Troubles getting a software RAID 5 to work correctly

Kenneth Burgener kenneth at
Fri Nov 24 18:06:18 MST 2006

Nicholas Leippe wrote:
> [UU_] indicates that the array is degraded.  Either a device has failed (or is 
> in failure mode because you told it so), or it has not completed 
> reconstruction (it remains degraded until this is done).
> When you first create a raid5 array, it computes the parity across the entire 
> volume. (I know, kinda dumb when the data is all junk.)
> from man mdadm(8):
> <quote>
> When creating a RAID5 array, mdadm will automatically create a degraded array 
> with an extra spare drive.  This is because building the spare into a 
> degraded array is in general faster than resyncing the parity on a 
> non-degraded, but not clean, array.  This feature can be over-ridden with 
> the --force option.
> </quote>
> Until it finishes adding the third drive into the array, removing either of 
> the others fails the array completely--IOW all of the data is now lost.
> I don't recall if you need to manually tell it to use the spare (I usually 
> use --force on a new array).  If /proc/mdadm does not show that 
> reconstruction is in progress, then you'll have to tell it to do it.

Thank you for your response.

If I have already created the array, and it does not appear to be
"processing" the spare into the array, how do I force it to begin
processing?  I did not see an option in the man pages about forcing the
processing of a spare.

Thank you,

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