Re: [RFC v2] ext4: Don't send extra barrier during fsync if there are no dirty pages.

[Ted Ts'o <tytso@mit.edu>]

On Fri, Aug 06, 2010 at 12:13:56AM -0700, Darrick J. Wong wrote:
> Yes, it's a proxy for something else.  One of our larger products would like to
> use fsync() to flush dirty data out to disk (right now it looks like they use
> O_SYNC), but they're concerned that the many threads they use can create an
> fsync() storm.  So, they wanted to know how to mitigate the effects of those
> storms.  Not calling fsync() except when they really need to guarantee a disk
> write is a good start, but I'd like to get ahead of them to pick off more low
> hanging fruit like the barrier coordination and not sending barriers when
> there's no dirty data ... before they run into it. :)

Do they need a barrier operation, or do they just want to initiate the
I/O?  One of the reasons I found it hard to believe you would have
multiple threads all fsync()'ing the same file is that keeping the the
file consistent is very hard to do in such a scenario.  Maintaining
ACID-level consistency without a single thread which coordinates when
commit records gets written is I'm sure theoretically possible, but in
practice, I wasn't sure any applications would actually be _written_
that way.

If the goal is just to make sure I/O is getting initiated, without
necessarily waiting for assurance that a specific file write has hit
the disk platters, it may be that the Linux-specific
sync_file_range(2) system call might be a far more efficient way of
achieving those ends.  Without more details about what this product is
doing, it's hard to say, of course.

						- Ted