Re: What is WAL used for?

Holger Hoffstaette wrote:

> On Fri, 28 Nov 2003 15:19:36 +0100, Thierry Missimilly wrote:
>
> > I have tried a little bench with pgbench on my 2 proc 2.4 Gb with 4 GB RAM
> > and Linux RH 9.0.
> > ...
>
> Which filesystem in which mode? Yes, that's relevant and in fact the
> make-or-break factor here, at least from the POV of the hard drive.
> I guess RH9 uses ext3 in journaled mode by default, which does data as
> well as metadata journaling. Retry your benchmarks with both ext2 and ext3
> in data=writeback mode; both results should be much closer to each other.
>

You are right, my filesystem types are ext3.

With the data=writeback mode, I increase the TPS by 18% and dicrease the wait
I/O from 54% to 30%.
I did not change my filesystem to ext2 as I have to have to cancel the partition
and recreate all the database. Futhermore, i have understood that journaled
filesystem allowed better and faster fsck after a Power off crash and it is not
redundant with the WAL Crash recovery.
I think that "journaling" is at file system level and WAL is above in the
Database level. What happen if the xlog filesystem has been breakdown by a power
off. All the Data concisentcy done by PG will be lost. I hope that data stored
in the FS journal, can avoid that.

Thierry Missimilly