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

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Message-ID: <BANLkTin35yOg+ndy3i4vTT7RHxtFso+HCA@mail.gmail.com>
Date:	Fri, 29 Apr 2011 22:45:23 +0300
From:	Amir Goldstein <amir73il@...il.com>
To:	"Ted Ts'o" <tytso@....edu>
Cc:	Ding Dinghua <dingdinghua85@...il.com>,
	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 3:25 PM, Ted Ts'o <tytso@....edu> wrote:
> 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

Actually, the original paper has no mention of revoke records.
I went out to look for useful documentation on journal forget/revoke
and came back empty handed as well.

>
> if you'd like more details.
>
> Hope this helps!
>
>                                                - Ted
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