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

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Message-ID: <20080526111015.GB20623@logfs.org>
Date:	Mon, 26 May 2008 13:10:16 +0200
From:	Jörn Engel <joern@...fs.org>
To:	Theodore Tso <tytso@....EDU>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [Bug 421482] Firefox 3 uses fsync excessively

On Mon, 26 May 2008 06:07:51 -0400, Theodore Tso wrote:
> 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.

Don't we already have this bit since Linux 2.4.0-test12?  I_DIRTY_SYNC
is admittedly not well-named for "smudged".  But it used to mean just
that.  I_DIRTY_DATASYNC was the real dirty bit.  Which, in I_DIRTY_PAGES,
has been split into I_DIRTY_DATASYNC and I_DIRTY_PAGES.

Now we just have to use sane names.

Jörn

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