If you want to migrate data from one ApsaraDB for Redis instance to another ApsaraDB for Redis 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 Redis instances, which can be self-managed Redis databases or ApsaraDB for Redis 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 ApsaraDB for Redis 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 full data migration, DTS can synchronize incremental data from the source database to the destination database in real time. To perform incremental migration, you must run the
PSYNCorSYNCcommand in the source database. Otherwise, you can perform only full migration. You are charged for incremental data migration based on the duration of the migration, rather than the volume of data being 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
The destination Redis instance is created 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 ApsaraDB for Redis instance.
We recommend that you keep the total memory of the destination database at least 10% larger than the memory used by the source database. If the memory of the destination database is insufficient during migration, it can lead to issues such as data inconsistency or task failures. In such cases, empty the destination database and reconfigure the migration task.
Precautions
During the migration process, do not scale or change the specifications or endpoint of the source or destination database. Otherwise, the task fails. If the task fails, reconfigure the task to account for the changes. In addition, the migration operation consumes resources of the source and destination databases. We recommend that you perform the migration operation during off-peak hours.
Procedure
Go to the Data Migration Tasks page.
Log on to the DMS console.
In the top navigation bar, click DTS.
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 have registered a source database with Data Management (DMS), you can select the source database. After you select the source database, you do not need to enter the source database information. If no source database is registered with DMS, ignore this option.
Database Type
The type of the source database. Set this parameter to ApsaraDB for Redis Enhanced Edition (Tair).
Access Method
The access method of the source database. In this example, Alibaba Cloud Instance is selected.
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. In this example, No is selected.
Instance ID
The ID of the source instance.
Database Password
The password of the source Redis 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 account that you use to log on to the source Redis database is admin and the password is Rp829dlwa, the database password is admin:Rp829dlwa.
Destination Database
Select a DMS database instance
If you have registered a destination database with DMS, you can select the destination database. After you select the destination database, you do not need to enter the destination database information. If no destination database is registered with DMS, ignore this option.
Database Type
By default, ApsaraDB for Redis Enhanced Edition (Tair) 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.
Database Password
The password of the destination Redis database.
NoteSpecify the database password in the <user>:<password> format. For example, if the username of the account that you use to log on to the destination Redis 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.
Parameter
Description
Migration Types
The migration type. Select a 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 an in-memory snapshot. This way, 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 instance 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 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 to be migrated from the Source Objects section and click
to move the objects to the Selected Objects section. To remove a selected object, click the selected object in the Selected Objects section and click the 
icon to move the object to the Source Objects section. NoteYou can select databases (DB 0 to DB 255) as the objects to be migrated.
Configure the advanced settings and click Next: Verification Configurations in the lower part of the page.
In most cases, you can retain the default settings. For more information, see Configure data verification and Appendix: Advanced settings.
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 then click Next: Purchase Instance in the lower part of the page.
If Warning or Failed items are generated during the precheck, check the items one by one. You can click View Details and troubleshoot the issues as prompted. 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 the precheck again.
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. The higher the specifications, the faster the migration speed and the higher the cost. 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 data migration instance, the migration task starts. You can view the progress on the data migration page.
What to do next
If you perform incremental migration, you must manually terminate or release the task in the console after you complete the migration.
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 ApsaraDB for Redis instance to a new ApsaraDB for Redis 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 ApsaraDB for Redis 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?
Consider the following aspects for troubleshooting:
The account password is invalid. The password must be in the
user:passwordformat. For more information, see Logon methods.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 the access from DTS servers. In this case, manually add the CIDR blocks of DTS servers in the corresponding region to allow access from DTS servers. For more information, see Add the CIDR blocks of DTS servers.
Why does the migration task fail to run?
During the migration process, if you make changes such as scaling or changing the specifications or endpoint of the source or destination database, the migration task fails. In this case, reconfigure the task to account for the changes.
If the destination instance has insufficient available memory, or if the destination instance is a cluster instance whose specific shard has reached the upper memory limit, the DTS 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 in the source and destination databases inconsistent?
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. Therefore, the number of keys in the destination database may be less than that 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 migration, DTS may perform multiple full migrations upon reestablishing the connection. In this case, DTS automatically overwrites existing keys with 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 an ApsaraDB for Redis instance that runs Redis 2.8?
DTS does not support ApsaraDB for Redis instances that run Redis 2.8.
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 ApsaraDB for Redis instances. If the destination database has insufficient 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 maxmemory-policy to noeviction for the destination database. This way, the data migration task fails if the destination database has insufficient memory, but data loss can be prevented for the destination database. For more information about data eviction policies, see How does ApsaraDB for Redis evict data by default?
Why is DTS_REDIS_TIMESTAMP_HEARTBEAT available in the source database?
To ensure the quality of data migration, DTS inserts a key whose prefix is DTS_REDIS_TIMESTAMP_HEARTBEAT into the source database to record the timestamp of the last update. If the source database uses the cluster architecture, DTS inserts the key into each shard. The key is filtered out during data migration. After the data migration task is completed, the key expires.
Which commands are supported for incremental migration?
The following commands are supported for incremental 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
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.