Date: Tue, 15 Jan 2013 19:40:01 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: <201301151940.r0FJe1qg077279@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 21:37:55 +0200 On 15.01.2013 20:53, Allen Landsidel wrote: > On 1/15/2013 13:10, Alexander Motin wrote: >> You may have some point from the boot side, but do you have reliable >> information about which controllers support RAID0+1 and which RAID10? > > Not beyond what the techdocs say for a given card. Is that a valid > reason to present them as the same to the user? I see no reason to implement crappy RAID0+1 in software, when I can do RAID10, just because some unknown salesman told so. As result, I have to either lie about my code capabilities that I know for sure, or lie about RAID BIOS which I know nothing about. >> Also, if user got single failure in RAID10, it >> should not feel much more comfortable then if it would be RAID0+1, as >> second failure still can destroy the data > > This is simply not true. I currently have two 12-disk RAID-10 arrays. > A failure of one disk (which has already happened) leaves ten other > in-use disks that could potentially fail without causing data loss. If > that system were RAID0+1, after a single disk fails the chance that > another disk failure will result in downtime and data loss is 100% -- > not 9%. That "100%" depends on how to calculate it. Since number of drives reduced from 12 to 6, total failure rate from age may also reduce. > RAID-10 is *much* safer than RAID0+1. The more disks you add, the safer > it gets. The more disks you add to a 0+1, the *less* safe it gets. Yes, it is safer, I am not challenging that. But with one disk already down, there is a chance that one more failure will trash everything. It is just not safe. It is just not RAID6 or triple mirror where you can quietly tolerate two failures. >> all that required >> is replace failed disks, boot from any FreeBSD install disk and run >> rebuild from the command line > > This strikes me as a comment from someone not experienced in working > with colocated/remote systems. Without an IPMI subsystem that can > remotely mount disk images, you're talking minutes (or hours) of > downtime while a support technician brings a bootable optical or usb > device to the machine and sets up the KVM-over-IP. > > Presenting RAID10 and RAID0+1 as the same thing is *wrong*. They aren't > the same. > > I will leave it at that. The project and maintainers can decide to fix > the issue or not. I've long since abandoned the machine that had that > controller and have no vested interest any longer. Agreed. :) ataraid is almost dead now, and I see no point to polish cosmetic things there. In graid I've implemented RAID10 algorithm and use it in all cases, wherever RAID BIOS claims RAID10 or RAID0+1. If some BIOS can't boot after second disk failure, that is bad, but data still can be restored if it is possible at all. Thank you for explaining your position. Truth seems to be in the middle, as always. :) -- Alexander Motin
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