Re: [rfc] fsync_range?

[Chris Mason <chris.mason@oracle.com>]

On Wed, 2009-01-21 at 09:12 -0500, Theodore Tso wrote:
> 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.

Usually if they don't use O_DIRECT, they use O_SYNC.

> 
> 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".  :-)

Yes, I think btrfs' nodatacow option is pretty important for database
use.

> 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?

Eric, didn't you have a magic script for grepping the sources/binaries
in fedora for syscalls? 

-chris