Date: Mon, 9 Jul 2012 12:58:14 -0700 (PDT) From: Jason Usher <jusher71@yahoo.com> To: Zaphod Beeblebrox <zbeeble@gmail.com> Cc: freebsd-fs@freebsd.org Subject: Re: vdev/pool math with combined raidzX vdevs... Message-ID: <1341863894.36655.YahooMailClassic@web122501.mail.ne1.yahoo.com>
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Hello again, --- On Fri, 7/6/12, Zaphod Beeblebrox <zbeeble@gmail.com> wrote: > ... so, again with simplistic assumptions, > > p(36drz3 --- 12 drives, 3 groups) = p(12drz3) * 3 > > A "vanilla" RAID-Z2 (if I make an assumption to what you're > saying) is: > > p(36drz2) = 36 * p(f) * 35 * p(f) > > ... but I can't directly answer you question without knowing > a) the > structure of the RAID-Z2 array and p(f). If we use a > 1% figure for > p(f), then P(36drz3,12,3) = 0.035% and p(36drz2) = 4.3% (snip) > Put simply, you add the probabilities of things where any > can cause > the failure (either drive of R0 failing, any one of the 3 > plexes of a > complex array failing) and you multiply things where all > must fail to > produce failure. Ok. So let's start with those numbers from that hardforum link I posted: (probability of data loss during a rebuild) RAID-10: F = 5% RAID-Z1: 1 - (1 - F)^(9 - 1) = 33.7% F= 33.7% RAID-Z2: 1 - (1 - F)^(10 - 1) - (10 - 1) F (1 - F)^(10 - 2) = 7.1% F=7.1% RAID-Z3: 1 - (1 - F)^(11 - 1) - (11 - 1) F (1 - F)^(11 - 2) - (11 - 1)(11 - 2) F^2 (1 - F)^(11 - 3) / 2 F = 1.15% Again, doesn't really matter what F is, since we are only interested in the comparison... From what you said, above, striping 3 different raidz3 arrays together into one pool is ADDITIVE ... so the 1.15% rises to 3.45%. Yes ? So we triple our risk by running all three raidz3 arrays in one pool, but we still have less than half the risk of a single raidz2 vdev (with no striping) which is 7.1%. Am I on the right track here ? I think I'm missing something because with one raidz3, I have a 1.15% chance of "losing a drive during rebuild" but I am thinking about competely healthy arrays who have a larger chance of blowing up because ONE OF THE OTHER vdevs blows four drives simultaneously. So I am really comparing 0% probability (if they aren't combined in a zpool, I can take one vdev out and run over it with a train and the other vdev is unharmed) with X% probability, because now something happening in the other vdev can ruin the healthy one... Am I really the only person worrying about the interactive failure properties of combining vdevs into a pool ?home | help
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