Date: Tue, 19 Apr 2016 11:22:30 -0500 (CDT) From: "Valeri Galtsev" <galtsev@kicp.uchicago.edu> To: "Steve O'Hara-Smith" <steve@sohara.org> Cc: freebsd-questions@freebsd.org Subject: Re: Raid 1+0 Message-ID: <59485.128.135.52.6.1461082950.squirrel@cosmo.uchicago.edu> In-Reply-To: <20160419153553.86e9a2990094cfcbc1302915@sohara.org> References: <571533F4.8040406@bananmonarki.se> <57153E6B.6090200@gmail.com> <20160418210257.GB86917@neutralgood.org> <20160419153553.86e9a2990094cfcbc1302915@sohara.org>
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On Tue, April 19, 2016 9:35 am, Steve O'Hara-Smith wrote: > On Mon, 18 Apr 2016 17:02:57 -0400 > "Kevin P. Neal" <kpn@neutralgood.org> wrote: > >> On Mon, Apr 18, 2016 at 09:07:07PM +0100, Shamim Shahriar wrote: >> > On 18/04/2016 20:22, Bernt Hansson wrote: >> > > Hello list >> > > >> > > >> > > Used gstripe to stripe the arrays raid/r0 + r1 into stripe0 >> > > >> > Hi >> > >> > I'm sure there are people with more expertise than I am, and they can >> > confirm either ways. But in my mind, given that you used RAID1 first >> > (mirror) and then used those two RAID1 to create a RAID0, this is >> > logically RAID 1+0. In other words, if you lose one disc from each of >> > the RAID1 you are still safe. If you lose both from one single mirror >> > array (highly unlikely), the stripe is unlikely to be of any use. >> >> Not that unlikely. If you take identical disks from the same company and >> subject them to identical load then the probability that they will fail >> around the same time is much higher than random. > > The classic case is not so much same model and same company but > same batch which (at least in the early days of RAID) was quite likely. > The > more similar the drives and their long term load the more likely they are > to fail at around the same time especially under the added load of > replacing the first to fail. Never happens to me that way. So my own small statistics of only a couple of dozens or RAIDs used over decade and a half time is kind ofin agreement with probability theory which IMHO says "double failure event" is much less likely than single failure event. (See my longer post about what I do to avoid apparent "double failure event", which are not actually such). Valeri > Perturbing that similarity is the goal. > >> That's why when I set up a mirror I always build it with drives from >> different companies. And I make it a three way mirror if I can. > > I like to use a variety of drives and ages if I can, and yes two > drives of redundancy either three way mirror or some variant of RAID6 (I > like RAIDZ2). > > -- > Steve O'Hara-Smith <steve@sohara.org> > > _______________________________________________ > freebsd-questions@freebsd.org mailing list > https://lists.freebsd.org/mailman/listinfo/freebsd-questions > To unsubscribe, send any mail to > "freebsd-questions-unsubscribe@freebsd.org" > ++++++++++++++++++++++++++++++++++++++++ Valeri Galtsev Sr System Administrator Department of Astronomy and Astrophysics Kavli Institute for Cosmological Physics University of Chicago Phone: 773-702-4247 ++++++++++++++++++++++++++++++++++++++++
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