linux-ext4 - Re: [ext3] Changes to block device after an ext3 mount point has been remounted readonly

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Message-ID: <20100222230552.GB13882@atrey.karlin.mff.cuni.cz>
Date:	Tue, 23 Feb 2010 00:05:52 +0100
From:	Jan Kara <jack@...e.cz>
To:	Camille Moncelier <pix@...life.org>
Cc:	linux-fsdevel@...r.kernel.org, Andreas Dilger <adilger@....com>,
	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: [ext3] Changes to block device after an ext3 mount point has
	been remounted readonly

> > On Thu, Feb 18, 2010 at 10:41 PM, Andreas Dilger <adilger@....com> wrote:
> > 
> > > Are you sure this isn't because e2fsck has been run at boot time and changed
> > > e.g. the "last checked" timestamp in the superblock?
> > >
> > No, I replaced /sbin/init by something which compute the sha1sum of
> > the root partition, display it then call /sbin/init and I can see that
> > the hash has changed after mount -o remount,ro.
> > 
> > As little as I understand, I managed to make a diff between two
> > hexdump of small images where changes happened after I created a file
> > and remounted the fs ro and it seems that, the driver didn't wrote
> > changes to the disk until unmount  ( The hexdump clearly shows that
> > /lost+found and /test file has been written after the umount )
> > 
> > workaround: Is there some knob in /proc or /sys which can trigger all
> > pending changes to disk ? ( Like /proc/sys/vm/drop_caches but for
> > filesystems ? )
>   I've looked at your script. The problem is that "echo s >/proc/sysrq_trigger"
> isn't really a data integrity operation. In particular it does not wait on
> IO to finish (with the new writeback code it does not even wait for IO to be
> submitted) so you sometimes take the image checksum before the sync actually
> happens. If you used sync(1) instead, everything should work as expected...
  Hmm, and apparently there is some subtlety in the loopback device code
because even when I use sync(1), the first and second images sometimes differ
(although it's much rarer). But I see a commit block of the transaction already
in the first image (the commit block is written last) but the contents of the
transaction is present only in the second image.

								Honza
-- 
Jan Kara <jack@...e.cz>
SuSE CR Labs
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