIG SET client-output-buffer-limit 'slave 0 0 0'.
Prerequisites
A Tair (Redis OSS-compatible) instance is created as the destination instance and the memory allocated to the destination instance is larger than the memory used by the source instance. For more information, see Step 1: Create an instance.
We recommend that you keep the total amount of memory of the destination database at least 10% larger than the amount of memory used by the source database. If the amount of memory of the destination database is insufficient when you run the data migration task, issues such as data inconsistency or task failures may occur. In this case, empty the destination database and reconfigure the data migration task.
Precautions
When you run a data migration task, do not scale or change the specifications or endpoint of the source or destination database. Otherwise, the data migration task fails. If the data migration task fails, reconfigure the task to account for the changes. In addition, the data migration consumes resources of the source and destination databases. We recommend that you perform the data migration during off-peak hours.
Procedure
Go to the Data Migration Tasks page.
Log on to the Data Management (DMS) console.
In the top navigation bar, click Data Development.
In the left-side navigation pane, choose .
Click Create Task.
Configure the source and destination databases and click Test Connectivity and Proceed in the lower part of the page.
Section
Parameter
Description
N/A
Task Name
The name of the DTS task. DTS automatically generates a task name. We recommend that you specify a descriptive name that makes it easy to identify the task. You do not need to specify a unique task name.
Source Database
Select a DMS database instance
If you registered a source database with DMS, you can select the database. After you select the source database, you do not need to enter information about the database. If you did not register a source database with DMS, ignore this option.
Database Type
The type of the source database. Select Tair/Redis.
Access Method
The access method of the source database. In this example, select Alibaba Cloud Instance.
Instance Region
The region in which the source database resides.
Replicate Data Across Alibaba Cloud Accounts
Specifies whether to migrate data across Alibaba Cloud accounts. Select No.
Instance ID
The ID of the source instance.
Authentication Method
Select Password Login or Secret-free login based on your business requirements. In this example, select Password Login.
NoteIf the password-free access feature is not enabled for the instance, select Password Login.
Database Password
The password used to connect to the source database.
NoteThis parameter is optional. You can leave the parameter empty.
Specify the database password in the <user>:<password> format. For example, if the username of the custom account that you use to log on to the source database is admin and the password is Rp829dlwa, the database password is admin:Rp829dlwa.
Destination Database
Select a DMS database instance
If you registered a destination database with DMS, you can select the database. After you select the destination database, you do not need to enter information about the database. If you did not register a destination database with DMS, ignore this option.
Database Type
The type of the destination database. By default, Tair/Redis is selected.
Access Method
The access method of the destination database. Select Alibaba Cloud Instance.
Instance Region
The region in which the destination instance resides.
Instance ID
The ID of the destination instance.
Authentication Method
Select Password Login or Secret-free login based on your business requirements. In this example, select Password Login.
NoteIf the password-free access feature is not enabled for the instance, select Password Login.
Database Password
The password used to connect to the destination database.
NoteSpecify the database password in the <user>:<password> format. For example, if the username of the custom account that you use to log on to the destination database is admin and the password is Rp829dlwa, the database password is admin:Rp829dlwa.
Configure task objects and click Next: Advanced Settings in the lower part of the page. The following table describes the parameters.
Parameter
Description
Migration Types
The data migration type. Select a data migration type based on your business requirements.
Full Data Migration and Incremental Data Migration (default): uses the native synchronization logic of Redis to write data to the destination database by means of memory snapshots. This way, data from the source database is migrated without downtime.
If you do not have the SYNC or PSYNC permission on the source database, select Full Data Migration.
Full Data Migration: runs the SCAN command to traverse the source database and writes the traversed data to the destination database. To ensure data consistency, do not write new data to the source database during migration.
Processing Mode of Conflicting Tables
Precheck and Report Errors (default): checks whether keys exist in the destination database.
If keys exist, an error is returned during the precheck and the data migration task cannot be started. If keys do not exist, the precheck is passed.
Ignore Errors and Proceed: skips the Check the existence of objects in the destination database check item. If a key with the same name already exists in the destination database, the key is overwritten.
Source Objects and Selected Objects
Select the objects that you want to migrate in the Source Objects section and click
to move the objects to the Selected Objects section. To remove a selected object, select the object in the Selected Objects section and click 
to move the object to the Source Objects section. NoteYou can select databases (DB 0 to DB 255) as the objects that you want to migrate.
Configure the advanced settings and click Next Step: Data Verification in the lower part of the page.
In most cases, you can retain the default settings. For more information, see the "Appendix: Advanced settings" section of this topic.
Configure data verification and click Next: Save Task Settings and Precheck in the lower part of the page.
In most cases, you can retain the default settings. For more information, see Configure data verification.
Perform a precheck, and click Next: Purchase Instance in the lower part of the page.
If Warning or Failed items are generated during the precheck, check the items individually. You can click View Details and troubleshoot the issues. You can also click Confirm Alert Details and ignore the check items. However, issues such as data inconsistency may occur, which may pose risks to your business. For more information, see FAQ. After you complete the preceding operations, perform another precheck.
On the buy page, configure the parameters and click Buy and Start.
(Optional) Select the resource group to which the DTS data migration instance belongs. The default value is default resource group.
