linux-kernel - Re: [PATCH 21/21] nvme: Support atomic writes

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Date:   Thu, 9 Nov 2023 16:46:19 +0100
From:   Christoph Hellwig <hch@....de>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Christoph Hellwig <hch@....de>,
        John Garry <john.g.garry@...cle.com>, axboe@...nel.dk,
        kbusch@...nel.org, sagi@...mberg.me, jejb@...ux.ibm.com,
        martin.petersen@...cle.com, djwong@...nel.org,
        viro@...iv.linux.org.uk, brauner@...nel.org,
        chandan.babu@...cle.com, dchinner@...hat.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, tytso@....edu, jbongio@...gle.com,
        linux-api@...r.kernel.org, Alan Adamson <alan.adamson@...cle.com>
Subject: Re: [PATCH 21/21] nvme: Support atomic writes

On Thu, Nov 09, 2023 at 03:42:40PM +0000, Matthew Wilcox wrote:
> That wasn't the model we had in mind.  In our thinking, it was fine to
> send a write that crossed the atomic write limit, but the drive wouldn't
> guarantee that it was atomic except at the atomic write boundary.
> Eg with an AWUN of 16kB, you could send five 16kB writes, combine them
> into a single 80kB write, and if the power failed midway through, the
> drive would guarantee that it had written 0, 16kB, 32kB, 48kB, 64kB or
> all 80kB.  Not necessarily in order; it might have written bytes 16-32kB,
> 64-80kB and not the other three.

I can see some use for that, but I'm really worried that debugging
problems in the I/O merging and splitting will be absolute hell.