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====== ZFS - Troubleshooting - Data Recovery ======

Assume we have a 2 x 2 mirrored zpool:

<code bash>
sudo zpool create testpool mirror /dev/sdc /dev/sdd mirror /dev/sde /dev/sdf -f
sudo zpool status
  pool: testpool
 state: ONLINE
  scan: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        testpool    ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            sdc     ONLINE       0     0     0
            sdd     ONLINE       0     0     0
          mirror-1  ONLINE       0     0     0
            sde     ONLINE       0     0     0
            sdf     ONLINE       0     0     0
</code>

Now populate it with some data and check sum the data:

<code bash>
dd if=/dev/urandom of=/testpool/random.dat bs=1M count=4096
md5sum /testpool/random.dat
f0ca5a6e2718b8c98c2e0fdabd83d943  /testpool/random.dat
</code>

Now we simulate catastrophic data loss by overwriting one of the VDEV devices with zeros:

<code bash>
sudo dd if=/dev/zero of=/dev/sde bs=1M count=8192
</code>

And now initiate a scrub:

<code bash>
sudo zpool scrub testpool
</code>

And check the status:

<code bash>
sudo zpool status
  pool: testpool
 state: ONLINE
status: One or more devices has experienced an unrecoverable error.  An
        attempt was made to correct the error.  Applications are unaffected.
action: Determine if the device needs to be replaced, and clear the errors
        using 'zpool clear' or replace the device with 'zpool replace'.
   see: http://zfsonlinux.org/msg/ZFS-8000-9P
  scan: scrub in progress since Tue May 12 17:34:53 2015
    244M scanned out of 1.91G at 61.0M/s, 0h0m to go
    115M repaired, 12.46% done
config:

        NAME        STATE     READ WRITE CKSUM
        mypool      ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            sdc     ONLINE       0     0     0
            sdd     ONLINE       0     0     0
          mirror-1  ONLINE       0     0     0
            sde     ONLINE       0     0   948  (repairing)
            sdf     ONLINE       0     0     0
</code>

...now let us remove the drive from the pool:

<code bash>
sudo zpool detach testpool /dev/sde
</code>

..hot swap it out and add a new one back:

<code bash>
sudo zpool attach testpool /dev/sdf /dev/sde  -f
</code>

..and initiate a scrub to repair the 2 x 2 mirror:

<code bash>
sudo zpool scrub testpool
</code>