linux-kernel - Re: [PATCH 01/21] block: Add atomic write operations to request

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Message-ID: <yq1o7heqmk0.fsf@ca-mkp.ca.oracle.com>
Date:   Wed, 04 Oct 2023 14:26:42 -0400
From:   "Martin K. Petersen" <martin.petersen@...cle.com>
To:     Bart Van Assche <bvanassche@....org>
Cc:     "Martin K. Petersen" <martin.petersen@...cle.com>,
        John Garry <john.g.garry@...cle.com>, axboe@...nel.dk,
        kbusch@...nel.org, hch@....de, sagi@...mberg.me,
        jejb@...ux.ibm.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,
        Himanshu Madhani <himanshu.madhani@...cle.com>
Subject: Re: [PATCH 01/21] block: Add atomic write operations to
 request_queue limits


Bart,

> In my opinion there is a contradiction between the above reply and
> patch 19/21 of this series. Data written with the SCSI WRITE ATOMIC
> command is not guaranteed to survive a power failure.

That is not the intent. The intent is to ensure that for any given
application block (say 16KB), the application block on media will
contain either 100% old data or 100% new data. Always.

If a storage device offers no such guarantee across a power failure,
then it is not suitable for use by applications which do not tolerate
torn writes. That is why the writes-are-atomic-unless-there's-a-problem
variant of the values reports in NVMe are of no interest.

-- 
Martin K. Petersen	Oracle Linux Engineering