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I have two different computers with different RAID5 arrays. When I run the command

mdadm --detail /dev/md0

one drive reports "active" while the other reports "clean" for the "state" field. What is the difference? Should I worry about either state? Both seem to work fine.

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up vote 12 down vote accepted

From Documentation/md.txt in the Linux kernel documentation:

clean - no pending writes, but otherwise active.
    When written to inactive array, starts without resync
    If a write request arrives then
      if metadata is known, mark 'dirty' and switch to 'active'.
      if not known, block and switch to write-pending
    If written to an active array that has pending writes, then fails.
    fully active: IO and resync can be happening.
    When written to inactive array, starts with resync

So, no, you don't need to worry about either state; both are normal operation. If you saw inactive, that you'd have to worry about.

mdadm includes a raid monitor daemon that will alert you (via email by default) of any conditions you need to be aware of, in particular a failed disk. You should make sure it is configured & running.

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The metadevice (md0) should report clean and the physical disks (f.e. /dev/sda2 and /dev/sdb2 for a simple mirror) should be active sync. Then all is fine.

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