Why does a ZFS resilver scan the entire pool

hard drivehardwareomniossolariszfs

I have a zpool where I have just replaced a failed disk, and started a resilvering to the new disk.

What I don't understand is, why zpool status says it want to scan 129TB, when the size of the vdev is ~30TB. When I look at iostat -nx 1 then I can see the 5 disks in the vdev are getting heavy reads, and the new disk equal heavy writes. So zfs doesn't scan all the data as it says. 

# zpool status tank3 |head
  pool: tank3
 state: ONLINE
status: One or more devices is currently being resilvered.  The pool will
        continue to function, possibly in a degraded state.
action: Wait for the resilver to complete.
  scan: resilver in progress since Thu Apr 30 09:59:15 2015
    61.2T scanned out of 129T at 3.03G/s, 6h23m to go
    946G resilvered, 47.34% done

Question

I would say that each vdev is independent of each other, so a resilver of one should not require any scan of the others. Why does zfs scan all used disk space when resilvering?

Best Answer

Resilvering is a vdev operation; as you implied only the storage devices in that vdev are used to rebuild the new device. I'm not sure why it quotes the full size of the zpool, but I suspect the developers borrowed code from the scrub functions, or that it simply quotes the full zpool size as that would be the worst case scenario.

Related Topic