Error email from ZFS: "ZFS has detected an io error"

Question

I receive error emails from ZFS telling me that ZFS has detected an io error, but, the details listed do not show any checksum, read or write errors. A zpool status does not show any errors and a scrub does not report any errors.

Email content:

ZFS has detected an io error: 

eid: 7 
class: io 
host: ubuntu 
time: 2016-08-10 18:18:37+0100 
vtype: disk 
vpath: /dev/disk/by-id/ata-WDC_WD30EFRX-68EUZN0_WD-WCC4N7FSX6F9-part1 
vguid: 0xFE98965C2E168896 
cksum: 0 
read: 0 
write: 0 
pool: unas 

The zpool is made up of two mirrors:

NAME                                           STATE     READ WRITE CKSUM
unas                                           ONLINE       0     0     0
  mirror-0                                     ONLINE       0     0     0
    ata-WDC_WD30EFRX-68EUZN0_WD-WCC4N1VUU0LX   ONLINE       0     0     0
    ata-WDC_WD30EFRX-68EUZN0_WD-WCC4N7FSX6F9   ONLINE       0     0     0
  mirror-1                                     ONLINE       0     0     0
    ata-WDC_WD3200AAKX-001CA0_WD-WMAYUW798623  ONLINE       0     0     0
    ata-ST3320418AS_6VM5GXHJ                   ONLINE       0     0     0

errors: No known data errors

Answer 1

This could possibly be due to hard drive spin down.

See this answer for details on how to check and adjust your hdd spindown settings. How can I control HDD spin down time?