My problem is the result of working late and not informing myself
previously, I'm fully aware that I should have had a backup, be less
spontaneous and more cautious.
The initial situation is a RAID-5 array with three disks. I assume it to
look follows:
| Disk 1 | Disk 2 | Disk 3 |
|----------|----------|----------|
| out | Block 2 | P(1,2) |
| of | P(3,4) | Block 4 | degenerated but working
| sync | Block 5 | Block 6 |
Then I started the re-sync:
| Disk 1 | Disk 2 | Disk 3 |
|----------|----------|----------|
| Block 1 | Block 2 | P(1,2) |
| Block 3 | P(3,4) | Block 4 | already synced
| P(5,6) | Block 5 | Block 6 |
. . .
| out | Block b | P(a,b) |
| of | P(c,d) | Block d | not yet synced
| sync | Block e | Block f |
But I didn't wait for it to finish as I actually wanted to add a fourth
disk and so started a grow process. But I just changed the size of the
array, I didn't actually add the fourth disk (don't ask why I cannot
recall it). I assume that both processes - re-sync and grow - raced
through the array and did their job.
| Disk 1 | Disk 2 | Disk 3 |
|----------|----------|----------|
| Block 1 | Block 2 | Block 3 |
| Block 4 | Block 5 | P(4,5,6) | with four disks but degenerated
| Block 7 | P(7,8,9) | Block 8 |
. . .
| Block a | Block b | P(a,b) |
| Block c | P(c,d) | Block d | not yet grown but synced
| P(e,f) | Block e | Block f |
. . .
| out | Block V | P(U,V) |
| of | P(W,X) | Block X | not yet synced
| sync | Block Y | Block Z |
And after running for a while - my NAS is very slow (partly because all
disks are LUKS'd), mdstat showed around 1GiB of Data processed - we had
a blackout. Water dropped in a distribution socket and *poff*. After a
reboot I wanted to resemble everything, didn't know what I was doing so
the RAID superblock is now lost and I failed to reassemble (this is the
part I really can't recall, I panicked). I never wrote anything to the
actual array so I assume, better hope that no actual data is lost.
I have a plan but wanted to check with you before doing anything stupid
again.
My idea is to look for that magic number of the ext4-fs to find the
beginning of Block 1 on Disk 1, then I would copy an reasonable amount
of data and try to figure out how big Block 1 and hence chunk-size is -
perhaps fsck.ext4 can help do that? After that I copy another reasonable
amount of data from Disks 1-3 to figure out the border between the grown
Stripes and the synced Stripes. And from there on I'd have my data in a
defined state from which I can save the whole file system.
One thing I'm wondering is if I got the layout right. And the other
might be rather a case for the ext4-mailing list but I'd ask it anyway:
how can I figure where the file system starts to be corrupted?
embarrassed Greetings,
Peter Hoffmann
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