bin/166589: atacontrol(8) incorrectly treats RAID10 and 0+1 the same
Alexander Motin
mav at FreeBSD.org
Tue Jan 15 16:30:03 UTC 2013
The following reply was made to PR bin/166589; it has been noted by GNATS.
From: Alexander Motin <mav at FreeBSD.org>
To: Allen Landsidel <landsidel.allen at gmail.com>
Cc: bug-followup at 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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