Re: [RFC PATCH] fpathconf() for fsync() behavior

[Valerie Aurora Henson <vaurora@redhat.com>]

On Wed, Apr 22, 2009 at 10:17:48PM -0700, Andrew Morton wrote:
> On Wed, 22 Apr 2009 20:12:57 -0400 Valerie Aurora Henson <vaurora@redhat.com> wrote:
> 
> > In the default mode for ext3 and btrfs, fsync() is both slow and
> > unnecessary for some important application use cases - at the same
> > time that it is absolutely required for correctness for other modes of
> > ext3, ext4, XFS, etc.  If applications could easilyl distinguish
> > between the two cases, they would be more likely to be correct and
> > fast.
> > 
> > How about an fpathconf() variable, something like _PC_ORDERED?  E.g.:
> > 
> > 	/* Unoptimized example optional fsync() demo */
> > 	write(fd);
> > 	/* Only fsync() if we need it */
> > 	if (fpath_conf(fd, _PC_ORDERED) != 1)
> > 		fsync(fd);
> > 	rename(tmp_path, new_path);
> > 
> > I know of two specific real-world cases in which this would
> > significantly improve performance: (a) fsync() before rename(), (b)
> > fsync() of the parent directory of a newly created file.  Case (b) is
> > particularly nasty when you have multiple threads creating files in
> > the same directory because the dir's i_mutex is held across fsync() -
> > file creates become limited to the speed of sequential fsync()s.
> > 
> > Conceptual libc patch below.
> 
> Would it be better to implement new syscall(s) with finer-grained control
> and better semantics?  Then userspace would just need to to:
> 
> 	fsync_on_steroids(fd, FSYNC_BEFORE_RENAME);
> 
> and that all gets down into the filesystem which can then work out what
> it needs to do to implement the command.

You and Jamie have a good point: fsync() is a very big hammer used for
many different purposes, and it would be nice to have finer-grained
tools.  There are distinct limits to what you can do to optimize a
full fsync(); we should be thrilled to get fewer of them from userspace.

Like others, I am concerned about the complexity for the programmer.
Perhaps in addition to the various fine-grained options, there is a:

	fsync_on_steroids(fd, FSYNC_DO_WHAT_ORDERED_WOULD_DO);

The idea is that we've currently got a lot of code that assumes ext3
data=ordered semantics (btrfs will fulfill these assumptions too).  It
would be nice if we had one simple drop-in test to distinguish between
ext3-ordered/btrfs/reiserfs and all other fs's; I think we'd get a lot
more adoption that way.

All that being said, I'd be thrilled to have fine-grained fsync().

-VAL