Re: New Linux xfs/reiser file systems

Bruce Momjian <pgman@candle.pha.pa.us> wrote:
>> > Yes, this double-writing is a problem.  Suppose you have your WAL on a
>> > separate drive.  You can fsync() WAL with zero head movement.  With a
>> > log based file system, you need two head movements, so you have gone
>> > from zero movements to two.
>> 
>> It may be worse depending on how the filesystem actually does
>> journalling.  I wonder if an fsync() may cause ALL pending
>> meta-data to be updated (even metadata not related to the 
>> postgresql files).
>> 
>> Do you know if reiser or xfs have this problem?

> I don't know, but the Linux user reported xfs was really slow.

i think this should be tested in more detail: i once tried this
lightly (running pgbench against postgresql 7.1beta4) with
different filesystems: ext2, reiserfs and XFS and reproducable
i got about 15% better results running on XFS ... ok - it's
not a very big test, but i think it might be worth to really
do an a/b test before seing it as a fact that postgresql is
slow on XFS (and maybe reiserfs too ... but reiserfs has had
performance problems in certain situations anyway)

XFS is a journaling fs, but it does all it's work in a very
clever way (delayed allocation etc.) - so usually you should
under normal conditions get decent performance out of it -
otherwise it might be worth sending a mail to the XFS
mailinglist (resierfs maybe dito)

t

-- 
thomas graichen <tgr@spoiled.org> ... perfection is reached, not
when there is no longer anything to add, but when there is no
longer anything to take away. --- antoine de saint-exupery