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ZFS Day (2012-10-02): Y4K? – George Wilson, Delphix.
At 16:12 on the timeline, under the heading Drawbacks of 4K and ZFS, RAID-Z and 4K is "not recommended". Then, spoken:
… a hard choice to make. RAID-Z and 4K has really become a problem, and I'm not sure that I would recommend it – at least today, but I think, as a community we need to go off and invest and understand – "What does it mean to be using 4K sectors in a RAID-Z world?". It probably works fine as long as you're doing large records, so if you're doing 128K you might be good. If you're doing 8K you're probably throwing away a lot of space …
– there's more to it, I recommend taking time to watch and listen to the whole of that half-hour session (and others from the day).
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Highly inefficient use of space observed when using raidz2 with ashift=12 · Issue #548 · zfsonlinux/zfs – referred from a post by Reinis Rozitis under RaidZ and 4k sectors in zfs-discuss on Google Groups.
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In Stack Exchange there's an answered question, software raid - Mixing 4k/512 drives with ZFS (FreeNAS) - Unix and Linux
Elsewhere there's a recommendation to set ashift=12 at time of creation for any pool that might in the future include a drive with those characteristics.