On 8/10/07, Decibel! <decibel@decibel.org> wrote:
> On Thu, Aug 09, 2007 at 05:50:10PM -0400, Merlin Moncure wrote:
> > Raid 10 is usually better for databases but in my experience it's a
> > roll of the dice. If you factor cost into the matrix a SAS raid 05
> > might outperform a SATA raid 10 because you are getting better storage
> > utilization out of the drives (n - 2 vs. n / 2). Then again, you
> > might not.
>
> It's going to depend heavily on the controller and the workload.
> Theoretically, if most of your writes are to stripes that the controller
> already has cached then you could actually out-perform RAID10. But
> that's a really, really big IF, because if the strip isn't in cache you
> have to read the entire thing in before you can do the write... and that
> costs *a lot*.
>
> Also, a good RAID controller can spread reads out across both drives in
> each mirror on a RAID10. Though, there is an argument for not doing
> that... it makes it much less likely that both drives in a mirror will
> fail close enough to each other that you'd lose that chunk of data.
>
> Speaking of failures, keep in mind that a normal RAID5 puts you only 2
> drive failures away from data loss, while with RAID10 you can
> potentially lose half the array without losing any data. If you do RAID5
> with multiple parity copies that does change things; I'm not sure which
> is better at that point (I suspect it matters how many drives are
> involved).
when making hardware recommendations I always suggest to buy two
servers and rig PITR with warm standby. This allows to adjust the
system a little bit for performance over fault tolerance.
Regarding raid controllers, I've found performance to be quite
variable as stated, especially with regards to RAID 5. I've also
unfortunately found bonnie++ to not be very reflective of actual
performance in high stress environments. We have a IBM DS4200 that
bangs out some pretty impressive numbers with our app using sata while
the bonnie++ numbers fairly suck.
merlin