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15.17.2 InnoDB Recovery

This section describes InnoDB recovery. Topics include:

Point-in-Time Recovery

To recover an InnoDB database to the present from the time at which the physical backup was made, you must run MySQL server with binary logging enabled, even before taking the backup. To achieve point-in-time recovery after restoring a backup, you can apply changes from the binary log that occurred after the backup was made. See Section 7.5, “Point-in-Time (Incremental) Recovery Using the Binary Log”.

Recovery from Data Corruption or Disk Failure

If your database becomes corrupted or disk failure occurs, you must perform the recovery using a backup. In the case of corruption, first find a backup that is not corrupted. After restoring the base backup, do a point-in-time recovery from the binary log files using mysqlbinlog and mysql to restore the changes that occurred after the backup was made.

In some cases of database corruption, it is enough to dump, drop, and re-create one or a few corrupt tables. You can use the CHECK TABLE statement to check whether a table is corrupt, although CHECK TABLE naturally cannot detect every possible kind of corruption.

In some cases, apparent database page corruption is actually due to the operating system corrupting its own file cache, and the data on disk may be okay. It is best to try restarting the computer first. Doing so may eliminate errors that appeared to be database page corruption. If MySQL still has trouble starting because of InnoDB consistency problems, see Section 15.20.2, “Forcing InnoDB Recovery” for steps to start the instance in recovery mode, which permits you to dump the data.

InnoDB Crash Recovery

To recover from a MySQL server crash, the only requirement is to restart the MySQL server. InnoDB automatically checks the logs and performs a roll-forward of the database to the present. InnoDB automatically rolls back uncommitted transactions that were present at the time of the crash. During recovery, mysqld displays output similar to this:

InnoDB: The log sequence number 664050266 in the system tablespace does not match 
the log sequence number 685111586 in the ib_logfiles!
InnoDB: Database was not shutdown normally!
InnoDB: Starting crash recovery.
InnoDB: Using 'tablespaces.open.2' max LSN: 664075228
InnoDB: Doing recovery: scanned up to log sequence number 690354176
InnoDB: Doing recovery: scanned up to log sequence number 695597056
InnoDB: Doing recovery: scanned up to log sequence number 700839936
InnoDB: Doing recovery: scanned up to log sequence number 706082816
InnoDB: Doing recovery: scanned up to log sequence number 711325696
InnoDB: Doing recovery: scanned up to log sequence number 713458156
InnoDB: Applying a batch of 1467 redo log records ...
InnoDB: 10%
InnoDB: 20%
InnoDB: 30%
InnoDB: 40%
InnoDB: 50%
InnoDB: 60%
InnoDB: 70%
InnoDB: 80%
InnoDB: 90%
InnoDB: 100%
InnoDB: Apply batch completed!
InnoDB: 1 transaction(s) which must be rolled back or cleaned up in total 561887 row 
operations to undo
InnoDB: Trx id counter is 4096
...
InnoDB: 8.0.1 started; log sequence number 713458156
InnoDB: Waiting for purge to start
InnoDB: Starting in background the rollback of uncommitted transactions
InnoDB: Rolling back trx with id 3596, 561887 rows to undo
...
./mysqld: ready for connections....

InnoDB crash recovery consists of several steps:

Tablespace discovery
Tablespace discovery is the process that InnoDB uses to identify tablespaces that require redo log application. See Tablespace Discovery During Crash Recovery.
Redo log application
Redo log application is performed during initialization, before accepting any connections. If all changes are flushed from the buffer pool to the tablespaces (ibdata* and *.ibd files) at the time of the shutdown or crash, redo log application is skipped. InnoDB also skips redo log application if redo log files are missing at startup.
- The current maximum auto-increment counter value is written to the redo log each time the value changes, which makes it crash-safe. During recovery, InnoDB scans the redo log to collect counter value changes and applies the changes to the in-memory table object.
  For more information about how InnoDB handles auto-increment values, see Section 15.6.1.4, “AUTO_INCREMENT Handling in InnoDB”, and InnoDB AUTO_INCREMENT Counter Initialization.
- When encountering index tree corruption, InnoDB writes a corruption flag to the redo log, which makes the corruption flag crash-safe. InnoDB also writes in-memory corruption flag data to an engine-private system table on each checkpoint. During recovery, InnoDB reads corruption flags from both locations and merges results before marking in-memory table and index objects as corrupt.
- Removing redo logs to speed up recovery is not recommended, even if some data loss is acceptable. Removing redo logs should only be considered after a clean shutdown, with innodb_fast_shutdown set to 0 or 1.
Roll back of incomplete transactions
Incomplete transactions are any transactions that were active at the time of crash or fast shutdown. The time it takes to roll back an incomplete transaction can be three or four times the amount of time a transaction is active before it is interrupted, depending on server load.
You cannot cancel transactions that are being rolled back. In extreme cases, when rolling back transactions is expected to take an exceptionally long time, it may be faster to start InnoDB with an innodb_force_recovery setting of 3 or greater. See Section 15.20.2, “Forcing InnoDB Recovery”.
Change buffer merge
Applying changes from the change buffer (part of the system tablespace) to leaf pages of secondary indexes, as the index pages are read to the buffer pool.
Purge
Deleting delete-marked records that are no longer visible to active transactions.

The steps that follow redo log application do not depend on the redo log (other than for logging the writes) and are performed in parallel with normal processing. Of these, only rollback of incomplete transactions is special to crash recovery. The insert buffer merge and the purge are performed during normal processing.

After redo log application, InnoDB attempts to accept connections as early as possible, to reduce downtime. As part of crash recovery, InnoDB rolls back transactions that were not committed or in XA PREPARE state when the server crashed. The rollback is performed by a background thread, executed in parallel with transactions from new connections. Until the rollback operation is completed, new connections may encounter locking conflicts with recovered transactions.

In most situations, even if the MySQL server was killed unexpectedly in the middle of heavy activity, the recovery process happens automatically and no action is required of the DBA. If a hardware failure or severe system error corrupted InnoDB data, MySQL might refuse to start. In this case, see Section 15.20.2, “Forcing InnoDB Recovery”.

For information about the binary log and InnoDB crash recovery, see Section 5.4.4, “The Binary Log”.

Tablespace Discovery During Crash Recovery

If, during recovery, InnoDB encounters redo logs written since the last checkpoint, the redo logs must be applied to affected tablespaces. The process that identifies affected tablespaces during recovery is referred to as tablespace discovery.

Tablespace discovery relies on the innodb_directories setting, which defines the directories to scan at startup for tablespace files. Directories defined by innodb_data_home_dir, innodb_undo_directory, and datadir are automatically appended to the innodb_directories argument value, regardless of whether the innodb_directories option is configured explicitly. Tablespace files defined with an absolute path or that reside outside of the directories automatically appended to the innodb_directories setting should be added to the innodb_directories setting. Recovery is terminated if any tablespace file referenced in a redo log has not been discovered previously.

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