linux-kernel - Re: [RFC PATCH 0/8] xfs: single block atomic writes for buffered IO

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Message-ID: <aRb0WQJi4rQQ-Zmo@li-dc0c254c-257c-11b2-a85c-98b6c1322444.ibm.com>
Date: Fri, 14 Nov 2025 14:50:25 +0530
From: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
To: Dave Chinner <david@...morbit.com>
Cc: Ritesh Harjani <ritesh.list@...il.com>, Christoph Hellwig <hch@....de>,
        Christian Brauner <brauner@...nel.org>, djwong@...nel.org,
        john.g.garry@...cle.com, tytso@....edu, willy@...radead.org,
        dchinner@...hat.com, linux-xfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, jack@...e.cz,
        nilay@...ux.ibm.com, martin.petersen@...cle.com, rostedt@...dmis.org,
        axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/8] xfs: single block atomic writes for buffered IO

On Thu, Nov 13, 2025 at 09:32:11PM +1100, Dave Chinner wrote:
> On Thu, Nov 13, 2025 at 11:12:49AM +0530, Ritesh Harjani wrote:
> > Christoph Hellwig <hch@....de> writes:
> > 
> > > On Thu, Nov 13, 2025 at 08:56:56AM +1100, Dave Chinner wrote:
> > >> On Wed, Nov 12, 2025 at 04:36:03PM +0530, Ojaswin Mujoo wrote:
> > >> > This patch adds support to perform single block RWF_ATOMIC writes for
> > >> > iomap xfs buffered IO. This builds upon the inital RFC shared by John
> > >> > Garry last year [1]. Most of the details are present in the respective 
> > >> > commit messages but I'd mention some of the design points below:
> > >> 
> > >> What is the use case for this functionality? i.e. what is the
> > >> reason for adding all this complexity?
> > >
> > > Seconded.  The atomic code has a lot of complexity, and further mixing
> > > it with buffered I/O makes this even worse.  We'd need a really important
> > > use case to even consider it.
> > 
> > I agree this should have been in the cover letter itself. 
> > 
> > I believe the reason for adding this functionality was also discussed at
> > LSFMM too...  
> > 
> > For e.g. https://lwn.net/Articles/974578/ goes in depth and talks about
> > Postgres folks looking for this, since PostgreSQL databases uses
> > buffered I/O for their database writes.
> 
> Pointing at a discussion about how "this application has some ideas
> on how it can maybe use it someday in the future" isn't a
> particularly good justification. This still sounds more like a
> research project than something a production system needs right now.

Hi Dave, Christoph,

There were some discussions around use cases for buffered atomic writes
in the previous LSFMM covered by LWN here [1]. AFAIK, there are 
databases that recommend/prefer buffered IO over direct IO. As mentioned
in the article, MongoDB being one that supports both but recommends
buffered IO. Further, many DBs support both direct IO and buffered IO
well and it may not be fair to force them to stick to direct IO to get
the benefits of atomic writes.

[1] https://lwn.net/Articles/1016015/
> 
> Why didn't you use the existing COW buffered write IO path to
> implement atomic semantics for buffered writes? The XFS
> functionality is already all there, and it doesn't require any
> changes to the page cache or iomap to support...

This patch set focuses on HW accelerated single block atomic writes with
buffered IO, to get some early reviews on the core design.

Just like we did for direct IO atomic writes, the software fallback with
COW and multi block support can be added eventually.

Regards,
ojaswin

> 
> -Dave.
> -- 
> Dave Chinner
> david@...morbit.com