I've read all the info I could find re running PG on ZFS: turn off full page writes, turn on lz4, tweak recordsize so as to take advantage of compression, etc. One thing I haven't seen is whether a separate volume for WAL would benefit from a larger recordsize. Or any other tweaks???
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Scott Ribe
scott_ribe(at)elevated-dev(dot)com
https://www.linkedin.com/in/scottribe/