Re: RAID6 Reshape Gone Awry

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David writes:
As I read it, he has this (prior to adding the new disk):

md0 = raid6(sda5, sdb5, sdc5, sdd5, sde5)
md1 = raid6(sda6, sdb6, sdc6, sdd6, sde6)
md9 = raid6(sda14, sdb14, sdc14, sdd14, sde14)

That's correct (although it's md5 - md14, to match the partition numbers). You're also correct that the LVM is a concatenation rather than striped. It performs just fine for its use case: occasional large writes (mostly with scp), lots of reading.

David continues:
I have sometimes used multiple arrays like this:

md0 = raid1,n4(sda1, sdb1, sdc1, sdd1) for /boot (makes grub happy)
md1 = raid5(sda2, sdb2, sdc2, sdd2) for everything else

But this particular setup seems very odd to me - I would love to know the
reasoning behind it.

In fact, there is also a RAID1 md0 for grub's sake as well, but it's not relevant to the problem.

I first built this array about four years ago, when CentOS 5.2 was current. It started life as a RAID5 (not 6) of 4 500GB drives, and I knew when I created it that I'd need to grow it over time by adding drives.

At that time, though, mdadm as shipped with CentOS 5.2 couldn't reshape a RAID5 -- IIRC, the most recent version of mdadm at the time listed it as an experimental feature that would eat your data and give you bad breath. But LVM + md + multiple partitions makes it possible, as long as you hold some space in reserve (a good idea for snapshot support anyway). Use pvmove to clear a given md device, pull the md out of the LVM, dissassemble it, reassemble it in whatever new configuration you need, and then put it back into the LVM.

Yes, it is an administrative mess. But it was a powerful administrative mess. [ :) ] This array has gone from a 4x500GB RAID5 to a 4x1500GB RAID5 to a 5x1500GB RAID6, without ever running anything in degraded mode, or taking the array as whole offline for any significant time.

Of course, the downside is that pvmove + recreating the array spends a lot of time hammering the drives: for 5x1500 RAID6 to 6x1500 RAID6, it was looking like a few weeks. Since mdadm _can_ reshape RAID6 now, and it was past time to get off CentOS anyway, spending a few weeks beating on the disk drives didn't much appeal to me.

To preempt a few other obvious questions: CentOS was a plus because I worked at a shop that made heavy use of RHEL at the time. Getting CentOS to boot off RAID sucked, though; that plus my tendency towards sysadmin by not screwing with a working system made me disinclined, for a long time, to go to a newer OS or mdadm. And it's a rather stripped-down system, to make security simpler to manage.

At this point, the system boots Ubuntu off CF, sidestepping the whole booting-off-RAID issue completely.

Stan notes:
What it can do is cause massive problems for the elevator when you try
to reshape 10 arrays simultaneously...

Note, though, that mdadm _did not_ try to reshape ten arrays simultaneously. It marked all but one as "pending" and then started into reshaping the one, which isn't any more abuse of the elevator algorithm than it normally gets...

Stan also suggests:
Backup what you need to external storage [and] [s]tart over from

to which David concurs:
If the OP can manage it, then I agree.

Nope, the OP cannot, especially not with arrays that can't be started. [ :) ] As noted, in theory it's all replaceable data anyway, but it would be much more pleasant to not have to make the experiment.

<deep breath> OK. All that being said, can we perhaps take the honor of the list as upheld, and return to the question of recovery? Is there a way to recover a RAID6 where the event counters and checksums and all that are consistent, but where the superblock is marked as version 0.91.00, and where it complains about failing to restore the critical section, even though it said it got past the critical section before?

Thanks much!

-- Flynn

Never let your sense of morals get in the way of doing what's right.
                                                           (Isaac Asimov)

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