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 11:00:22 -0500

 I don't know of a single RAID controller that supports both levels.  I 
 know of many that support 0+1 and many that support 10.
 
 If the controller supports 10 and you call it 0+1 in the software, the 
 user is being lied to, and may incorrectly think their controller or 
 FreeBSD does not support RAID-10.
 
 If the controller supports 0+1 and you call it 10 in the software, the 
 user is being lied to, and my incorrectly think their data is more 
 protected than it really is.
 
 Why so much pushback?  Since when did FreeBSD start trying to make users 
 decisions for them, rather than simply allowing them to choose for 
 themselves amongst the options their hardware supports, anyway?
 
 A better question:  Can you name one good reason why the RAID level, 
 whatever it is, should be misrepresented to the user?
 
 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.
 >