Re: question about xfs_fsync on linux

[Dave Chinner <david@fromorbit.com>]

On Wed, Jul 16, 2008 at 03:58:55PM -0600, Chris Torek wrote:
> >Well, you are pretty much on your own, then. Really, we cannot help
> >diagnose problems on old kernels with a random set of backported
> >patches in them.
> 
> Definitely understood.  I just wanted to ask that original
> question, really.  I had assumed that the file system itself
> had to start any dirty-page writes, having missed the top level
> filemap_fdatawrite() call.
> 
> We finally got a test case and did a bunch of analysis, and it
> turns out that the DB software is missing an fsync() call.  Of
> course XFS won't fsync the file if you don't *ask* it to!

Yes, that would help ;)

Thanks for following up and letting us know you found the
problem.

> As long as I am sending mail, there is something else I am curious
> about though.  While this is not XFS specific, I wonder if there
> is any desire to have different background write frequencies on
> different file systems.  By default, mm/page-writeback.c will start
> writebacks after a 30-second delay.  One can tune this to any other
> number (via /proc/sys/vm/dirty_{expire,writeback}_centisecs), but
> this affects the entire system.  It might be useful to be able
> to tune this per-FS instead.

Wouldn't be too difficult, but really if you have  data that needs
to go to disk quickly from a given application then the application
should be triggering the flush. e.g. іssuing
posix_fadvise(fd, ...., POSIX_FADV_DONTNEED) will trigger an
immediate async flush of the file....

> (On the other hand, perhaps if one really wants one's data journaled,
> one should just use a data-journaling file system....)

Or use the sync mount option.....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com