[PaulDavisThe1st]

Chance of Postgres commit mapping 1:1 onto posix fsync or equivalent: slim.

[anarazel]

Without parallelism, each commit will be at least one fdatasync (or fsync, O_SYNC/O_DSYNC write, depending on configuration). With parallelism, concurrent transaction might be flushed together, reducing the total number of fsyncs.