Date: Tue, 15 Jan 2013 16:30:02 GMT From: Alexander Motin <mav@FreeBSD.org> To: freebsd-bugs@FreeBSD.org Subject: Re: bin/166589: atacontrol(8) incorrectly treats RAID10 and 0+1 the same Message-ID: <201301151630.r0FGU2N2043262@freefall.freebsd.org>
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The following reply was made to PR bin/166589; it has been noted by GNATS. From: Alexander Motin <mav@FreeBSD.org> To: Allen Landsidel <landsidel.allen@gmail.com> Cc: bug-followup@FreeBSD.org Subject: Re: bin/166589: atacontrol(8) incorrectly treats RAID10 and 0+1 the same Date: Tue, 15 Jan 2013 18:20:56 +0200 On 15.01.2013 18:03, Allen Landsidel wrote: > I'm also extremely interested to hear how you intend to "handle it as > RAID10 at the OS level" since that is, in fact, impossible. Easily! > If it's a RAID0+1 in the controller, than it's a RAID0+1. Period. The > OS can't do anything about it. A single disk failure is still knocking > half the array offline (the entire failed RAID-0) and you are left with > a functioning RAID-0 with no redundancy at all. ataraid(8) in question (and its new alternative graid(8)) controls software RAIDs. It means that I can do anything I want in software as long as it fits into existing on-disk metadata format. If RAID BIOS wants to believe that two failed disks of four always mean failed array -- it is their decision I can't change. But after OS booted nothing will prevent me from accessing still available data replicas. > On 1/15/2013 10:55, Alexander Motin wrote: >> Their on-disk formats are identical. Even if RAID BIOS supports RAID0+1, >> there is no problem to handle it as RAID10 at the OS level. That gives >> better reliability without any downsides. I think there is much higher >> chance that inexperienced user will choose RAID0+1 by mistake, then >> experienced wish do to it on intentionally. Do you know any reason why >> RAID0+1 can't be handled as RAID10? >> >> On 15.01.2013 17:28, Allen Landsidel wrote: >>> Most devices typically only support one level or the other, but not >>> both. I don't "Insist that it should exist", it *does* exist. Both >>> levels do, and they are not the same thing. >>> >>> As for why it should be "available" to the user, I think that's a pretty >>> silly question. If their hardware supports one or both levels, they >>> should be available to the user -- and called by their correct names. >>> >>> On 1/15/2013 03:12, Alexander Motin wrote: >>>> That is clear and I had guess you mean it, but why do you insist that >>>> such RAID0+1 variant should even exist if it has no benefits over >>>> RAID10, and why it should be explicitly available to user? >>>> >>>> On 15.01.2013 04:51, Allen Landsidel wrote: >>>>> They are not variants in terminology, they are different raid levels. >>>>> Raid0+1 is two RAID-0 arrays, mirrored into a RAID-1. if one of the >>>>> disks fails, that entire RAID-0 is offline and must be rebuilt, and >>>>> all >>>>> redundancy is lost. A RAID-10 is composed of N raid-1 disks combined >>>>> into a RAID-0. If one disk fails, only that particular RAID-1 is >>>>> degraded, and the redundancy of the others is maintained. >>>>> >>>>> 0+1 cannot survive two failed disks no matter how many are in the >>>>> array. 10 can survive half the disks failing, if it's the right half. >>>>> >>>>> This is something people who've never used more than 4 disks fail to >>>>> grasp, but those of us with 6 (or many many more) know very well. >>>>> >>>>> On 1/14/2013 21:46, Alexander Motin wrote: >>>>>> There could be variants in terminology, but in fact for most of users >>>>>> they are the same. If you have opinion why they should be treated >>>>>> differently, please explain it. >> > -- Alexander Motin
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