Re: [PATCH 0/5] block/scsi/lio support for COMPARE_AND_WRITE

[Bart Van Assche <bvanassche@acm.org>]

On 10/16/14 07:37, michaelc@cs.wisc.edu wrote:
> The following patches implement the SCSI command COMPARE_AND_WRITE as a new
> bio/request type REQ_CMP_AND_WRITE. COMPARE_AND_WRITE is defined in the
> SCSI SBC (SCSI block command) specs as:
>
> The COMPARE AND WRITE command requests that the device server perform the
> following as an uninterrupted series of actions:
>
> 1) perform the following operations:
>          A) read the specified logical blocks; and
>          B) transfer the specified number of logical blocks from the Data-Out
>          Buffer (i.e., the verify instance of the data is transferred from the
>          Data-Out Buffer);
>
> 2) compare the data read from the specified logical blocks with the verify
> instance of the data; and
> 3) If the compared data matches, then perform the following operations:
>          1) transfer the specified number of logical blocks from the Data-Out
>          Buffer (i.e., the write instance of the data transferred from the
>          Data-Out Buffer); and
>          2) write those logical blocks.

Hello Mike,

Just below the above text one can find the following additional 
requirement in SBC-4:
<quote>
4) if the compare operation does not indicate a match, then terminate
    the command with CHECK CONDITION status with the sense key set to
    MISCOMPARE and the additional sense code set to MISCOMPARE DURING
    VERIFY OPERATION. In the sense data (see 4.18 and SPC-4) the offset
    from the start of the Data-Out Buffer to the first byte of data that
    was not equal shall be reported in the INFORMATION field.
</quote>

What I'm wondering now is how requirement (4) can be supported if 
REQ_CMP_AND_WRITE doesn't return the offset of the first byte that 
didn't match ? Additionally, shouldn't compare_and_write_callback() be 
fixed such that it returns the miscompare offset to its caller ?

Thanks,

Bart.