oops
On 8/9/07, Decibel! <decibel(at)decibel(dot)org> wrote:
> You forgot the list. :)
>
> On Thu, Aug 09, 2007 at 05:29:18PM -0500, Scott Marlowe wrote:
> > On 8/9/07, Decibel! <decibel(at)decibel(dot)org> wrote:
> >
> > > 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.
> >
> > I'd think that kind of failure mode is pretty uncommon, unless you're
> > in an environment where physical shocks are common. which is not a
> > typical database environment. (tell that to the guys writing a db for
> > a modern tank fire control system though :) )
> >
> > > Speaking of failures, keep in mind that a normal RAID5 puts you only 2
> > > drive failures away from data loss,
> >
> > Not only that, but the first drive failure puts you way down the list
> > in terms of performance, where a single failed drive in a large
> > RAID-10 only marginally affects performance.
> >
> > > while with RAID10 you can
> > > potentially lose half the array without losing any data.
> >
> > Yes, but the RIGHT two drives can kill EITHER RAID 5 or RAID10.
> >
> > > 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).
> >
> > That's RAID6. The primary advantages of RAID6 over RAID10 or RAID5
> > are two fold:
> >
> > 1: A single drive failure has no negative effect on performance, so
> > the array is still pretty fast, especially for reads, which just suck
> > under RAID 5 with a missing drive.
> > 2: No two drive failures can cause loss of data. Admittedly, by the
> > time the second drive fails, you're now running on the equivalent of a
> > degraded RAID5, unless you've configured >2 drives for parity.
> >
> > On very large arrays (100s of drives), RAID6 with 2, 3, or 4 drives
> > for parity makes some sense, since having that many extra drives means
> > the RAID controller (SW or HW) can now have elections to decide which
> > drive might be lying if you get data corruption.
> >
> > Note that you can also look into RAID10 with 3 or more drives per
> > mirror. I.e. build 3 RAID-1 sets of 3 drives each, then you can lose
> > any two drives and still stay up. Plus, on a mostly read database,
> > where users might be reading the same drives but in different places,
> > multi-disk RAID-1 makes sense under RAID-10.
> >
> > While I agree with Merlin that for OLTP a faster drive is a must, for
> > OLAP, more drives is often the real key. The high aggregate bandwidth
> > of a large array of SATA drives is an amazing thing to watch when
> > running a reporting server with otherwise unimpressive specs.
> >
>
> --
> Decibel!, aka Jim C. Nasby, Database Architect decibel(at)decibel(dot)org
> Give your computer some brain candy! www.distributed.net Team #1828
>
>