On Sat, 1 Mar 2008, Franck Routier wrote:
> Well, am I just wrong, or the file system might also heavily rely on
> cache, especially as I use XFS ? So anyway Postgresql has no way to know
> if the data is really on the disk, and in case of a brutal outage, the
> system may definitely lose data, wether there is another level of
> caching (Raid controller) or not...
After PostgreSQL writes to the WAL, it calls fsync. If your filesystem
doesn't then force a real write to disk at that point and clear whatever
cache it might have, it's broken and unsuitable for database use. XFS is
smart enough to understand that.
The only thing people typically run into that will hear fsync and lie
about the data actually being written to disk are a) caching controllers
with the write cache turned on and b) cheap hard drives. In case (a),
having a battery backup for the cache is sufficient to survive most
classes of outage without damage--if the system is without power for
longer than the battery lasts you're in trouble, otherwise is shouldn't be
a problem. In case (b), you have to turn the disk cache off to get
reliable database operation.
I've put all the interesting trivia on this topic I've ever come across at
http://www.westnet.com/~gsmith/content/postgresql/TuningPGWAL.htm if
you're looking for some really exciting reading.
--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD
