Re: Slow restoration question - Mailing list pgsql-performance

BTW, you should be able to check to see what the controller is actually
doing by pulling one of the drives from a running array. If it only
hammers 2 drives during the rebuild, it's RAID10. If it hammers all the
drives, it's 0+1.

As for Xserve raid, it is possible to eliminate most (or maybe even all)
of the overhead associated with RAID5, depending on how tricky the
controller wants to be. I believe many large storage appliances actually
use RAID5 internally, but they perform a bunch of 'magic' behind the
scenes to get good performance from it. So, it is possible that the
XServe RAID performs quite well on RAID5. If you provided the results
from bonnie as well as info about the drives I suspect someone here
could tell you if you're getting close to RAID10 performance or not.

On Tue, May 02, 2006 at 02:34:16PM -0500, Will Reese wrote:
> RAID 10 is better than RAID 0+1.  There is a lot of information on
> the net about this, but here is the first one that popped up on
> google for me.
>
> http://www.pcguide.com/ref/hdd/perf/raid/levels/multLevel01-c.html
>
> The quick summary is that performance is about the same between the
> two, but RAID 10 gives better fault tolerance and rebuild
> performance.  I have seen docs for RAID cards that have confused
> these two RAID levels.  In addition, some cards claim to support RAID
> 10, when they actually support RAID 0+1 or even RAID 0+1 with
> concatenation (lame, some of the Dell PERCs have this).
>
> RAID 10 with 6 drives would stripe across 3 mirrored pairs.  RAID 0+1
> with 6 drives is a mirror of two striped arrays (3 disks each).  RAID
> 0+1 (with concatenation) using 6 drives is a mirror of two volumes
> (kind of like JBOD) each consisting of 3 drives concatenated together
> (it's a cheap implementation, and it gives about the same performance
> as RAID 1 but with increased storage capacity and less fault
> tolerance).  RAID 10 is better than RAID 5 (especially with 6 or less
> disks) because you don't have the performance hit for parity (which
> dramatically affects rebuild performance and write performance) and
> you get better fault tolerance (up to 3 disks can fail in a 6 disk
> RAID 10 and you can still be online, with RAID 5 you can only lose 1
> drive).  All of this comes with a higher cost (more drives and higher
> end cards).
>
> -- Will Reese http://blog.rezra.com
>
>
> On May 2, 2006, at 1:49 PM, Mark Lewis wrote:
>
> >They are not equivalent.  As I understand it, RAID 0+1 performs about
> >the same as RAID 10 when everything is working, but degrades much less
> >nicely in the presence of a single failed drive, and is more likely to
> >suffer catastrophic data loss if multiple drives fail.
> >
> >-- Mark
> >
> >On Tue, 2006-05-02 at 12:40 -0600, Brendan Duddridge wrote:
> >>Everyone here always says that RAID 5 isn't good for Postgres. We
> >>have an Apple Xserve RAID configured with RAID 5. We chose RAID 5
> >>because Apple said their Xserve RAID was "optimized" for RAID 5. Not
> >>sure if we made the right decision though. They give an option for
> >>formatting as RAID 0+1. Is that the same as RAID 10 that everyone
> >>talks about? Or is it the reverse?
> >>
> >>Thanks,
> >>
> >>____________________________________________________________________
> >>Brendan Duddridge | CTO | 403-277-5591 x24 |  brendan@clickspace.com
> >>
> >>ClickSpace Interactive Inc.
> >>Suite L100, 239 - 10th Ave. SE
> >>Calgary, AB  T2G 0V9
> >>
> >>http://www.clickspace.com
> >>
> >>On May 2, 2006, at 11:16 AM, Jim C. Nasby wrote:
> >>
> >>>On Wed, Apr 26, 2006 at 05:14:41PM +0930, Eric Lam wrote:
> >>>>all dumpfiles total about 17Gb. It has been running for 50ish hrs
> >>>>and up
> >>>>to about the fourth file (5-6 ish Gb) and this is on a raid 5
> >>>>server.
> >>>
> >>>RAID5 generally doesn't bode too well for performance; that could be
> >>>part of the issue.
> >>>--
> >>>Jim C. Nasby, Sr. Engineering Consultant      jnasby@pervasive.com
> >>>Pervasive Software      http://pervasive.com    work: 512-231-6117
> >>>vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461
> >>>
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--
Jim C. Nasby, Sr. Engineering Consultant      jnasby@pervasive.com
Pervasive Software      http://pervasive.com    work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461