Zpool status -- why does a suboptimal pool show as "ONLINE"?

Hey there all,

I have a nagios check that assumes that if I have a suboptimal zfs zpool, that the word “DEGRADED” will appear in the output.  One disk of a two-disk mirror seems to have faulted, but the pool still shows as “ONLINE”.  I know I’ve seen the word “DEGRADED” in the past.  What’s different?

  pool: zroot
 state: ONLINE
status: One or more devices are faulted in response to persistent errors.
        Sufficient replicas exist for the pool to continue functioning in a
        degraded state.
action: Replace the faulted device, or use 'zpool clear' to mark the device
        repaired.
config:

        NAME        STATE     READ WRITE CKSUM
        zroot       ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            ada0p3  FAULTED      4   372     0  too many errors
            ada1p3  ONLINE       0     0     0

errors: No known data errors

14.1, if it matters, the disks are two innolite SATADOM’s.

-Dan