I checked my weekly scrub report and found, lo and behold, one of my disks experienced an unrecoverable error! The message is nicely presented with a very professional and informative descriptive page:
pool: tank state: ONLINE status: One or more devices has experienced an unrecoverable error. An attempt was made to correct the error. Applications are unaffected. action: Determine if the device needs to be replaced, and clear the errors using 'zpool clear' or replace the device with 'zpool replace'. see: http://illumos.org/msg/ZFS-8000-9P scan: scrub repaired 16K in 2h25m with 0 errors on Mon Nov 23 04:25:24 2015
See, now that’s a nice file system right there. Very understandable, readable, and that link is fantastic. The page explains this may be an indication of a drive beginning to fail, or it could be that fearsome bit rot which OpenZFS has successfully thwarted in its vigilant stand against corruption.
I’ll be keeping an eye on this disk here. It has basically been powered on for 3 years solid, providing various levels of service, so it’s done pretty well even if this is an indication of failure.