Running MySQL slave is quite common and regular task which we do every day, taking backups from slave is often recommended solution. However the current state of MySQL replication makes restoring slave a bit tricky (if possible at all). The main problem is that InnoDB transaction state and replication state are not synchronized. If we speak about backup and you can execute SHOW SLAVE STATUS command you can get reliable information about current state, but some solutions does not allow that. Look for example Sun Storage 7410, which provides storage via NFS and where you can make ZFS snapshots without any info what kind of data you are storing there. What makes situation worse is that files with replication state (relay-log.info, master.info) are not synchronized on disk after each update, and even wrose – in case with NFS they are stored on client side OS/NFS cache for long time. As solution we can do patch to execute fsync() for these files after each write, but I can’t predict how much performance penalty we will see here, I expect it will be very significant.
Our idea is not new, it was taken from TransactionalReplication http://code.google.com/p/google-mysql-tools/wiki/TransactionalReplication patch and http://bugs.mysql.com/bug.php?id=34058 bug report. Basically we want to store the state of replication in InnoDB transactional log file, in this case we will be able to see what position in replication the last executed transaction corresponds to. Of course it will work if you have writes exclusively to InnoDB storage engine, case with mix of storage engines is much more complex and I do not see easy way to solve it.
So we propose overwrite_relay_log_info extension for XtraDB storage engine, the name comes from fact that XtraDB will try to rewrite relay-log.info by replication position, or at least this info is available in error-log output, so you can repoint your slave to correct position by executing CHANGE MASTER command.
Currently patch is available in Launchpad lp:~percona-dev/percona-xtradb/overwrite-relay-log-info , and after testing it will go to main XtraDB tree. If there is interest the patch will be ported to 5.0 and 5.1 trees. Some info available at this documentation page.
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