Re: Perfomance Tuning

Neil Conway wrote:
> On Mon, Aug 11, 2003 at 06:59:30PM -0400, Bruce Momjian wrote:
> > Uh, the ext2 developers say it isn't 100% reliable --- at least that is
> > that was told. I don't know any personally, but I mentioned it while I
> > was visiting Red Hat, and they didn't refute it.
>
> IMHO, if we're going to say "don't use X on production PostgreSQL
> systems", we need to have some better evidene than "no one has
> said anything to the contrary, and I heard X is bad". If we can't
> produce such evidence, we shouldn't say anything at all, and users
> can decide what to use for themselves.
>
> (Not that I'm agreeing or disagreeing about ext2 in particular...)

I don't use Linux and was just repeating what I had heard from others,
and read in postings. I don't have any first-hand experience with ext2
(except for a laptop I borrowed that wouldn't boot after being shut
off), but others on this mailing list have said the same thing.

Here is another email talking about corrupting ext2 file systems:

http://groups.google.com/groups?q=ext2+corrupt+%22power+failure%22&start=10&hl=en&lr=&ie=UTF-8&selm=20021128061318.GE18980%40ursine&rnum=11

From his wording, I assume he is not talking about fsck-correctable
corrupting.

From what I remember, the ext2 failure cases were quite small, but known
by the ext2 developers, and considered too large a performance hit to
correct.

> > > My
> > > untested interpretation was that the update bookkeeping as well as data
> > > update were all getting journalled, the journal space would fill, get
> > > sync'd, then repeat. In effect, all blocks were being written TWICE just
> > > for the journalling, never mind the overhead for PostgreSQL
> > > transactions.
>
> Journalling may or may not have been the culprit, but I doubt everything
> was being written to disk twice:
>
> (a) ext3 does metadata-only journalling by default

If that is true, why was I told people have to mount their ext3 file
systems with metadata-only. Again, I have no experience myself, but why
are people telling me this?

> (b) PostgreSQL only fsyncs WAL records to disk, not the data itself

Right. WAL recovers the data.

--
Bruce Momjian | http://candle.pha.pa.us
pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073