[Wylug-discuss] Help needed with 'failed' Linux software RAID 10

[CStreet at tsys.com]

I've not had chance to read it all through and will comment later when I 
have but one thought...

I've had a lot of success sorting these problems out by booting off a 
LiveCD and then imaging the disks with DD etc and then trying to sort it 
allout in a virtual machine environment. This obviously means of course 
you need a lot of spare disc in order to do this. How much space are we 
talking about here?

Chris Street MBCS
Senior Server Engineer, European Datacentre.
+44 (0)1423 580173; +44 (0)7717 366641



Dave Fisher <wylug-discuss at davefisher.co.uk> 
Sent by: wylug-discuss-bounces at wylug.org.uk
29/05/2010 19:43

To
wylug-discuss at wylug.org.uk
cc

Subject
[Wylug-discuss] Help needed with 'failed' Linux software RAID 10






Dear all,

The other day, I posted a request to wylug-help concerning a serious
(for me) RAID failure, but got no reply, so I'm trying again via
wylug-discuss (with appologies).

I know that many wylug members are RAID experts (at least in
comparison with me), so I guess that either I didn't express myself
properly,  no-one uses wylug-help any more, or everyone hates me ;-)

In the first instance, the only help I'm asking for is advice on how
to carry out safe diagnostics and how to interpret the results.

To give you some idea of the facts that I've already established, I've
copied the output from some simple diagnostics in a text attachment.

I might be completely missing the obvious, but some salient points seem to 
be:

 a) LVM2 logical volumes sit on top of a RAID 10 array
 b) mdadm --examine shows partition names/IDs and statuses to be mixed
up, like my last RAI failure
 c) That last failure was 'fixed' by finding the right incantation to
re-assemble and re-sync
 d) One problem, then as now, was to identify which pairs of
partitions make up the each side of the mirror
 e) There seems to be no superblock detected for the whole device (md1)
 f) The spare (sdj4) seems to be invisible in proc
 g) The report of md0 on sdi2 is a red herring - left over from the
incomplete removal of md0


Some questions
-------------

a) My, admittedly dodgy, understanding of RAID 10 is that if I can ID
the two halves of a pair, I should be able to mount the pair like
normal partitions or LVM volumes. Is this correct?

b) Is it possible to readonly mount the md1 partitions in their
current state - safely, without screwing-up further?

Any interpretation of the attached data, or advice on further safe
diagnostics would be appreciated.

Dave
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