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SubjectRe: [PATCH v3 2/2] dm unstripe: Add documentation for unstripe target
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On 12/13/2017 01:33 PM, Scott Bauer wrote:
> Signed-off-by: Scott Bauer <scott.bauer@intel.com>
> ---
> Documentation/device-mapper/dm-unstripe.txt | 130 ++++++++++++++++++++++++++++
> 1 file changed, 130 insertions(+)
> create mode 100644 Documentation/device-mapper/dm-unstripe.txt
>
> diff --git a/Documentation/device-mapper/dm-unstripe.txt b/Documentation/device-mapper/dm-unstripe.txt
> new file mode 100644
> index 000000000000..01d7194b9075
> --- /dev/null
> +++ b/Documentation/device-mapper/dm-unstripe.txt
> @@ -0,0 +1,130 @@
> +Device-Mapper Unstripe
> +=====================
> +

[snip]

> +==============
> +
> +
> + Another example:
> +
> + Intel NVMe drives contain two cores on the physical device.
> + Each core of the drive has segregated access to its LBA range.
> + The current LBA model has a RAID 0 128k chunk on each core, resulting
> + in a 256k stripe across the two cores:
> +
> + Core 0: Core 1:
> + __________ __________
> + | LBA 512| | LBA 768|
> + | LBA 0 | | LBA 256|
> + ⎻⎻⎻⎻⎻⎻⎻⎻⎻⎻ ⎻⎻⎻⎻⎻⎻⎻⎻⎻⎻

Use ASCII characters ___ or ---, not whatever those bottom block characters are.

> +
> + The purpose of this unstriping is to provide better QoS in noisy
> + neighbor environments. When two partitions are created on the
> + aggregate drive without this unstriping, reads on one partition
> + can affect writes on another partition. This is because the partitions
> + are striped across the two cores. When we unstripe this hardware RAID 0
> + and make partitions on each new exposed device the two partitions are now
> + physically separated.
> +
> + With the module we were able to segregate a fio script that has read and
> + write jobs that are independent of each other. Compared to when we run
> + the test on a combined drive with partitions, we were able to get a 92%
> + reduction in five-9ths read latency using this device mapper target.

5/9ths
although I can't quite parse that sentence.


--
~Randy

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