linux-ext4 - Re: Need of revoke mechanism in JBD

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Message-ID: <20110426122558.GF9486@thunk.org>
Date:	Tue, 26 Apr 2011 08:25:58 -0400
From:	Ted Ts'o <tytso@....edu>
To:	Ding Dinghua <dingdinghua85@...il.com>
Cc:	Yongqiang Yang <xiaoqiangnk@...il.com>,
	Niraj Kulkarni <kulkarniniraj14@...il.com>,
	linux-ext4@...r.kernel.org
Subject: Re: Need of revoke mechanism in JBD

On Tue, Apr 26, 2011 at 05:23:21PM +0800, Ding Dinghua wrote:
> I think it's not only a performance issue but more important, a
> correctness issue.
> Revoke table is used for preventing the wrong replay of journal which
> cause data corruption:
> If block A has been journalled its modification, committed to journal
> and hasn't been checkpointed,
> and in later transactions block A is freed and reused for data in
> no-journalled-data mode, then If
> we don't have revoke table which recording the releasing event, replay
> of journal will overwrite the new data,
> which causing data corruption.

Yes, this is correct.  It should be covered fairly well in Stephen
Tweedie's, "Journaling the ext2fs file system" paper, which you can
find at:

https://ext4.wiki.kernel.org/index.php/Publications

if you'd like more details.

Hope this helps!

						- Ted
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