ZFS: what does it mean if all disks are ONLINE but resilvering takes place?

Question

I have a pool of four raidz1 on Solaris 10, three of which look like this:

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 Wed Jun 17 16:46:31 2015
    536G scanned out of 43.9T at 389M/s, 32h26m to go
    536G scanned out of 43.9T at 389M/s, 32h26m to go
    27.3G resilvered, 1.19% done
config:

        NAME                                       STATE     READ WRITE CKSUM
        volume                                     ONLINE       0     0     0
          raidz1-0                                 ONLINE       0     0     0
            c0t600C0FF00012FBB109A2154F01000000d0  ONLINE       0     0     0
            c0t600C0FF00012FBB115A2154F01000000d0  ONLINE       0     0     0
            c0t600C0FF00012FBB139A2154F01000000d0  ONLINE       0     0     0
            c0t600C0FF00012FC7DFCA1154F01000000d0  ONLINE       0     0     0
            c0t600C0FF00012FBB151A2154F01000000d0  ONLINE       0     0     0
            c0t600C0FF00012FBB1E630025201000000d0  ONLINE       0     0     0  (resilvering)
            c0t600C0FF00012FBB169A2154F01000000d0  ONLINE       0     0     0
          ...
    spares
      c0t600C0FF00012FBB11555EC5401000000d0    AVAIL   

What does it mean if I see that resilvering kicked in although no disk is marked OFFLINE or UNAVAIL? Is it safe to remove one of the disks from the array? Should I replace the resilvering device by a spare?

Answer 1

There are a couple of explanations. One of them is that the system lost contact to the disk for a while and is now updating the latest changes to that disk.

You should not remove a disk while it is resilvering, because RAIDz1 has only a single disk fault tolerance.