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To:     Bart Van Assche <bvanassche@acm.org>
Cc:     Dave Chinner <david@fromorbit.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
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Subject: Re: [PATCH 10/21] block: Add fops atomic write support
From:   "Martin K. Petersen" <martin.petersen@oracle.com>
Organization: Oracle Corporation
Message-ID: <yq1v8bjpaz5.fsf@ca-mkp.ca.oracle.com>
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Date:   Fri, 06 Oct 2023 21:21:01 -0400
In-Reply-To: <2bb2a4d0-4f1f-45f1-9196-f5d0d8ee1878@acm.org> (Bart Van Assche's
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Bart,

> The above implies that this parameter will always be equal to the
> logical block size.

It does not. Being able to write each individual block in an I/O without
tearing does not imply that a device can write two blocks as a single
atomic operation.

> Writes to a single physical block happen atomically. If there are
> multiple logical blocks per physical block, the block device must
> serialize read/modify/write cycles internally.

This is what SBC has to say:

"If any write command that is not an atomic write command, does not
complete successfully (e.g., the command completed with CHECK CONDITION
status, or the command was being processed at the time of a power loss
or an incorrect demount of a removable medium), then any data in the
logical blocks referenced by the LBAs specified by that command is
indeterminate."

SBC defines "atomic write command" like this:

"An atomic write command performs one or more atomic write operations.
 The following write commands are atomic write commands:

 a) WRITE ATOMIC (16) (see 5.48); and
 b) WRITE ATOMIC (32) (see 5.49)."

You will note that none of the regular WRITE commands appear in that
list.

Now, in practice we obviously rely heavily on the fact that most devices
are implemented in a sane fashion which doesn't mess up individual
logical blocks on power fail. But the spec does not guarantee this; it
is device implementation dependent. And again, we have seen both hard
disk drives and SSDs that cause collateral damage to an entire physical
block when power is lost at the wrong time.

-- 
Martin K. Petersen	Oracle Linux Engineering