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14.4.3. Handling MySQL Recovery with ZFS

When using ZFS replication to provide a constant copy of your data, you should ensure that you can recover your tables, either manually or automatically, in the event of a failure of the original system.

In the event of a failure, you should follow this sequence:

  1. Stop the script on the master, if it is still up and running.

  2. Set the slave filesystem to be read/write: 

    root-shell> zfs set readonly=off slavepool

  3. Start up mysqld on the slave. If you are using InnoDB, Falcon or Maria you should get auto-recovery, if it is needed, to make sure the table data is correct, as shown here when I started up from our mid-INSERT snapshot: 

    InnoDB: The log sequence number in ibdata files does not match
InnoDB: the log sequence number in the ib_logfiles!
081109 15:59:59  InnoDB: Database was not shut down normally!
InnoDB: Starting crash recovery.
InnoDB: Reading tablespace information from the .ibd files...
InnoDB: Restoring possible half-written data pages from the doublewrite
InnoDB: buffer...
081109 16:00:03  InnoDB: Started; log sequence number 0 1142807951
081109 16:00:03 [Note] /slavepool/mysql-5.0.67-solaris10-i386/bin/mysqld: ready for connections.
Version: '5.0.67'  socket: '/tmp/mysql.sock'  port: 3306  MySQL Community Server (GPL)

On MyISAM, or other tables, you may need to run REPAIR TABLE, and you might even have lost some information. You should use a recovery-capable storage engine and a regular synchronization schedule to reduce the risk for significant data loss.