The pool has experienced I/O failures. Since the ZFS pool property 'failmode' is set to 'continue', read I/Os will continue to be serviced, but write I/Os are blocked. See the zpool(8) manpage for more information on the 'failmode' property. Manual intervention is required for write I/Os to be serviced. You can see which devices are affected by running 'zpool status -x':

# zpool status -x
  pool: test
 state: FAULTED
status: There are I/O failures.
action: Make sure the affected devices are connected, then run 'zpool clear'.
   see: https://zfsonlinux.org/msg/ZFS-8000-HC
 scrub: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        test        FAULTED      0    13     0  insufficient replicas
          sda9      FAULTED      0     7     0  experienced I/O failures
          sdb9      ONLINE       0     0     0

errors: 1 data errors, use '-v' for a list

After you have made sure the affected devices are connected, run 'zpool clear' to allow write I/O to the pool again:

# zpool clear test

If I/O failures continue to happen, then applications and commands for the pool may hang. At this point, a reboot may be necessary to allow I/O to the pool again.