Re: Huge iowait during checkpoint finish

On Mon, Jan 11, 2010 at 2:59 PM, Greg Smith <greg(at)2ndquadrant(dot)com> wrote:
> Scott Marlowe wrote:

> If you can shoehorn one more drive, you could run RAID-10 and get much
> better performance.
>
>
> And throwing drives at the problem may not help.  I've see a system with a
> 48 disk software RAID-10 that only got 100 TPS when running a commit-heavy
> test, because it didn't have any way to cache writes usefully for that
> purpose.

A 4 disk RAID-10 will be about 4 to 8 times faster than a RAID-5 of 3
disks. It won't be as fast as a good sized RAID-10 with HW caching,
but it would be a big improvement.

> If the old system had a write caching card, and the new one doesn't, that's
> certainly your most likely suspect for the source of the slowdown.  As for

Agreed. If the new machine is limited to 3 disks, and any one is big
enough to hold the db, I'd look at a two disk mirror with a hot spare
on a HW RAID controller with battery backed chat. If they can't get a
HW RAID controller than switching to SW RAID-1 would be a positive
step.