Yes. If you were doing multiple WAL writes before transaction fsync, you would be fsyncing every write, rather than doing two writes and fsync'ing them both. I wonder if larger transactions would find open_sync slower?
No hard numbers, but I remember testing fsync vs open_sync something ago on 7.3.x.
open_sync was blazingly fast for pgbench, but for when we switched our development database over to open_sync, things slowed to a crawl.
This was some months ago, and I might be wrong, so take it with a grain of salt. It was on Red Hat 8's Linux kernel 2.4.18, I think. YMMV.
Will be testing it real soon tonight, if possible.
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