Date: Tue, 15 Jan 2013 16:10:02 GMT From: Allen Landsidel <landsidel.allen@gmail.com> To: freebsd-bugs@FreeBSD.org Subject: Re: bin/166589: atacontrol(8) incorrectly treats RAID10 and 0+1 the same Message-ID: <201301151610.r0FGA2AP039057@freefall.freebsd.org>
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The following reply was made to PR bin/166589; it has been noted by GNATS. From: Allen Landsidel <landsidel.allen@gmail.com> To: Alexander Motin <mav@FreeBSD.org> Cc: bug-followup@FreeBSD.org Subject: Re: bin/166589: atacontrol(8) incorrectly treats RAID10 and 0+1 the same Date: Tue, 15 Jan 2013 11:03:16 -0500 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. 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. 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. >
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