support quality (Re: dump | restore fails: unknown tape
peter.schuller at infidyne.com
Thu Mar 26 08:40:57 PDT 2009
> Exactly right - if you backup a database by relying on storage snapshots
> then the best you will end up with is a database that needs to be
> recovered when you restore those volumes (crash consistent). That's not a
> good place to be in when your production DB has just blown up.
If you have a production db that you care about, your database better
not have trouble recoverying from a crash consistent state. But again
I'm not suggesting that snapshot based backups be the primary method
of backing up.
In terms of time-to-recovery, having a crash consistent DB can be a
lot quicker to recover than grabbing a dump, whose restoration will
tend to be a lot slower than copying files.
> Absolutely. You really must use a tool that interacts with the database
> to perform the backup. Most commercial DBs have hooks that allow the
> backup routines to call out to custom snapshot facilities. One would
> usually request a backup through the database, which would then freeze IO
> to its data files and maybe log files, deal with flushing caches etc and
> then call your snapshot routine. I'm not aware that MySQL and Postgres do
> though so the best you can do is a dump.
I do not think "really must" is appropriate since clearly you can
recover without DB specific integration. There may be reasons why it's
better to have DB specific integration though (for example, limiting
the amount of log reply that will be needed at recovery). The
implication above that you cannot use snapshot based mechanisms with
PostgreSQL and MySQL is not true; it's just that if you do you have to
know what you're doing.
/ Peter Schuller
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