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

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Message-ID: <4DB6A2B0.7050708@gmail.com>
Date:	Tue, 26 Apr 2011 16:17:12 +0530
From:	Niraj Kulkarni <kulkarniniraj14@...il.com>
To:	Ding Dinghua <dingdinghua85@...il.com>
CC:	Yongqiang Yang <xiaoqiangnk@...il.com>, linux-ext4@...r.kernel.org
Subject: Re: Need of revoke mechanism in JBD

If I am thinking correctly, journal would be checkpointed  on filesystem 
unmount calls.
This implies the given scenario would be pretty rare.

ie first filesystem should be mounted in full-journal mode, and crashed 
prior to checkpoint.
then it should be remounted in no-journalled-data mode without recovery
and again remounted in full journalled mode with recovery.

Am I thinking on correct lines?

On Tuesday 26 April 2011 02:53 PM, 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.
>
> 2011/4/26 Yongqiang Yang<xiaoqiangnk@...il.com>:
>> AFAIK, it can accelerate the recovering process.  If a block is in the
>> revoke table of a transaction t1 and t1 is committed, then the there
>> is no need to recover the block in transactions which is earlier than
>> t1.
>>
>> On Tue, Apr 26, 2011 at 4:29 PM, Niraj Kulkarni
>> <kulkarniniraj14@...il.com>  wrote:
>>> Hi all,
>>>       I am new to fs development. I am trying to modify the journal structure
>>> of JBD. While analyzing the code, I could understand most of the things, but
>>> I am not able to understand the need of revoke mechanism. Can anybody
>>> enlighten me on this issue?
>>>
>>> Regards
>>> Niraj
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>>
>>
>> --
>> Best Wishes
>> Yongqiang Yang
>> --
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>
>

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