RAID5 demise or coma? after re-creating with a spare

RAID5 demise or coma? after re-creating with a spare

[Lucian Șandor]

Hi all,
There is a problem with my Linux installation, and the drives get
renamed and reordered all the time. Now, it just happened that the two
degraded RAID5s won't return to life. The system would not boot, so I
panicked and deleted: fstab, mdadm.conf, and some of the superblocks.
Now Linux boots, but RAIDs are, of course, dead. I tried to re-create
the arrays, but I cannot recall the correct order and my attempts
failed. I believe that the partitions are OK, because I don't recall
re-creating without "missing", but surely the superblocks are damaged
and certanily most of them are zero now.
Is there a short way to recover the degraded RAIDs without knowing the
order of drives? I have 6 drives in one (including "missing"), that
gives 720 permutations. Also, clearing the superblocks is recoverable,
isn't it?
Thank you,
Lucian

RAID5 demise or coma? after re-creating with a spare

[Lucian Șandor]

Hi all,
There is a problem with my Linux installation, and the drives get
renamed and reordered all the time. Now, it just happened that the two
degraded RAID5s won't return to life. The system would not boot, so I
panicked and deleted: fstab, mdadm.conf, and some of the superblocks.
Now Linux boots, but RAIDs are, of course, dead. I tried to re-create
the arrays, but I cannot recall the correct order and my attempts
failed. I believe that the partitions are OK, because I don't recall
re-creating without "missing", but surely the superblocks are damaged
and certainly most of them are zero now.
Is there a short way to recover the degraded RAIDs without knowing the
order of drives? I have 6 drives in one (including "missing"), that
gives 720 permutations. Also, clearing the superblocks is recoverable,
isn't it?
Thank you,
Lucian

Re: RAID5 demise or coma? after re-creating with a spare

[Neil Brown]

On Fri, 4 Dec 2009 14:46:39 -0500
Lucian Șandor <lucisandor@gmail.com> wrote:

> Hi all,
> There is a problem with my Linux installation, and the drives get
> renamed and reordered all the time. Now, it just happened that the two
> degraded RAID5s won't return to life. The system would not boot, so I
> panicked and deleted: fstab, mdadm.conf, and some of the superblocks.
> Now Linux boots, but RAIDs are, of course, dead. I tried to re-create
> the arrays, but I cannot recall the correct order and my attempts
> failed. I believe that the partitions are OK, because I don't recall
> re-creating without "missing", but surely the superblocks are damaged
> and certanily most of them are zero now.
> Is there a short way to recover the degraded RAIDs without knowing the
> order of drives? I have 6 drives in one (including "missing"), that
> gives 720 permutations. Also, clearing the superblocks is recoverable,
> isn't it?

Yes, 720 permutations.  But you can probably write a script
to generate them all ... how good are your programming skills?
Use "--assume-clean" to create the array so that it doesn't
auto-resync.  Then "fsck -n" to see of the data is even close
to correct.

And why would you think that erasing the superblocks is a recoverable
operation?  It isn't.

NeilBrown
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Re: RAID5 demise or coma? after re-creating with a spare

[Lucian Șandor]

2009/12/4 Neil Brown <neilb@suse.de>:
> On Fri, 4 Dec 2009 14:46:39 -0500
> Lucian Șandor <lucisandor@gmail.com> wrote:
>
>> Hi all,
>> There is a problem with my Linux installation, and the drives get
>> renamed and reordered all the time. Now, it just happened that the two
>> degraded RAID5s won't return to life. The system would not boot, so I
>> panicked and deleted: fstab, mdadm.conf, and some of the superblocks.
>> Now Linux boots, but RAIDs are, of course, dead. I tried to re-create
>> the arrays, but I cannot recall the correct order and my attempts
>> failed. I believe that the partitions are OK, because I don't recall
>> re-creating without "missing", but surely the superblocks are damaged
>> and certanily most of them are zero now.
>> Is there a short way to recover the degraded RAIDs without knowing the
>> order of drives? I have 6 drives in one (including "missing"), that
>> gives 720 permutations. Also, clearing the superblocks is recoverable,
>> isn't it?
>
> Yes, 720 permutations.  But you can probably write a script
> to generate them all ... how good are your programming skills?
> Use "--assume-clean" to create the array so that it doesn't
> auto-resync.  Then "fsck -n" to see of the data is even close
> to correct.
>
> And why would you think that erasing the superblocks is a recoverable
> operation?  It isn't.
>
> NeilBrown
>

Thanks for your reply.

I didn't realize why googling "recovery after zero superblock" was so
inefficient. Sounds very very troubling.

I will script it then for the one array with non-zeroes superblocks.
One issue is that I didn't use -assume-clean in my early attempts of
re-creation. I know this overwrites the superblock. Didn't it make my
superblocks as useless as if I zeroed them?

Thanks,
Lucian Sandor
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