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Backup & Restore FAQs

This document lists the frequently asked questions (FAQs) and the solutions of TiDB Backup & Restore (BR).

What should I do to quickly recover data after mistakenly deleting or updating data?

TiDB v6.4.0 introduces the flashback feature. You can use this feature to quickly recover data within the GC time to a specified point in time. Therefore, if misoperations occur, you can use this feature to recover data. For details, see Flashback Cluster and Flashback Database.

In TiDB v5.4.0 and later versions, when backup tasks are performed on the cluster under a heavy workload, why does the speed of backup tasks become slow?

Starting from TiDB v5.4.0, BR introduces the auto-tune feature for backup tasks. For clusters in v5.4.0 or later versions, this feature is enabled by default. When the cluster workload is heavy, the feature limits the resources used by backup tasks to reduce the impact on the online cluster. For more information, refer to Backup Auto-Tune.

TiKV supports dynamically configuring the auto-tune feature. You can enable or disable the feature by the following methods without restarting your cluster:

  • Disable auto-tune: Set the TiKV configuration item backup.enable-auto-tune to false.
  • Enable auto-tune: Set backup.enable-auto-tune to true. For clusters upgraded from v5.3.x to v5.4.0 or later versions, the auto-tune feature is disabled by default. You need to manually enable it.

To use tikv-ctl to enable or disable auto-tune, refer to Use auto-tune.

In addition, auto-tune reduces the default number of threads used by backup tasks. For details, see backup.num-threads](/tikv-configuration-file.md#num-threads-1). Therefore, on the Grafana Dashboard, the speed, CPU usage, and I/O resource utilization used by backup tasks are lower than those of versions earlier than v5.4.0. Before v5.4.0, the default value of backup.num-threads was CPU * 0.75, that is, the number of threads used by backup tasks makes up 75% of the logical CPU cores. The maximum value of it was 32. Starting from v5.4.0, the default value of this configuration item is CPU * 0.5, and its maximum value is 8.

When you perform backup tasks on an offline cluster, to speed up the backup, you can modify the value of backup.num-threads to a larger number using tikv-ctl.

PITR issues

What is the difference between PITR and cluster flashback?

From the perspective of use cases, PITR is usually used to restore the data of a cluster to a specified point in time when the cluster is completely out of service or the data is corrupted and cannot be recovered using other solutions. To use PITR, you need a new cluster for data recovery. The cluster flashback feature is specifically designed for the data error scenarios caused by user mis-operations or other factors, which allows you to restore the data of a cluster in-place to the latest timestamp before the data errors occur.

In most cases, flashback is a better recovery solution than PITR for data errors caused by human mistakes, as it has a much shorter RPO (close to zero) and RTO. However, when a cluster is completely unavailable, because flashback cannot run at this time, PITR is the only solution to recover the cluster in this case. Therefore, PITR is always a must-have solution when you develop database disaster recovery strategies, even though it has a longer RPO (up to 5 minutes) and RTO than flashback.

When the upstream database imports data using TiDB Lightning in the physical import mode, the log backup feature becomes unavailable. Why?

Currently, the log backup feature is not fully adapted to TiDB Lightning. Therefore, data imported in the physical mode of TiDB Lightning cannot be backed up into log data

In upstream clusters where you create log backup tasks, avoid using the TiDB Lightning physical mode to import data. Instead, you can use TiDB Lightning logical mode. If you do need to use the physical mode, perform a snapshot backup after the import is complete, so that PITR can be restored to the time point after the snapshot backup.

The cluster has recovered from the network partition failure, but the checkpoint of the log backup task progress still does not resume. Why?

Issue: #13126

After a network partition failure in the cluster, the backup task cannot continue backing up logs. After a certain retry time, the task will be set to ERROR state. At this point, the backup task has stopped.

To resolve this issue, you need to manually execute the br log resume command to resume the log backup task.

After restoring a downstream cluster using the br restore point command, data cannot be accessed from TiFlash. What should I do?

Currently, PITR does not support writing data directly to TiFlash during the restore phase. Instead, br command-line tool executes the ALTER TABLE table_name SET TIFLASH REPLICA *** DDL to replicate the data. Therefore, TiFlash replicas are not available immediately after PITR completes data restore. Instead, you need to wait for a certain period of time for the data to be replicated from TiKV nodes. To check the replication progress, check the progress information in the INFORMATION_SCHEMA.tiflash_replica table.

What should I do if the status of a log backup task becomes ERROR?

