Re: wal_sync_method=fsync_writethrough

On Sat, Aug 27, 2022 at 12:17 AM Magnus Hagander <magnus(at)hagander(dot)net> wrote:
> So, I don't know how it works now, but the history at least was this:
> it was not about the disk caches, it was about raid controller caches.
> Basically, we determined that windows didn't fsync it all the way. But
> it would with But if we changed wal_sync_method=fsync to actually
> *do* that, then people who had paid big money for raid controllers
> with flash or battery backed cache would lose a ton of performance. So
> we needed one level that would sync out of the OS but not through the
> RAID cache, and another one that would sync it out of the RAID cache
> as well. Which would/could be different from the drive caches
> themselves, and they often behaved differently. And I think it may
> have even been dependent on the individual RAID drivers what the
> default would be.

Thanks for the background. Yeah, that makes sense to motivate
open_datasync for Windows. Not sure what you meant about fsync or
meant to write after "would with".

It seems like the 2005 discussions were primarily about open_datasync
but also had the by-product of introducing the name
fsync_writethrough. If I'm reading between the lines[1] correctly,
perhaps the logic went like this:

1. We noticed that _commit() AKA FlushFileBuffers() issued
SYNCHRONIZE CACHE (or equivalent) on Windows.

2. At that time in history, Linux (and other Unixes) probably did not
issue SYNCHRONIZE CACHE when you called fsync()/fdatasync().

3. We concluded therefore that Windows was strange and we needed to
use a different level name for the setting to reflect this extra
effect.

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...)

[1] https://www.postgresql.org/message-id/flat/26109.1111084860%40sss.pgh.pa.us#e7f8c2e14d76cad76b1857e89c8a6314