Re: fdatasync(2) on macOS

On Fri, Jan 15, 2021 at 12:55:52PM -0500, Tom Lane wrote:
> > So... does this unreleased function flush drive caches? We know that
> > fsync(2) doesn't, based on Apple's advice[1] for databases hackers to
> > call fcntl(fd, F_FULLSYNC, 0) instead. We do that.
>
> pg_test_fsync results make it clear that fdatasync is the same or a shade
> faster than fsync, which is pretty much what you'd expect. On my
> late-model Macbook Pro:
>
> Compare file sync methods using two 8kB writes:
> (in wal_sync_method preference order, except fdatasync is Linux's default)
> open_datasync 14251.416 ops/sec 70 usecs/op
> fdatasync 25345.103 ops/sec 39 usecs/op
> fsync 24677.445 ops/sec 41 usecs/op
> fsync_writethrough 41.519 ops/sec 24085 usecs/op
> open_sync 14188.903 ops/sec 70 usecs/op
>
> and on an old Mac mini with spinning rust:
>
> Compare file sync methods using two 8kB writes:
> (in wal_sync_method preference order, except fdatasync is Linux's default)
> open_datasync 2536.535 ops/sec 394 usecs/op
> fdatasync 4602.192 ops/sec 217 usecs/op
> fsync 4600.365 ops/sec 217 usecs/op
> fsync_writethrough 12.135 ops/sec 82404 usecs/op
> open_sync 2506.674 ops/sec 399 usecs/op
>
> So it's not a no-op, but on the other hand it's not succeeding in getting
> bits down to the platter. I'm not inclined to dike it out, but it does
> seem problematic that we're defaulting to open_datasync, which is also
> not getting bits down to the platter.
>
> I have a vague recollection that we discussed changing the default
> wal_sync_method for Darwin years ago, but I don't recall why we
> didn't pull the trigger. These results certainly suggest that
> we oughta.

Is this with an SSD? We used to be able to know something wasn't
flushing to durable storage because magnetic disk was so slow you could
tell from the numbers, but with SSDs, it might be harder to guess.
Maybe time to use:

https://brad.livejournal.com/2116715.html
diskchecker.pl

or find a way to automate that test.

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EnterpriseDB https://enterprisedb.com

The usefulness of a cup is in its emptiness, Bruce Lee