OS6 Data Recovery - How to Mount BTRFS Volumes

How to Mount Degraded BTRFS Volumes - RAID 6 - Two Hard Disks Missing

I just did a quick test of the degraded array with one disconnected hard disk. I shut down the machine normally and reconnected the missing hard disk. Upon start up, the array was identified and assembled normally. The array behaved as it did in the first test of the FlexRAID RAID 6 array and the array was assembled automatically during boot up. All that was needed was just to mount the array using "mount -t btrfs -o ro /dev/md127 /mnt" and then the test files were accessible. So an array can apparently be started up with a missing disk and if the missing disk is added back in, it rejoins the array without requiring a rebuild or any repairs.

Then I shut down the system and disconnected ReadyNAS disks 2 and 4 from the motherboard to simulate a double hard disk failure.

Following the exact same procedure as used for the "One Hard Disk Missing" recovery above - everything worked the same.

Stop the auto-assembled arrays.

Re-assemble the data array using the UUID and the "--run" switch.

Then mount it the same way. The data was again copied to the home folder and it passed verification with SHA1 sums. :D :D :D

RAID 6 does allow 2 disks to be removed from the array and the degraded array can still be mounted and the data recovered. :)

Note the following cautions:

1) This is a pretty ideal situation. There is no data corruption from viruses, worms, malware or hardware malfunction. The drives were cleanly removed from a shut down system and suffered no lingering data corruption or slow failure. The success of this recovery method does NOT mean that backups aren't necessary - backup your data! :x

2) The array gets very slow when it's in a degraded state. Copying 17 Gb of test data is not the same as moving 3.6 Tb of data (the full formatted size of the array). Data recovery will need lots of space and lots of time. A fast processor and plenty of RAM will help (ie. don't assume you can use an old, slow, junk PC to recover your data). And don't forget the cooling or you might have a catastrophic failure in the middle of your recovery process.

3) The mdadm and underlying BTRFS filesystem appears to be pretty robust (keeping in mind caution number 1!) But I'm not enough of a Linux expert to know where the "gotchas" are in this process. This is definitely a "last resort" process. Try all the normal ReadyNAS repairs and recovery techniques first before resorting to this method.

4) Don't be afraid to ask for help. There are a lot of knowledgeable people on this forum (thanks everyone! :) ) and a lot of related information floating around at various sites. Most important of all - Don't Panic! Always start by gathering information first and if you're not sure about the changes you're making, ask questions and get help BEFORE you make them. Some are irreversible. I always try all the non-destructive methods first before I start changing settings or zeroing superblocks. I didn't risk any data in this testing and I'm glad I found out how to do this BEFORE I might need to do it. Test thoroughly before making permanent changes.

-Stan