On 14-07-28 01:22 PM, Gilberto Nunes wrote:
In other words, nothing to be so concerned, at all...
I wouldn't go *quite* that far.
It *should* work, but make sure you do backups the regular way (i.e.
mysqldump, an agent or some sort, or shut down the database first) until
you've proven that snapshots are adequate for your situation, i.e. don't
rely on them until you've confirmed they work for your exact scenario.
Worst-case: MySQL writes to the DB every second, you've turned off
transactional guarantees (i.e. set MySQL to cache writes for ~10sec,
mounted ext3 "nobarrier, data=writeback") to increase performance. You
take a disk-level snapshot at *exactly* the wrong instant, as MySQL is
expanding a file, or as ext3 is writing a dirent to disk ahead of the
data, or something like that, and you could lose access to the entire
database if you're very, very unlucky.
You CAN experience data loss with snapshots if you get very unlucky and
the deck is stacked against you.
If you have not done anything strange with mount options, and ext3 is
able to use barriers (check your dmesg!!!) inside the VM, and MySQL is
flush()ing data correctly, and everything else works correctly, then
snapshots are fine.
I'm not saying don't use snapshots, I'm saying "trust, but verify" so
you don't get burned later on when a critical snapshot turns out to be
unusable.
(FWIW: somewhat counter-intuitively, Oracle and Sybase seem to be much
more vulnerable to this sort of problem than MySQL or PgSQL, at least in
my experience.)
--
-Adam Thompson
athom...@athompso.net
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