linux-kernel - Re: [PATCH -next v2 00/22] ext4: use iomap for regular file's buffered I/O path

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Message-ID: <xxr5myyy2dkumgqmqk3qpwkkvwiwxntx2ovl6cuxifn7ody4bv@2ni65ps2mhjy>
Date: Thu, 5 Feb 2026 13:46:38 +0100
From: Jan Kara <jack@...e.cz>
To: Baokun Li <libaokun1@...wei.com>
Cc: Jan Kara <jack@...e.cz>, Theodore Tso <tytso@....edu>, 
	Zhang Yi <yi.zhang@...weicloud.com>, Christoph Hellwig <hch@...radead.org>, 
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	adilger.kernel@...ger.ca, ojaswin@...ux.ibm.com, ritesh.list@...il.com, djwong@...nel.org, 
	Zhang Yi <yi.zhang@...wei.com>, yizhang089@...il.com, yangerkun@...wei.com, 
	yukuai@...-78bjiv52429oh8qptp.cn-shenzhen.alb.aliyuncs.com, libaokun9@...il.com
Subject: Re: [PATCH -next v2 00/22] ext4: use iomap for regular file's
 buffered I/O path

On Thu 05-02-26 10:55:59, Baokun Li wrote:
> > I don't see how you want to get rid of data=journal mode - perhaps that's
> > related to the COW functionality?
> 
> Yes. The only real advantage of data=journal mode over data=ordered is
> its guarantee of data atomicity for overwrites.
> 
> If we can achieve this through COW-based software atomic writes, we can
> move away from the performance-heavy data=journal mode.

Hum, I don't think data=journal actually currently offers overwrite
atomicity - even in data=journal mode you can observe only part of the
write completed after a crash. But what it does offer and why people tend
to use it is that it offers strict linear ordering guarantees between all
data and metadata operations happening in the system. Thus if you can prove
that operation A completed before operation B started, then you are
guaranteed that even after crash you will not see B done and A not done.
This is a very strong consistency guarantee that makes life simpler for the
applications so people that can afford the performance cost and care a lot
about crash safety like it.

I think you are correct that with COW and a bit of care it could be
possible to achieve these guarantees even without journalling data. But I'd
need to think more about it.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR