If we do a scrub while a leaf device is offline (via "zpool offline"),
we will inadvertently clear the DTL (dirty time log) of the offline
device, even though it is still damaged. When the device comes back
online, we will incompletely resilver it, thinking that the scrub
repaired blocks written before the scrub was started. The incomplete
resilver can lead to data loss if there is a subsequent failure of a
different leaf device.
The fix is to never clear the DTL of offline devices. Note that if a
device is onlined while a scrub is in progress, the scrub will be
restarted.
The problem can be worked around by running "zpool scrub" after
"zpool online".
See also https://github.com/zfsonlinux/zfs/issues/5806