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Kees Cook
Asked: 2010-12-03 21:26:24 +0800 CST

How do I unmark a "failed" md device?

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In an effort to clean up from a software bug in md, I marked half of a RAID (/dev/sda1 and /dev/sdb1 in RAID1) as faulty using:

mdadm /dev/md2 --fail /dev/sdb1
mdadm /dev/md2 --remove /dev/sdb1

But now I want to use the "failed" half instead. i.e. instead of using /dev/sda1, I want to use /dev/sdb1, but mdadm refuses to bring it online:

mdadm --stop /dev/md2
mdadm -A /dev/md2 /dev/sdb1 --run --force

This fails, as it considers /dev/sdb1 a "spare" and not an actual member of the array any more.

disk raid mdadm

3 Answers

  1. You're using -A which is --assemble, but you should be using -a which is --add.

    If I recall correctly, devices (re)added to an existing array are always added as spares, then shortly afterwards the kernel will pick up that the array is both degraded and has a spare, and then it will start re-mirroring. You can see this with watch cat /proc/mdstat.

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  2. The entire command that should be run is one of the following assuming the array is not assembled or running (which the OP shows as not running):

    If the array is not running:

    mdadm --assemble /dev/md2 /dev/sdb1

    However if the array is running:

    mdadm --manage /dev/md2 -a /dev/sdb1
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  3. I think that the simpliest way to unmark the partition would be to add it again to the array. By the way, if you need to do a test, don't remove the drive from the array. Flag it as faulty and if you reboot, it will fall back on the other partition.

    mdadm /dev/md2 --add /dev/sdb1
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