Re: FreeBSD UFS & fsync

On Tue, Feb 23, 2021 at 5:49 AM Luca Ferrari <fluca1978(at)gmail(dot)com> wrote:
> I'm running a virtual machine with FreeBSD 12.2, PostgreSQL 12.5 and
> UFS as filesystem.
> I was experimenting with fsync = off and pgbench, and I see no
> particular difference in tps having fsync enabled or disabled.
> Now, the same tiny test on a linux box provides a 10x tps, while on
> FreeBSD is a 1% increase.
> I'm trying to figure out why, and I suspect there is something related
> to how UFS handles writes.

Do you have WCE enabled? In that case, modern Linux file systems
would do a synchronous SYNCHRONIZE CACHE for our WAL fdatasync(), but
FreeBSD UFS wouldn't as far as I know. It does know how to do that
(there's a BIO_FLUSH operation, also used by ZFS), but as far as I can
see UFS uses it just for its own file system meta-data crash safety
currently (see softdep_synchronize()). (There is also no FUA flag for
O_[D]SYNC writes, an even more modern invention.)