I had a hardware raid array, a disk failed during migration and the controller flipped out and is unable to recover. I've written a small python script which has correctly mapped out the state of the array as it is currently (about 160GB was migrated the remaining 5.something TB wasn't) and I'm about to use that information to rebuild the file system onto a new disk. This process is likely to take a LOOOOONNNNGGGG time.
So my question is, if I recover say the first 250GB can I mount this partial file system and check that the script is doing the right thing (e.g. if I've got the block/stripe order incorrect the result will be garbage) before processing the whole array? If so what mount commands will I need to run to mount it.
As an aside, I assume I only need the file system and not the partition table?
Edit:
Some more specific details on the failure: I had a 4 disk raid 5, I did an online capacity expansion to 5 disks, the new disk failed.
What I have done so far is to calculate the point at which the migration got to (using A xor B xor C == D, true for old array false for new, ignoring results from empty space)
Calculated the 'void space' between the arrays using old stripe size, new stripe size and the migrated size; double checked this result by comparing blocks at end of void space with end of migrated chunk till I found matches.
Now I'm writing some code to reconstruct the stripes, glue the two halves of the array back together and write it out to new disks, Ideally I'd like to be able to check the result of this code without having to write out the full 6TB array. By writing out the first 250GB or so I can check and refine the procedure for recovering the whole array without having to wait several days for the recovery to complete.
Best Answer
You can't mount half a partition. Half a partition isn't a container that contains half the files in the filesystem, it's basically unusable. Reaching a file requires traversing several directories and inodes which may be spread all over the partition.
You can however check the beginning of the partition to see if it seems to have a valid filesystem header. This only requires the beginning of the partition (the first few kilobytes) to be ok.
file -s /dev/somethingwill tell you whether that particular device seems to contain a partition (that's assuming that you already have a device entry for the RAID array that's being reassembled).