linux-ext4 - Re: [Bug 421482] Firefox 3 uses fsync excessively

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Message-ID: <20080526100751.GB24507@mit.edu>
Date:	Mon, 26 May 2008 06:07:51 -0400
From:	Theodore Tso <tytso@....EDU>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [Bug 421482] Firefox 3 uses fsync excessively

On Mon, May 26, 2008 at 12:05:06AM -0700, Andrew Morton wrote:
> It's purportedly showing that fdatasync() on ext3 is syncing the whole
> world in fsync()-fashion even with an application which does not grow
> the file size.
> 
> But fdatasync() shouldn't do that.  Even if the inode is dirty from
> atime or mtime updates, that shouldn't cause fdatasync() to run an
> ext3 commit?

Well, ideally it shouldn't, although POSIX allows fdatasync() to be
implemented in terms of fsync().  It is at the moment.  :-/

The problem is we don't currently have a way of distinguishing between
a "smudged" inode (only the mtime/atime has changed) and a "dirty"
inode (even if the number of blocks hasn't changed, if i_size has
changed, or i_mode, or anything else, including extended attributes
inline in the inode).  We're not tracking that difference.  If we only
allow mtime/atime changes through setattr (see Cristoph's patches),
and don't set the VFS dirty bit, but our own "smudged" bit, we could
do it --- but at the moment, we're not.

							- Ted
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