Re: wal_sync_method=fsync_writethrough

On Tue, Aug 30, 2022 at 3:44 AM Magnus Hagander <magnus(at)hagander(dot)net> wrote:
> On Fri, Aug 26, 2022 at 11:29 PM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
> > Now it looks strange: we have both "fsync" and "fsync_writethrough"
> > doing exactly the same thing while vaguely implying otherwise, and the
> > contrast with other operating systems (if I divined that aspect
> > correctly) mostly doesn't apply. How flush commands affect various
> > caches in modern storage stacks is also not really OS-specific AFAIK.
> >
> > (Obviously macOS is a different story...)
>
> Given that it does vary (because macOS is actually an OS :D), we might
> need to start from a matrix of exactly what happens in different
> states, and then try to map that to a set? I fully agree that if
> things actually behave the same, they should be called the same.

Thanks, I'll take that as a +1 for dropping the redundant level for
Windows. (Of course it stays for macOS).

I like that our current levels are the literal names of standard
interfaces we call, since the rest is out of our hands. I'm not sure
what you could actually *do* with the information that some OS doesn't
flush write caches, other than document it and suggest a remedy (e.g.
turn it off). I would even prefer it if fsync_writethrough were
called F_FULLFSYNC, following that just-say-what-it-does-directly
philosophy, but that horse is already over the horizon.

> And it may also be that there is no longer a difference between
> direct-drive and RAID-with-battery-or-flash, which used to be the huge
> difference back then, where you had to tune for it. For many cases
> that has been negated by just not using that (and using NVME and
> possibly software raid instead), but there are certainly still people
> using such systems...

I believe modern systems are a lot better at negotiating the need for
flushes (i.e. for *volatile* caches). In contrast, the FUA situation
(as used for FILE_FLAG_WRITE_THROUGH) seems like a multi-level
dumpster fire on anything but high-end gear, from what I've been able
to figure out so far, though I'm no expert.