this past weekend i upgraded my computer to Sierra (from Mavericks), and to 1.6.1 from 1.5.2.
after i finished with the checklist items for my OS upgrade, i noticed that when doing reads on my zpool (2-disk mirror + ZIL & cache on SSD), only one of the disks was being read instead of the usual "both" almost all the time (but not entirely all the time, but like 100:1). i tried rebooting a few times and the favored disk wasn't consistent boot-to-boot, but it did stick for that boot. i didn't see any tunables that sounded likely. i tried several parallel large reads and they were all funneled through the one favored drive, while the other stayed mostly idle.
writes still went to both disks simultaneously, as expected.
a "zpool scrub" read from both disks simultaneously also, as expected. interestingly, after running a scrub for about 30 seconds and then stopping it, ZFS started reading from both disks for ordinary reads like the good old days.
today i ran a scrub to completion (since it was due), and afterward the read behavior had reverted to one favored disk. running a scrub again for 30ish seconds and stopping it restored read-from-both-disks.
has anyone else seen this?