(Optional) Select the specifications of the DTS data migration instance. Higher specifications result in faster migration speed and higher costs. The default value is large. For more information, see Specifications of data migration instances.
Read and select the terms of service.
After you purchase the DTS data migration instance, the data migration task starts. You can view the progress of the data migration task on the data migration page.
What to do next
If you perform incremental data migration, you must manually terminate or release the data migration task in the console after the migration is complete.
You can verify the data. For more information, see Verify migrated Redis data.
References
If you want to clone the full data of an existing Tair (Redis OSS-compatible) instance to a new Tair (Redis OSS-compatible) instance, you can use the backup and restoration feature. For information about the differences between cloning data to a new instance by means of backup and restoration and migrating data by using DTS, see Comparison of methods for data migration between Tair (Redis OSS-compatible) instances.
For more information, see Restore data from a backup set to a new instance and Use data flashback to restore data by point in time.
FAQ
Why does the connectivity test fail?
Take note of the following factors when you perform troubleshooting:
The account password is invalid. The password must be in the
user:passwordformat. For more information, see Connect to an instance.If the source database is a self-managed database that is deployed in an on-premises data center or on a third-party cloud platform, a network firewall may block access from DTS servers. In this case, manually add the CIDR blocks of DTS servers in the corresponding region to allow access from the servers. For more information, see Add the CIDR blocks of DTS servers.
Why does the migration task fail to run?
If you scale or change the specifications or endpoint of the source or destination database when you run a data migration task, the task fails. In this case, reconfigure the data migration task to account for the changes.
If the destination instance does not have sufficient available memory or the destination instance is a cluster instance whose specific shard has reached the upper memory limit, the DTS data migration task fails due to an out of memory (OOM) error.
If transparent data encryption (TDE) is enabled for the destination instance, you cannot use DTS to migrate data.
Why are data volumes different in the source and destination databases?
If an expiration policy is enabled for specific keys in the source database, these keys may not be deleted at the earliest opportunity after they expire. In this case, the number of keys in the destination database may be smaller than the number of keys in the source database.
When you run the PSYNC or SYNC command to transmit list data, DTS does not perform the FLUSH operation on the existing data in the destination database. As a result, duplicate data may exist.
If the network is interrupted during a full data migration, DTS may perform multiple full data migrations after the connection is reestablished. In this case, DTS automatically overwrites existing keys that have the same name in the destination database. If you perform a delete operation on the source database at this time, the command is not synchronized to the destination database. As a result, the destination database may have more keys than the source database.
Why am I unable to select a Tair (Redis OSS-compatible) 2.8 instance?
DTS does not support Tair (Redis OSS-compatible) 2.8 instances.
Why should I check whether the eviction policy is noeviction?
By default, the maxmemory-policy parameter that specifies how data is evicted is set to volatile-lru for Tair (Redis OSS-compatible) instances. If the destination database does not have sufficient memory, data inconsistency may occur between the source and destination databases due to data eviction. In this case, the data migration task does not stop running. To prevent data inconsistency, we recommend that you set the maxmemory-policy parameter to noeviction for the destination database. This way, the data migration task fails if the destination database does not have sufficient memory, but you can prevent data loss in the destination database. For more information about data eviction policies, see What is the default eviction policy of Tair?
Why does a key whose prefix is DTS_REDIS_TIMESTAMP_HEARTBEAT exist in the source database?
To ensure the efficiency of data migration and synchronization, DTS inserts a key whose prefix is DTS_REDIS_TIMESTAMP_HEARTBEAT into the source database to record the points in time when updates occur. If the source database uses the cluster architecture, DTS inserts the key into each shard. DTS filters out the key during data migration. After the data migration is complete, the key automatically expires.
Which commands are supported for incremental data migration?
The following commands are supported for incremental data migration:
APPEND
BITOP, BLPOP, BRPOP, and BRPOPLPUSH
DECR, DECRBY, and DEL
EVAL, EVALSHA, EXEC, EXPIRE, and EXPIREAT
FLUSHALL and FLUSHDB
GEOADD and GETSET
HDEL, HINCRBY, HINCRBYFLOAT, HMSET, HSET, and HSETNX
INCR, INCRBY, and INCRBYFLOAT
LINSERT, LPOP, LPUSH, LPUSHX, LREM, LSET, and LTRIM
MOVE, MSET, MSETNX, and MULTI
PERSIST, PEXPIRE, PEXPIREAT, PFADD, PFMERGE, PSETEX, and PUBLISH
RENAME, RENAMENX, RESTORE, RPOP, RPOPLPUSH, RPUSH, and RPUSHX
SADD, SDIFFSTORE, SELECT, SET, SETBIT, SETEX, SETNX, SETRANGE, SINTERSTORE, SMOVE, SPOP, SREM, and SUNIONSTORE
ZADD, ZINCRBY, ZINTERSTORE, ZREM, ZREMRANGEBYLEX, ZUNIONSTORE, ZREMRANGEBYRANK, and ZREMRANGEBYSCORE
XADD, XCLAIM, XDEL, XAUTOCLAIM, XGROUP CREATECONSUMER, and XTRIM
If you run the EVAL or EVALSHA command to call Lua scripts, DTS cannot identify whether these Lua scripts are executed in the destination database. This is because the destination database does not explicitly return the execution results of Lua scripts during incremental data migration.