Re: [rfc] fsync_range?

[Theodore Tso <tytso@mit.edu>]

On Wed, Jan 21, 2009 at 12:37:11PM +0000, Jamie Lokier wrote:
> 
> What about btrfs with data checksums?  Doesn't that count among
> data-retrieval metadata?  What about nilfs, which always writes data
> to a new place?  Etc.
> 
> I'm wondering what exactly sync_file_range() definitely writes, and
> what it doesn't write.
> 
> If it's just in use by Oracle, and nobody's sure what it does, that
> smacks of those secret APIs in Windows that made Word run a bit faster
> than everyone else's word processer...  sort of. :-)

Actually, I take that back; Oracle (and most other enterprise
databases; the world is not just Oracle --- there's also DB2, for
example) generally uses Direct I/O, so I wonder if they are using
sync_file_range() at all.

I do wonder though how well or poorly Oracle will work on btrfs, or
indeed any filesystem that uses WAFL-like or log-structutred
filesystem-like algorithms.  Most of the enterprise databases have
been optimized for use on block devices and filesystems where you do
write-in-place acesses; and some enterprise databases do their own
data checksumming.  So if I had to guess, I suspect the answer to the
question I posed is "disastrously".  :-)  After all, such db's
generally are happiest when the OS acts as a program loader than then
gets the heck out of the way of the filesystem, hence their use of
DIO.

Which again brings me back to the question --- I wonder who is
actually using sync_file_range, and what for?  I would assume it is
some database, most likely; so maybe we should check with MySQL or
Postgres?

						- Ted