Re: [LSF/MM/BPF TOPIC] Buffered atomic writes

[Andres Freund <andres@anarazel.de>]

Hi,

On 2026-02-17 13:42:35 +0100, Pankaj Raghav wrote:
> On 2/17/2026 1:06 PM, Jan Kara wrote:
> > On Mon 16-02-26 10:45:40, Andres Freund wrote:
> > > (*) As it turns out, it often seems to improves write throughput as well, if
> > > writeback is triggered by memory pressure instead of SYNC_FILE_RANGE_WRITE,
> > > linux seems to often trigger a lot more small random IO.
> > > 
> > > > So immediately writing them might be ok as long as we don't remove those
> > > > pages from the page cache like we do in RWF_UNCACHED.
> > > 
> > > Yes, it might.  I actually often have wished for something like a
> > > RWF_WRITEBACK flag...
> > 
> > I'd call it RWF_WRITETHROUGH but otherwise it makes sense.
> > 
> 
> One naive question: semantically what will be the difference between
> RWF_DSYNC and RWF_WRITETHROUGH? So RWF_DSYNC will be the sync version and
> RWF_WRITETHOUGH will be an async version where we kick off writeback
> immediately in the background and return?

Besides sync vs async:

If the device has a volatile write cache, RWF_DSYNC will trigger flushes for
the entire write cache or do FUA writes for just the RWF_DSYNC write. Which
wouldn't be needed for RWF_WRITETHROUGH, right?

I don't know if there will be devices that have a volatile write cache with
atomicity support for > 4kB, so maybe that's a distinction that's irrelevant
in practice for Postgres.  But for 4kB writes, the difference in throughput
and individual IO latency you get from many SSDs between using FUA writes /
cache flushes and not doing so are enormous.

Greetings,

Andres Freund