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    SubjectRe: [PATCH 3/3] blkdev: honor discard_granularity in blkdev_issue_discard()
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    "Martin K. Petersen" <martin.petersen@oracle.com> writes:

    >>>>>> "Lukas" == Lukas Czerner <lczerner@redhat.com> writes:
    >
    > Lukas> As Jeff Moyer pointed out we do not honor discard granularity
    > Lukas> while submitting REQ_DISCARD bios of size smaller than
    > Lukas> max_discard_sectors. That fact might have unwanted consequences
    > Lukas> of device ignoring the request, or even worse if device firmware
    > Lukas> is buggy.
    >
    > We've discussed this before and the consensus was not to do it. The
    > granularity is a hint, not a hard limit like max_discard_sectors.
    >
    > We want the reporting to be comprehensive throughout the block layer. If
    > we start aligning to the granularity at the top we lose information for
    > stacked devices below with a finer granularity.
    >
    > So if we were to align to the granularity we'd want to do it at the
    > bottom of the stack when we issue the command to the device. We've had a
    > few proposed patches to did that but so far we've only found one device
    > where it made a difference. And that case didn't justify adding a quirk.

    Hm, I wonder where it makes sense to document this. Maybe
    Documentation/block/queue-sysfs.txt, but I admit I wouldn't have looked
    there while reviewing this code.

    Cheers,
    Jeff


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