Re: questions about disk configurations

On Mon, 9 Dec 2002, eric soroos wrote:

> >
> > Er, no, it's not.  In fact, for a 3-disk config, I reccommend:
> >
> > Disk 1: OS, swap, system logs
> > Disk 2: Data + Indexes
> > Disk 3: Transaction Log
>
> What is the accepted way of splitting the data from pg_xlog?

You really can't split it so to speak.  It all needs to be in one place.
Or do you mean splitting the load?  Maybe putting it onto a RAID0
partition, but that's chancy.

> I've been testing some configurations for low budget performance, and I
> haven't been able to make this help vs. one disk. (under osx, ymmv)

I haven't found anything that helps much either, except for fast drives.

You can, however, turn on the noatime mounting option under Linux (BSD has
something similar) and it should help speed things up on any file system.
You can also try turning on the async option, but I'm not sure this is a
problem or not for data integrity on a transaction log file system.
Comments?

> I rsync'd the pg_xlog directory to another disk, then set up a symlink
> pointing from the data/pg_xlog to /other/disk/pg_xlog.
>
> I then got tps numbers that were 2/3 of the single ide drive speed. The
> only explanation I can come up with is that something is seeking to the
> symlink, then doing the actual write on the other drive.

rsync isn't still running is it?  you can just use the cp command while
the database is shut down to move the pg_xlog dir.  like so:

pg_ctl stop
mkdir /mnt/bigdog/pg_xlog
chown postgres.postgres /mnt/bigdog/pg_xlog
chmod 700 /mnt/bigdog/pg_xlog
cd $PGDATA
cp -Rfp pg_xlog/* /mnt/bigdog/pg_xlog/
mv pg_xlog pg_xlog.old (I always keep stuff till I'm sure I really don't
need it.)
ln -s /mnt/bigdog/pg_xlog pg_xlog
pg_ctl start



Don't forget, noatime in the mount options, makes a big difference.