O_SYNC

Posted May 21, 2019 23:10 UTC (Tue) by nix (subscriber, #2304)
In reply to: O_SYNC by rweikusat2
Parent article: DAX semantics

Nevertheless, it refers to a timeout for periodically flushing all dirty page cache pages to stable storage, just like the sync but not the fsync system call would do

Except that's not what it does. Every dirty_writeback_centisecs, dirty inodes that became dirty longer ago than dirty_writeback_centisecs are flushed. inodes that were dirtied more recently than that are *not* flushed. That's nothing like sync(), which flushes everything regardless of first dirty time, and you can't do it with fsync() either (even if you track the timing yourself, good luck with that) because there is extra behaviour regarding flushes that take too long which cannot be mimicked with fsync().