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Subject: Re: [PATCH v5 0/3] ext4, jbd2: journal cycled record transactions
 between each mount
To:     Andreas Dilger <adilger@dilger.ca>,
        Zhang Yi <yi.zhang@huaweicloud.com>
CC:     Ext4 Developers List <linux-ext4@vger.kernel.org>,
        Theodore Ts'o <tytso@mit.edu>, Jan Kara <jack@suse.cz>,
        <yukuai3@huawei.com>
References: <20230322013353.1843306-1-yi.zhang@huaweicloud.com>
 <06F8DFC9-26F5-475F-9428-06FED2CA01AA@dilger.ca>
From:   Zhang Yi <yi.zhang@huawei.com>
Message-ID: <62be1bf9-293a-7833-d13d-e7dee7d04742@huawei.com>
Date:   Thu, 23 Mar 2023 16:20:06 +0800
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On 2023/3/23 5:34, Andreas Dilger wrote:
> On Mar 21, 2023, at 7:33 PM, Zhang Yi <yi.zhang@huaweicloud.com> wrote:
>> This patch set add a new journal option 'JBD2_CYCLE_RECORD' and always
>> enable on ext4. It saves journal head for a clean unmounted file system
>> in the journal super block, which could let us record journal
>> transactions between each mount continuously. It could help us to do
>> journal backtrack and find root cause from a corrupted filesystem.
>> Current filesystem's corruption analysis is difficult and less useful
>> information, especially on the real products. It is useful to some
>> extent, especially for the cases of doing fuzzy tests and deploy in some
>> shout-runing products.
> 
> Another interesting side benefit of this change is that it gets a step
> closer to the "lazy ext4" (log-structured optimization) that had been
> described some time ago at FAST:
> 
> https://lwn.net/Articles/720226/
> https://www.usenix.org/system/files/conference/fast17/fast17-aghayev.pdf
> https://lists.openwall.net/linux-ext4/2017/04/11/1
> 
> Essentially, free space in the filesystem (or a large external device)
> could be used as a continuous journal, and metadata would only rarely
> be checkpointed to the actual filesystem.  If the "journal" is close to
> wrapping to the start, either the meta/data is checkpointed (if it is
> no longer actively used or can make a large write), or re-journaled to
> the end of the journal.  At remount time, the full journal is read into
> memory (discarding old copies of blocks) and this is used to identify
> the current metadata rather than reading from the filesystem itself.
> 
> This would allow e.g. very efficient flash caching of metadata (and also
> journaled data for small writes) for an HDD (or QLC) device.
> 

This is interesting, but current change looks like is just one small step.
It's been almost 6 years after the last talk I can found[1]. Is there
anyone still working on it?

[1] https://lore.kernel.org/linux-ext4/6B0F0C59-6930-41B3-8EE4-EA5BEECEB9F9@dilger.ca/

Thanks,
Yi.