--- On Tue, 7/14/09, Matthew Seaman <m.sea...@infracaninophile.co.uk> wrote:
> From: Matthew Seaman <m.sea...@infracaninophile.co.uk> > Subject: Re: ZFS or UFS for 4TB hardware RAID6? > To: mahle...@yahoo.com > Cc: "Free BSD Questions list" <freebsd-questions@freebsd.org> > Date: Tuesday, July 14, 2009, 4:23 AM > Richard Mahlerwein wrote: > > > With 4 drives, you could get much, much higher > performance out of > > RAID10 (which is alternatively called RAID0+1 or > RAID1+0 depending on > > the manufacturer > > Uh -- no. RAID10 and RAID0+1 are superficially > similar but quite different > things. The main differentiator is resilience to disk > failure. RAID10 takes > the raw disks in pairs, creates a mirror across each pair, > and then stripes > across all the sets of mirrors. RAID0+1 divides the > raw disks into two equal > sets, constructs stripes across each set of disks, and then > mirrors the > two stripes. > > Read/Write performance is similar in either case: both > perform well for the sort of small randomly distributed IO > operations you'ld get when eg. > running a RDBMS. However, consider what happens if > you get a disk failure. > In the RAID10 case *one* of your N/2 mirrors is degraded > but the other N-1 > drives in the array operate as normal. In the RAID0+1 > case, one of the > 2 stripes is immediately out of action and the whole IO > load is carried by > the N/2 drives in the other stripe. > > Now consider what happens if a second drive should > fail. In the RAID10 > case, you're still up and running so long as the failed > drive is one of > the N-2 disks that aren't the mirror pair of the 1st failed > drive. > In the RAID0+1 case, you're out of action if the 2nd disk > to fail is one > of the N/2 drives from the working stripe. Or in > other words, if two > random disks fail in a RAID10, chances are the RAID will > still work. If > two arbitrarily selected disks fail in a RAID0+1 chances > are basically > even that the whole RAID is out of action[*]. > > I don't think I've ever seen a manufacturer say RAID1+0 > instead of RAID10, > but I suppose all things are possible. My impression > was that the 0+1 terminology was specifically invented to > make it more visually distinctive > -- ie to prevent confusion between '01' and '10'. > > Cheers, > > Matthew > > [*] Astute students of probability will point out that this > really only > makes a difference for N > 4, and for N=4 chances are > evens either way that failure of two drives would take out > the RAID. > > -- Dr Matthew J Seaman MA, D.Phil. > 7 > Priory Courtyard > > > > Flat 3 > PGP: http://www.infracaninophile.co.uk/pgpkey > Ramsgate > > > > Kent, CT11 9PW > --- On Tue, 7/14/09, Matthew Seaman <m.sea...@infracaninophile.co.uk> wrote: > From: Matthew Seaman <m.sea...@infracaninophile.co.uk> > Subject: Re: ZFS or UFS for 4TB hardware RAID6? > To: mahle...@yahoo.com > Cc: "Free BSD Questions list" <freebsd-questions@freebsd.org> > Date: Tuesday, July 14, 2009, 4:23 AM > Richard Mahlerwein wrote: > > > With 4 drives, you could get much, much higher > performance out of > > RAID10 (which is alternatively called RAID0+1 or > RAID1+0 depending on > > the manufacturer > > Uh -- no. RAID10 and RAID0+1 are superficially > similar but quite different > things. The main differentiator is resilience to disk > failure. RAID10 takes > the raw disks in pairs, creates a mirror across each pair, > and then stripes > across all the sets of mirrors. RAID0+1 divides the > raw disks into two equal > sets, constructs stripes across each set of disks, and then > mirrors the > two stripes. > > Read/Write performance is similar in either case: both > perform well for the sort of small randomly distributed IO > operations you'ld get when eg. > running a RDBMS. However, consider what happens if > you get a disk failure. > In the RAID10 case *one* of your N/2 mirrors is degraded > but the other N-1 > drives in the array operate as normal. In the RAID0+1 > case, one of the > 2 stripes is immediately out of action and the whole IO > load is carried by > the N/2 drives in the other stripe. > > Now consider what happens if a second drive should > fail. In the RAID10 > case, you're still up and running so long as the failed > drive is one of > the N-2 disks that aren't the mirror pair of the 1st failed > drive. > In the RAID0+1 case, you're out of action if the 2nd disk > to fail is one > of the N/2 drives from the working stripe. Or in > other words, if two > random disks fail in a RAID10, chances are the RAID will > still work. If > two arbitrarily selected disks fail in a RAID0+1 chances > are basically > even that the whole RAID is out of action[*]. > > I don't think I've ever seen a manufacturer say RAID1+0 > instead of RAID10, > but I suppose all things are possible. My impression > was that the 0+1 terminology was specifically invented to > make it more visually distinctive > -- ie to prevent confusion between '01' and '10'. > > Cheers, > > Matthew > > [*] Astute students of probability will point out that this > really only > makes a difference for N > 4, and for N=4 chances are > evens either way that failure of two drives would take out > the RAID. Sorry, you are correct. Thanks for clearing that up. I *have,* by the way, stumbled across them a couple of times in the consumer/on-board market, and that's why I tend to remember that and include it even though it's incorrect now. IIRC (which is NOT certain :), I remember once perhaps back around 2000 that a major mag tested some and found that it was only nomenclature differences: all RAID10/1+0/0+1 that were available were all RAID10. And, if I recall, that was back in the PATA days. Anyway, NP. I could also be off my rockers. (Oh, and thanks for the addendum, I actually was following and thinking "...now wait a minute..." and then you clarified that last bit. :) ) _______________________________________________ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to "freebsd-questions-unsubscr...@freebsd.org"