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Re: question about xfs_fsync on linux

To: Chris Torek <chris.torek@xxxxxxxxxxxxx>
Subject: Re: question about xfs_fsync on linux
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Thu, 17 Jul 2008 10:22:53 +1000
Cc: xfs@xxxxxxxxxxx
In-reply-to: <200807162158.m6GLwtE00281@xxxxxxxxxxxxx>
Mail-followup-to: Chris Torek <chris.torek@xxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
References: <20080715024830.GZ29319@disturbed> <200807162158.m6GLwtE00281@xxxxxxxxxxxxx>
Sender: xfs-bounce@xxxxxxxxxxx
User-agent: Mutt/1.5.17+20080114 (2008-01-14)
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@xxxxxxxxxxxxx


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