Re: Use pg_pwritev_with_retry() instead of write() in dir_open_for_write() to avoid partial writes?

On Wed, Sep 28, 2022 at 1:03 AM Bharath Rupireddy
<bharath.rupireddyforpostgres@gmail.com> wrote:>
> On Tue, Sep 27, 2022 at 3:01 PM Thomas Munro <thomas.munro@gmail.com> wrote:
> > Something like the attached.
>
> Isn't it also better to add notes in win32pread.c and win32pwrite.c
> about the callers doing lseek(SEEK_SET) if they wish to and Windows
> implementations changing file position? We can also add a TODO item
> about replacing pg_ versions with pread and friends someday when
> Windows fixes the issue? Having it in the commit and include/port.h is
> good, but the comments in win32pread.c and win32pwrite.c make life
> easier IMO. Otherwise, the patch LGTM.

Thanks, will do.  FWIW I doubt the OS itself will change released
behaviour like that, but I did complain about the straight-up
misleading documentation and they agreed and fixed it[1].

After some looking around, the only way I could find to avoid this
behaviour is to switch to async handles, which do behave as documented
in this respect, but then you can't use plain read/write at all unless
you write replacement wrappers for those too, and AFAICT that adds
more system calls (start write, wait for write to finish) and
complexity/weirdness without any real payoff so it seems like the
wrong direction, at least without more research that I'm not pursuing
currently.  (FWIW in AIO porting experiments a while back we used
async handles to get IOs running concurrently with CPU work and
possibly other IOs so it was actually worth doing, but that was only
for smgr and the main wal writing code where there's no intermixed
plain read/write calls as you have here, so the problem doesn't even
come up.)

[1] https://github.com/MicrosoftDocs/sdk-api/pull/1309