Re: How to allocate 8 disks

On Sat, Mar 1, 2008 at 12:06 PM, Craig James <craig_james(at)emolecules(dot)com> wrote:
> We're upgrading to a medium-sized server, a Dell PowerEdge 2950, dual-quad CPU's and 8 GB memory. This box can hold at most 8 disks (10K SCSI 2.5" 146 GB drives) and has Dell's Perc 6/i RAID controller.
>
> I'm thinking of this:
>
> 6 disks RAID 1+0 Postgres data
> 1 disk WAL
> 1 disk Linux
>
> I've often seen RAID 1 recommended for the WAL. Is that strictly for reliability, or is there a performance advantage to RAID 1 for the WAL?
>
> It seems to me separating the OS and WAL on two disks is better than making a single RAID 1 and sharing it, from a performance point of view.

It's a trade off. Remember that if the single disk hold xlog fails
you've just quite possubly lost your database. I'd be inclined to
either using a RAID-1 of two disks for the OS and xlog, and having
pgsql log to the 6 disk RAID-10 instead of the OS / xlog disk set.

More important, do you have battery backed cache on the controller? A
good controller with a battery backed cache can usually outrun a
larger array with no write cache when it comes to transactions /
writing to the disks.