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    SubjectRe: [dm-devel] REQUEST for new 'topology' metrics to be moved out of the 'queue' sysfs directory.
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    >>>>> "Neil" == Neil Brown <neilb@suse.de> writes:

    Neil> Providing the fields are clearly and unambiguously documented so
    Neil> that it I can use the documentation to verify the implementation
    Neil> (in md at least), I will be satisfied.

    The current sysfs documentation says:

    /sys/block/<disk>/queue/minimum_io_size:
    [...] For RAID arrays it is often the stripe chunk size.

    /sys/block/<disk>/queue/optimal_io_size:
    [...] For RAID devices it is usually the stripe width or the internal
    block size.

    The latter should be "internal track size". But in the context of MD I
    think those two definitions are crystal clear.


    As far as making the application of these values more obvious I propose
    the following:

    What: /sys/block/<disk>/queue/minimum_io_size
    Date: April 2009
    Contact: Martin K. Petersen <martin.petersen@oracle.com>
    Description:
    Storage devices may report a granularity or minimum I/O
    size which is the device's preferred unit of I/O.
    Requests smaller than this may incur a significant
    performance penalty.

    For disk drives this value corresponds to the physical
    block size. For RAID devices it is usually the stripe
    chunk size.

    A properly aligned multiple of minimum_io_size is the
    preferred request size for workloads where a high number
    of I/O operations is desired.


    What: /sys/block/<disk>/queue/optimal_io_size
    Date: April 2009
    Contact: Martin K. Petersen <martin.petersen@oracle.com>
    Description:
    Storage devices may report an optimal transfer length or
    streaming I/O size which is the device's preferred unit
    of sustained I/O. This value is a multiple of the
    device's minimum_io_size.

    optimal_io_size is rarely reported for disk drives. For
    RAID devices it is usually the stripe width or the
    internal track size.

    A properly aligned multiple of optimal_io_size is the
    preferred request size for workloads where sustained
    throughput is desired.

    After contemplating for a bit I think I prefer to keep them I/O
    direction agnostic. Granted, the potential penalties mostly apply to
    writes. But I think the application of the values apply to reads as
    well. They will in a hw RAID context for sure.


    Neil> I'm looking forward to seeing how you justify the name
    Neil> "physical_block_size" in a way the encompasses possibilities like
    Neil> a device that stripes over a heterogeneous set of disk drives ;-)

    I explained that in my mails yesterday. But that is of no concern to
    MD.

    --
    Martin K. Petersen Oracle Linux Engineering


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