Re: What's better: Raid 0 or disk for seperate pg_xlog

Thanks to all for the tips.

On Thu, 2005-03-10 at 09:26 -0600, John A Meinel wrote:
> How critical is your data? How update heavy versus read heavy, etc are you?

Large, relatively infrequent uploads, with frequent reads. The
application is a web front-end to scientific research data. The
scientists have their own copy of the data, so if something went really
bad, we could probably get them to upload again.

> Do you have a way to restore the database if something fails? If
> you do nightly pg_dumps, will you survive if you lose a days worth of
> transactions?

For now, we have access to a terabyte backup server, and the DB is small
enough that my sysadmin lets me have hourly pg_dumps for last 24 hours
backed up nightly. Veritas is configured to save daily pg_dumps for the
last week, a weekly dump for the last month and a monthly version for
the last 6 months.

> In general I would recommend RAID1, because that is the safe bet. If
> your db is the bottleneck, and your data isn't all that critical, and
> you are read heavy, I would probably go with RAID1, if you are write
> heavy I would say 2 independent disks.

I feel that we have enough data safety such that I want to go for speed.
Some of the queries are very large joins, and I am going for pure
throughput at this point - unless someone can find a hole in my backup
tactic.

Of course, later we will have money to throw at more spindles. But for
now, I am trying gaze in to the future and maximize my current
capabilities.

Seems to me that the "best" solution would be:

* disk 0 partition 1..n - os mounts
partition n+1 - /var/lib/postgres/data/pg_xlog

* disk 1 partition 1 - /var/lib/postgres/data

* Further (safe) performance gains can be had by adding more spindles as
such:
- first disk: RAID1 to disk 1
- next 2 disks: RAID 0 across the above

Do I grok it?

Thanks again,
--
Karim Nassar
Department of Computer Science
Box 15600, College of Engineering and Natural Sciences
Northern Arizona University, Flagstaff, Arizona 86011
Office: (928) 523-5868 -=- Mobile: (928) 699-9221