Re: How to allocate 8 disks

On Sat, Mar 1, 2008 at 12:06 PM, Craig James <craig_james@emolecules.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
atmost 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
advantageto 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
performancepoint 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.