8. Reconstruction

If you have read the rest of this HOWTO, you should already have a pretty good idea about what reconstruction of a degraded RAID involves. Let us summarize:

• Power down the system
• Replace the failed disk
• Power up the system once again.
• Use raidhotadd /dev/mdX /dev/sdX to re-insert the disk in the array
• Have coffee while you watch the automatic reconstruction running

Well, it usually is, unless you're unlucky and your RAID has been rendered unusable because more disks than the ones redundant failed. This can actually happen if a number of disks reside on the same bus, and one disk takes the bus with it as it crashes. The other disks, however fine, will be unreachable to the RAID layer, because the bus is down, and they will be marked as faulty. On a RAID-5 where you can spare one disk only, loosing two or more disks can be fatal.

The following section is the explanation that Martin Bene gave to me, and describes a possible recovery from the scary scenario outlined above. It involves using the failed-disk directive in your /etc/raidtab (so for people running patched 2.2 kernels, this will only work on kernels 2.2.10 and later).

8.1 Recovery from a multiple disk failure

The scenario is:

• A controller dies and takes two disks offline at the same time,
• All disks on one scsi bus can no longer be reached if a disk dies,
• A cable comes loose...

If using mdadm, you could first try to run:

  mdadm --assemble --force

To get this to work, you'll need to have an up to date /etc/raidtab - if it doesn't EXACTLY match devices and ordering of the original disks this will not work as expected, but will most likely completely obliterate whatever data you used to have on your disks.

Look at the sylog produced by trying to start the array, you'll see the event count for each superblock; usually it's best to leave out the disk with the lowest event count, i.e the oldest one.

If you mkraid without failed-disk, the recovery thread will kick in immediately and start rebuilding the parity blocks - not necessarily what you want at that moment.

With failed-disk you can specify exactly which disks you want to be active and perhaps try different combinations for best results. BTW, only mount the filesystem read-only while trying this out... This has been successfully used by at least two guys I've been in contact with.