Have a look at
  mdadm -Q --detail /dev/md0
But I'd suspect the problematic disk is causing that slow resync.
I would add a new disk and then declare the one you suspect as bad.
If the other two are are ok it should resync to the new one and you're fine.
Otherwise you can re-add the declared failed disk and try again with  
another one.
I would use dd to verify which disk is giving trouble (e.g. "dd  
if=/dev/sda of=/dev/null" , use "iostat -d5 -k" to watch the read  
speed).
I hope this helps..
Alex.

----- Message from alberto@ggsys.net ---------
     Date: Sat, 13 Oct 2007 13:00:16 -0500
     From: Alberto Alonso [off-list ref]
Reply-To: Alberto Alonso [off-list ref]
  Subject: Kicking the right drive out
       To: linux-raid@vger.kernel.org

I have a need to kick a disk out of a RAID 5 array.

I can do a fdisk on 2 out of the 3 devices that form part
of the array, so I suspect I know which one is bad.

The problem is that mdstat shows the array as follows:

md3 : active raid5 sda6[0] sdc6[2] sdb6[1]
      960863488 blocks level 5, 64k chunk, algorithm 2 [3/3] [UUU]
      [>....................]  resync =  0.7% (3444348/480431744)
finish=582280.4min speed=1K/sec


Doesn't this mean that all three disks are active and in use?

mdadm -D /dev/md3 shows:

    Number   Major   Minor   RaidDevice State
       0       8        6        0      active sync   /dev/sda6
       1       8       22        1      active sync   /dev/sdb6
       2       8       38        2      active sync   /dev/sdc6

So, my question is as follows: can I remove sdc6 and be sure the
array will not stop?

To make things worse, I see no disk failures in any log file. So
I am not sure how sdc is actually failing.

Thanks,

Alberto


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