bin/166589: atacontrol(8) incorrectly treats RAID10 and 0+1 the same

[Allen Landsidel]

The following reply was made to PR bin/166589; it has been noted by GNATS.

From: Allen Landsidel <landsidel.allen at gmail.com>
To: Alexander Motin <mav at FreeBSD.org>
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 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.
 >