29 November, 2011

mysql rsync backup doh!

Well, I moved a big live database from one server to another for a customer. I was planning on taking the site offline first and doing an sql dump/move/restore, but someone accidentally deleted the old server before they were meant too (oops) - leaving me with some rsync backups that I'd made of the whole server over the previous days, to guard against such an eventuality.

Turns out rsyncing a live mysql database is not the right way to do things - I ended up with a database that managed to get mysqld to actually crash when I tried to dump out the database contents. I managed to fix this by dropping the tables that it was crashing on: it conveniently told me which ones it was crashing on, and it let me drop them, just not dump them. Luckily they were all cache tables so there didn't seem to be harm in dropping them and recreating them empty.

It also turns out (obviously, post facto) that mysql databases don't play nicely with rsync's --link-dest option, which is supposed to reduce disk space on the target file system when making multiple snapshots. Because it only optimises away duplicate data for identical files, but mysql stores the entire database (to a first approximation) in a single file, hiding its internal structure from rsync, and so the whole db gets duplicated each time.

Taking an SQL dump should solve the first problem. Not entirely sure what the right way to do an incremental backup of the database is. Maybe I shouldn't even try.


  1. I've been thinking about this too but haven't gotten around to it. How about a diff on your SQL dump to provide incremental. And then use patch to restore it.

  2. Could you use mysql replication and set up a replication proxy that just spits replication updates (chunked into some reasonable size / frequency) to the filesystem, and then back up those replichunks ? You'd need to figure out how to replay the chunks in order to restore the backup.