During a log backup task, the task status becomes ERROR if it fails and cannot be recovered after retrying. The following is an example:

br log status --pd x.x.x.x:2379 ● Total 1 Tasks. > #1 < name: task1 status: ○ ERROR start: 2022-07-25 13:49:02.868 +0000 end: 2090-11-18 14:07:45.624 +0000 storage: s3://tmp/br-log-backup0ef49055-5198-4be3-beab-d382a2189efb/Log speed(est.): 0.00 ops/s checkpoint[global]: 2022-07-25 14:46:50.118 +0000; gap=11h31m29s error[store=1]: KV:LogBackup:RaftReq error-happen-at[store=1]: 2022-07-25 14:54:44.467 +0000; gap=11h23m35s error-message[store=1]: retry time exceeds: and error failed to get initial snapshot: failed to get the snapshot (region_id = 94812): Error during requesting raftstore: message: "read index not ready, reason can not read index due to merge, region 94812" read_index_not_ready { reason: "can not read index due to merge" region_id: 94812 }: failed to get initial snapshot: failed to get the snapshot (region_id = 94812): Error during requesting raftstore: message: "read index not ready, reason can not read index due to merge, region 94812" read_index_not_ready { reason: "can not read index due to merge" region_id: 94812 }: failed to get initial snapshot: failed to get the snapshot (region_id = 94812): Error during requesting raftstore: message: "read index not ready, reason can not read index due to merge, region 94812" read_index_not_ready { reason: "can not read index due to merge" region_id: 94812 }

To address this problem, check the error message for the cause and perform as instructed. After the problem is addressed, run the following command to resume the task:

br log resume --task-name=task1 --pd x.x.x.x:2379

After the backup task is resumed, you can check the status using br log status. The backup task continues when the task status becomes NORMAL.

● Total 1 Tasks. > #1 < name: task1 status: ● NORMAL start: 2022-07-25 13:49:02.868 +0000 end: 2090-11-18 14:07:45.624 +0000 storage: s3://tmp/br-log-backup0ef49055-5198-4be3-beab-d382a2189efb/Log speed(est.): 15509.75 ops/s checkpoint[global]: 2022-07-25 14:46:50.118 +0000; gap=6m28s

What should I do if the error message ErrBackupGCSafepointExceeded is returned when using the br log resume command to resume a suspended task?

Error: failed to check gc safePoint, checkpoint ts 433177834291200000: GC safepoint 433193092308795392 exceed TS 433177834291200000: [BR:Backup:ErrBackupGCSafepointExceeded]backup GC safepoint exceeded

After you pause a log backup task, to prevent the MVCC data from being garbage collected, the pausing task program sets the current checkpoint as the service safepoint automatically. This ensures that the MVCC data generated within 24 hours can remain. If the MVCC data of the backup checkpoint has been generated for more than 24 hours, the data of the checkpoint will be garbage collected, and the backup task is unable to resume.

To address this problem, delete the current task using br log stop, and then create a log backup task using br log start. At the same time, you can perform a full backup for subsequent PITR.

Feature compatibility issues

Why does data restored using br command-line tool cannot be replicated to the upstream cluster of TiCDC or Drainer?

  • The data restored using BR cannot be replicated to the downstream. This is because BR directly imports SST files but the downstream cluster currently cannot obtain these files from the upstream.

  • Before v4.0.3, DDL jobs generated during the restore might cause unexpected DDL executions in TiCDC/Drainer. Therefore, if you need to perform restore on the upstream cluster of TiCDC/Drainer, add all tables restored using br command-line tool to the TiCDC/Drainer block list.

You can use filter.rules to configure the block list for TiCDC and use syncer.ignore-table to configure the block list for Drainer.

Why is new_collation_enabled mismatch reported during restore?

Since TiDB v6.0.0, the default value of new_collations_enabled_on_first_bootstrap has changed from false to true. BR backs up the new_collation_enabled configuration in the mysql.tidb table of the upstream cluster and then checks whether the value of this configuration is consistent between the upstream and downstream clusters. If the value is consistent, BR safely restores the data backed up in the upstream cluster to the downstream cluster. If the value is inconsistent, BR does not perform the data restore and reports an error.

Suppose that you have backed up the data in a TiDB cluster of an earlier version of v6.0.0, and you want to restore this data to a TiDB cluster of v6.0.0 or later versions. In this situation, you need to manually check whether the value of new_collations_enabled_on_first_bootstrap is consistent between the upstream and downstream clusters:

  • If the value is consistent, you can add --check-requirements=false to the restore command to skip this configuration check.
  • If the value is inconsistent, and you forcibly perform the restore, BR reports a data validation error.

Why does an error occur when I restore placement rules to a cluster?

Before v6.0.0, BR does not support placement rules. Starting from v6.0.0, BR supports placement rules and introduces a command-line option --with-tidb-placement-mode=strict/ignore to control the backup and restore mode of placement rules. With the default value strict, BR imports and validates placement rules, but ignores all placement rules when the value is ignore.

