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Re: question about xfs_fsync on linux

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: question about xfs_fsync on linux
From: Chris Torek <chris.torek@xxxxxxxxxxxxx>
Date: Wed, 16 Jul 2008 15:58:55 -0600
Cc: xfs@xxxxxxxxxxx
In-reply-to: Your message of "Tue, 15 Jul 2008 12:48:30 +1000." <20080715024830.GZ29319@disturbed>
Sender: xfs-bounce@xxxxxxxxxxx
>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!

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.

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

Chris


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