Re: [pgsql-hackers-win32] win32 performance - fsync question - Mailing list pgsql-hackers

Magnus Hagander wrote:
> > This indicated to me that open_sync did not require any
> > additional changes than our current fsync.
>
> fsync and open_sync both write through the write cache in the operating
> system. Only fsync=off turns this off.
>
> fsync also writes through the hardware write cache. o_sync does not.
> This is what causes the large slowdown with write cache enabled,
> *including* most battery backed write cache systems (pretty much making
> the write-cache a waste of money). This may be a good thing on IDE
> systems (for admins that don't know how to remove the little check in
> the box for "enable write caching on the disk" that MS provides, which
> *explicitly* warns that you may lose data if you enabled it), but it's a
> very bad thing for anything higher end.

I found the checkbox on XP looking at "Properties" for the drive, then
choosing "Hardware", the drive, "Properties", and "Policies".

> fsync also syncs the directory metadata. o_sync only cares about the
> files contents. (This is what causes the large slowdown with write cache
> *disabled*, because it requires multiple writes on multiple disk
> locations for each fsync).
>
> Basically, fsync hurts people who configure their box correctly, or who
> use things like SCSI disks. o_sync hurts people who configure their
> machine in an unsafe way.

So, it seems that Win32 open_sync is exactly the same as our
"wal_sync_method = open_datasync" on Unix (it needs to be renamed), and
"wal_sync_method = fsync" on Win32 is something we don't have that
writes through the disk write cache even if it is enabled.

I have developed the following patch which renames our wal_sync_method
Win32 support from open_sync to open_datasync:

    ftp://candle.pha.pa.us/pub/postgresql/mypatches

One issue with this patch is that if applied it would make open_datasync
the default sync method on Win32 because we prefer open_datasync over
all other sync methods.  If we don't want to do that, I think we should
still do the rename for accuracy and add a !WIN32 test to prevent
open_datasync from being the default.

However, I do prefer this patch and let Win32 have the same write cache
issues as Unix, for consistency.

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