Improbably I had two drives fail in the same Raid5 array within 2 weeks of one another. Which means the array is dead. Yes yes hot spares not being lazy about replacing the failed drive I know. But let's move past that.

The data is somewhat backed up and not of critical importance, so I am not particularly panicked by this. I would still like to try to salvage what I can anyway.

It is a 4-device Software RAID5 set up with mdadm. The drives are as follows:

/dev/sde - device 0, healthy 
/dev/sdf - device 1, first failure, hard failure, totally dead
/dev/sdg - device 2, second failure, badblocks reports a few bad sectors
/dev/sdc - device 3, healthy

I think you can see where I'm going with this. Given that sdg has only the few bad sectors I'd like to believe that most of the data is salvagable. When I reassemble the array with

mdadm --create /dev/md0 --assume-clean --level=5 --raid-devices=4 /dev/sde missing /dev/sdg /dev/sdc

I get no complaints and the device assembles and starts just fine in degraded mode. The problem occurs when I try to mount it. As soon as I run

mount -t ext4 /dev/md0 /mnt/raid

The bad blocks are detected at that point, /dev/sdg fails out of the array, and with only /dev/sde and /dev/sdc still operational the raid goes inactive and the mount fails.

Is there some way to prevent mdadm from failing the drive as soon as it detects a bad block? Some debug flag I can set? Something? I realize that some of the data will be corrupt, and some of the reads will fail.

I'm guessing what I am asking is impossible, although I don't see the theoretical reason that it needs to be. The RAID device could just say I/O error like the drive itself does. But I figure that if the only way to avoid dd failing on a normal hard drive's bad blocks is to use a different program dd_rescue instead, I sort of figure the same will end up being true with mdadm, except I doubt there is any such thing as "mdadm_rescue".

Still, I will ask anyway, and please enlighten me if I am wrong or if you can think of a way to pull some of the data out without the drive instantly crashing out of the array.

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    This is not as improbable as you'd have thought. Alas, RAID5 is not a reliable solution... If the data are backed up (somewhat??) the best bet is to fix your RAID (getting away from RAID5) and restore. Just my $.02... Feb 5, 2013 at 8:06
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    If I were you I would get disk images of all the devices in the array as no doubt they are consumer 7200 rpm drives and improbability nears one that you'll get a further error during the rebuild.
    – user9517
    Feb 5, 2013 at 8:28
  • I have nowhere to put the disk images at the moment unfortunately, and you're right they are Seagate Barracudas. As soon as I get the new Seagate Constellations that I ordered (which should be a bit more reliable) I will take some images of the old array.
    – cecilkorik
    Feb 5, 2013 at 18:41

1 Answer 1


Off hand, try doing a disk dump of the dying drive to a healthy drive, and then add the healthy drive to the array.

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    This should work, though then you have the mystery of which files are corrupt? Solving that problem will probably take much longer than just restoring from backup. Feb 5, 2013 at 8:23
  • Heh, I completely agree with you. I have the impression he'd rather not spend the time/effort to do the restore from backup; if the data really isn't that valuable to him, then I suppose it doesn't matter.
    – Stephan
    Feb 5, 2013 at 18:22
  • I don't have a spare healthy drive of adequate size at the moment (just ordered a bunch) so whenever I get them I might give this a try. Michael Hampton makes a very good point, but at this point I'm more or less resigned to the data being a half-broken mess. I guess it's just my digital-hoarder nature that I would like to try to recover whatever bits and pieces I can from it. I do have a backup, it's just not as recent as I would like.
    – cecilkorik
    Feb 5, 2013 at 18:39
  • I'm a bit of a digital horder too, to be fair, so I'll offer a few thoughts on how I manage it. First: checksumming. Have a script regularly crawl all of your files, creating an md5hash of each file. If it's a new file, the checksum gets written into a new file in a /checksums directory. If the file name matches a file in /checksum then they're compared. If they don't match, they email alert you. Do this once every couple of weeks; that way you have a record of what your 'good' data looks like. Second, if the data is valuable, Deer Hunter is right; raid5 isn't good. Automate backups. :)
    – Stephan
    Feb 5, 2013 at 18:58

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