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

[Allen Landsidel]

http://www.freebsd.org/cgi/query-pr.cgi?pr=166589&cat=

Can somebody else talk some sense into this guy? I'm losing my temper.

-------- Original Message --------
Subject: Re: bin/166589: atacontrol(8) incorrectly treats RAID10 and
0+1 the same
Date: Tue, 15 Jan 2013 17:55:57 +0200
From: Alexander Motin <***@FreeBSD.org>
To: Allen Landsidel <***@gmail.com>
CC: bug-***@FreeBSD.org



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?

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.

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?

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.

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.

[John Baldwin]

Post by Allen Landsidel
http://www.freebsd.org/cgi/query-pr.cgi?pr=166589&cat=
Can somebody else talk some sense into this guy? I'm losing my temper.

Well, is his last question correct? If the RAID BIOS writes the metadata
the same way regardless, then is there a reason (beyond "pure correctness") to
not just treat RAID0+1 as RAID10?

Post by Allen Landsidel
-------- Original Message --------
Subject: Re: bin/166589: atacontrol(8) incorrectly treats RAID10 and
0+1 the same
Date: Tue, 15 Jan 2013 17:55:57 +0200
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?

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.

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?

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.

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
_______________________________________________
http://lists.freebsd.org/mailman/listinfo/freebsd-hardware

[Allen Landsidel]

Several updates have occurred since the email, I think I've explained it
to him well enough.

Yes, there is a difference. A "fakeraid" / firmware RAID0+1 with two
failed disks will not boot. A RAID10 will, if it's not the "wrong" 2nd
disk. Even though the OS has no control over this behavior, it's not
doing anyone any favors to suppressed or obscure the what the hardware
is capable of.

A RAID0+1 is a no-buy for many, including myself. A controller sold
claiming to do RAID10 that actually does 0+1 is an RMA item.

Post by John Baldwin

Post by Allen Landsidel
http://www.freebsd.org/cgi/query-pr.cgi?pr=166589&cat=
Can somebody else talk some sense into this guy? I'm losing my temper.

Well, is his last question correct? If the RAID BIOS writes the metadata
the same way regardless, then is there a reason (beyond "pure correctness") to
not just treat RAID0+1 as RAID10?

Post by Allen Landsidel
-------- Original Message --------
Subject: Re: bin/166589: atacontrol(8) incorrectly treats RAID10 and
0+1 the same
Date: Tue, 15 Jan 2013 17:55:57 +0200
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?

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.

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?

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.

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
_______________________________________________
http://lists.freebsd.org/mailman/listinfo/freebsd-hardware