Data restore issues

What should I do to handle the Io(Os...) error?

Almost all of these problems are system call errors that occur when TiKV writes data to the disk, for example, Io(Os {code: 13, kind: PermissionDenied...}) or Io(Os {code: 2, kind: NotFound...}).

To address such problems, first check the mounting method and the file system of the backup directory, and try to back up data to another folder or another hard disk.

For example, you might encounter the Code: 22(invalid argument) error when backing up data to the network disk built by samba.

What should I do to handle the rpc error: code = Unavailable desc =... error occurred in restore?

This error might occur when the capacity of the cluster to restore is insufficient. You can further confirm the cause by checking the monitoring metrics of this cluster or the TiKV log.

To handle this issue, you can try to scale out the cluster resources, reduce the concurrency during restore, and enable the RATE_LIMIT option.

What should I do if the restore fails with the error message the entry too large, the max entry size is 6291456, the size of data is 7690800?

You can try to reduce the number of tables to be created in a batch by setting --ddl-batch-size to 128 or a smaller value.

When using BR to restore the backup data with the value of --ddl-batch-size greater than 1, TiDB writes a DDL job of table creation to the DDL jobs queue that is maintained by TiKV. At this time, the total size of all tables schema sent by TiDB at one time should not exceed 6 MB, because the maximum value of job messages is 6 MB by default (it is not recommended to modify this value. For details, see txn-entry-size-limit and raft-entry-max-size). Therefore, if you set --ddl-batch-size to an excessively large value, the schema size of the tables sent by TiDB in a batch at one time exceeds the specified value, which causes BR to report the entry too large, the max entry size is 6291456, the size of data is 7690800 error.

Where are the backed up files stored when I use local storage?

When you use local storage, backupmeta is generated on the node where BR is running, and backup files are generated on the Leader nodes of each Region.

What should I do if the error message could not read local://...:download sst failed is returned during data restore?

When you restore data, each node must have access to all backup files (SST files). By default, if local storage is used, you cannot restore data because the backup files are scattered among different nodes. Therefore, you have to copy the backup file of each TiKV node to the other TiKV nodes. It is recommended that you store backup data to Amazon S3, Google Cloud Storage (GCS), Azure Blob Storage, or NFS.

What should I do to handle the Permission denied or No such file or directory error, even if I have tried to run br using root in vain?

You need to confirm whether TiKV has access to the backup directory. To back up data, confirm whether TiKV has the write permission. To restore data, confirm whether it has the read permission.

During the backup operation, if the storage medium is the local disk or a network file system (NFS), make sure that the user to start br and the user to start TiKV are consistent (if br and TiKV are on different machines, the users' UIDs must be consistent). Otherwise, the Permission denied issue might occur.

Running br as the root user might fail due to the disk permission, because the backup files (SST files) are saved by TiKV.

Therefore, it is recommended to check the permission before data restore according to the following steps:

  1. Run the Linux command for process query:

    ps aux | grep tikv-server

    The output is as follows:

    tidb_ouo 9235 10.9 3.8 2019248 622776 ? Ssl 08:28 1:12 bin/tikv-server --addr 0.0.0.0:20162 --advertise-addr 172.16.6.118:20162 --status-addr 0.0.0.0:20188 --advertise-status-addr 172.16.6.118:20188 --pd 172.16.6.118:2379 --data-dir /home/user1/tidb-data/tikv-20162 --config conf/tikv.toml --log-file /home/user1/tidb-deploy/tikv-20162/log/tikv.log tidb_ouo 9236 9.8 3.8 2048940 631136 ? Ssl 08:28 1:05 bin/tikv-server --addr 0.0.0.0:20161 --advertise-addr 172.16.6.118:20161 --status-addr 0.0.0.0:20189 --advertise-status-addr 172.16.6.118:20189 --pd 172.16.6.118:2379 --data-dir /home/user1/tidb-data/tikv-20161 --config conf/tikv.toml --log-file /home/user1/tidb-deploy/tikv-20161/log/tikv.log

    Or you can run the following command:

    ps aux | grep tikv-server | awk '{print $1}'

    The output is as follows:

    tidb_ouo tidb_ouo
  2. Query the startup information of the cluster using the tiup command:

    tiup cluster list

    The output is as follows:

    [root@Copy-of-VM-EE-CentOS76-v1 br]# tiup cluster list Starting component `cluster`: /root/.tiup/components/cluster/v1.5.2/tiup-cluster list Name User Version Path PrivateKey ---- ---- ------- ---- ---------- tidb_cluster tidb_ouo v5.0.2 /root/.tiup/storage/cluster/clusters/tidb_cluster /root/.tiup/storage/cluster/clusters/tidb_cluster/ssh/id_rsa
  3. Check the permission for the backup directory. For example, backup is for backup data storage:

    ls -al backup

    The output is as follows:

    [root@Copy-of-VM-EE-CentOS76-v1 user1]# ls -al backup total 0 drwxr-xr-x 2 root root 6 Jun 28 17:48 . drwxr-xr-x 11 root root 310 Jul 4 10:35 ..

