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Re: FOLLOW-UP: RAID-1 (mirroring) disk failed; now what? [failed]



Will Partain wrote:
* A _big mistake_ that I made! --

  At some point, with my new disk (/dev/sda) in and booted off the
  rescue CD, I did something like...

  dd if=/dev/sdb2 of=/dev/sda2

  ... i.e. brute-force copy all of a raid-1 partition onto its
  presently-empty cousin.  Theory: "/dev/sda is empty and not in play;
  what harm can it do?"

  Answer (I think): Lots.  The RAID software snoops around on these
  (type 'fd') partitions and silently decides what to make of the
  situation.  This is really not what you want in this delicate
  state.  Information is OK ("I've spotted a degraded array on
  /dev/sdb2, which seems odd"), and doing nothing is OK, but "being
  helpful" isn't.  (Is there a kernel boot parameter to turn off RAID
  cleverness?)

Doing this copies the UUIDs that the RAID software uses to identify each partition of the RAID. It's the RAID equivalent of having identical filesystem labels on two partitions so that mount doesn't know which one to use when you use the "LABEL=" syntax in fstab. You're best off just creating the partitions and adding them to the array; the RAID software will then restore the contents from the other mirror in the background whilst the system is running.


Paul.


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