Re: Perfomance Tuning - Mailing list pgsql-performance

OK, I got some hard evidence.  Here is a discussion on the Linux kernel
mailing list with postings from Allen Cox (ac Linux kernels) and Stephen
Tweedie (ext3 author).

    http://www.tux.org/hypermail/linux-kernel/1999week14/subject.html#start

Search for "softupdates and ext2".

Here is the original email in the thread:

    http://www.tux.org/hypermail/linux-kernel/1999week14/0498.html

Summary is at:

    http://www.tux.org/hypermail/linux-kernel/1999week14/0571.html

and conclusion in:

    http://www.tux.org/hypermail/linux-kernel/1999week14/0504.html

I now remember the issue --- ext2 makes all disk changes asynchonously
(unless you mount it via sync, which is slow).  This means that the file
system isn't always consistent on disk.

UFS has always sync metadata (file/directory creation) to the disk so
the disk was always consistent, but doesn't sync the data to the disk,
for performance reasons.  With soft updates, the metadata writes are
delayed, and written to disk in an order that keeps the file system
consistent.

Is this enough evidence, or should I keep researching?

---------------------------------------------------------------------------

Neil Conway wrote:
> On Tue, Aug 12, 2003 at 12:52:46AM -0400, Bruce Momjian wrote:
>  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.
>
> Right, and I understand the need to answer users asking about
> which filesystem to use, but I'd be cautious of bad-mouthing
> another OSS project without any hard evidence to back up our
> claim (of course if we have such evidence, then fine -- I
> just haven't seen it). It would be like $SOME_LARGE_OSS
> project saying "Don't use our project with PostgreSQL, as
> foo@bar.org had data corruption with PostgreSQL 6.3 on
> UnixWare" -- kind of annoying, right?
>
> > > (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?
>
> Perhaps they were suggesting that people mount ext2 using
> data=writeback, rather than the default of data=ordered.
>
> BTW, I've heard from a couple different people that using
> ext3 with data=journalled (i.e. enabling journalling of both
> data and metadata) actually makes PostgreSQL faster, as
> it means that ext3 can skip PostgreSQL's fsync request
> since ext3's log is flushed to disk already. I haven't
> tested this myself, however.
>
> -Neil
>
>
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