    From the output of step 2, you can find that the tikv-server instance is started by the user tidb_ouo. But the user tidb_ouo does not have the write permission for backup. Therefore, the backup fails.

Why are tables in the mysql schema not restored?

Starting from BR v5.1.0, when you perform a full backup, BR backs up the tables in the mysql schema. Before BR v6.2.0, under default configuration, BR only restores user data, but does not restore tables in the mysql schema.

To restore a table created by the user in the mysql schema (not system tables), you can explicitly include the table using table filters. The following example shows how to restore the mysql.usertable table when BR performs a normal restore.

br restore full -f '*.*' -f '!mysql.*' -f 'mysql.usertable' -s $external_storage_url --with-sys-table

In the preceding command,

  • -f '*.*' is used to override the default rules
  • -f '!mysql.*' instructs BR not to restore tables in mysql unless otherwise stated.
  • -f 'mysql.usertable' indicates that mysql.usertable should be restored.

If you only need to restore mysql.usertable, run the following command:

br restore full -f 'mysql.usertable' -s $external_storage_url --with-sys-table

Note that even if you configures table filter, BR does not restore the following system tables:

  • Statistics tables (mysql.stat_*)
  • System variable tables (mysql.tidb, mysql.global_variables)
  • Other system tables

How to deal with the error of cannot find rewrite rule during restoration?

Examine whether there are tables in the restoration cluster sharing the same name as other tables in the backup data but having inconsistent structures. In most cases, this issue is caused by missing indexes in the tables of the restoration cluster. A recommended approach is to delete such tables in the restoration cluster first and then retry restoration.

Other things you may want to know about backup and restore

What is the size of the backup data? Are there replicas of the backup?

During data backup, backup files are generated on the Leader nodes of each Region. The size of the backup is equal to the data size, with no redundant replicas. Therefore, the total data size is approximately the total number of TiKV data divided by the number of replicas.

However, if you want to restore data from local storage, the number of replicas is equal to that of the TiKV nodes, because each TiKV must have access to all backup files.

Why is the disk usage shown on the monitoring node inconsistent after backup or restore using BR?

This inconsistency is caused by the fact that the data compression rate used in backup is different from the default rate used in restore. If the checksum succeeds, you can ignore this issue.

After BR restores the backup data, do I need to execute the ANALYZE statement on the table to update the statistics of TiDB on the tables and indexes?

BR does not back up statistics (except in v4.0.9). Therefore, after restoring the backup data, you need to manually execute ANALYZE TABLE or wait for TiDB to automatically execute ANALYZE.

In v4.0.9, BR backs up statistics by default, which consumes too much memory. To ensure that the backup process goes well, the backup for statistics is disabled by default starting from v4.0.10.

If you do not execute ANALYZE on the table, TiDB will fail to select the optimal execution plan due to inaccurate statistics. If query performance is not a key concern, you can ignore ANALYZE.

Can I start multiple restore tasks at the same time to restore the data of a single cluster?

It is strongly not recommended to start multiple restore tasks at the same time to restore the data of a single cluster for the following reasons:

  • When BR restores data, it modifies some global configurations of PD. Therefore, if you start multiple restore tasks for data restore at the same time, these configurations might be mistakenly overwritten and cause abnormal cluster status.
  • BR consumes a lot of cluster resources to restore data, so in fact, running restore tasks in parallel improves the restore speed only to a limited extent.
  • There has been no test for running multiple restore tasks in parallel for data restore, so it is not guaranteed to succeed.

Does BR back up the SHARD_ROW_ID_BITS and PRE_SPLIT_REGIONS information of a table? Does the restored table have multiple Regions?

Yes. BR backs up the SHARD_ROW_ID_BITS and PRE_SPLIT_REGIONS information of a table. The data of the restored table is also split into multiple Regions.

If the recovery process is interrupted, is it necessary to delete the already recovered data and start the recovery again?

No, it is not necessary. Starting from v7.1.0, BR supports resuming data from a breakpoint. If the recovery is interrupted due to unexpected circumstances, simply restart the recovery task, and it will resume from where it left off.

After the recovery is complete, can I delete a specific table and then recover it again?

Yes, after deleting a specific table, you can recover it again. But note that, you can only recover tables that are deleted using the DROP TABLE or TRUNCATE TABLE statement, not the DELETE FROM statement. This is because DELETE FROM only updates the MVCC version to mark the data to be deleted, and the actual data deletion occurs after GC.

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