Good morning Daniel,

On 3/30/20 10:09 PM, Daniel Jones wrote:

Hello Phil,

quoted

In particular, knowledge of the filesystem or nested structure (LVM?) present on the array will be needed to identify the real data offsets of the three mangled members.

I don't have the history of original creation, but I'm fairly certain
it was something straightforward like:

   mdadm --create /dev/md0 {parameters}
   sudo mkfs.ext4 /dev/md0
   mount /dev/md0 /mnt/raid5

After the array was corrupted I needed to comment out the mount from
my fstab, which was as follows (confirming ext4):

     /dev/md0                                      /mnt/raid5
    ext4    defaults        0       0

Ok.  This should be relatively easy, if a bit time consuming.  Things we 
know:

1) array layout, and chunk size: 512k or 1024 sectors
2) Active device #1 offset 261124 sectors.
3) The array had bad block logging turned on.  We won't re-enable this 
mis-feature.  It is default, so you must turn it off in your --create.

Things we don't know:

1) Data offsets for other drives.  However, the one we know appears to 
be the typical you'd get from one reshape after a modern default 
creation (262144).  There are good odds that the others are at this 
offset, except the newest one that might be at 262144.  You'll have to 
test four combinations: all at 261124 plus one at a time at 262144.

2) Member order for the other drives.  Three drives taken three at a 
time is six combinations.

3) Identity of the first drive kicked out. (Or do we know?)  If not 
known, there's four more combinations: whether to leave out or one of 
three left out.

That yields either twenty-four or 96 different --create --assume-clean 
combinations to test to find the one that gives you the cleanest 
filesystem in a read-only fsck.  (Do NOT mount!  Even a read-only mount 
will write to the filesystem.  Only test with fsck -n.)

Start by creating partitions on all devices, preferably at 2048 sectors. 
  (Should be the default offered.)  Use data offsets 259076 and 260100 
instead of 261124 and 262144.

I recommend writing out all the combinations before you start and 
keeping the fsck -n output from each until you have the final version 
you want.

Yeah, I'd write a script to do it all for me, if your best guess 
combination doesn't yield a good filesystem.

Cheers,
DJ

Phil