Re: [PATCH v2 2/2] dm unstripe: Add documentation for unstripe target

[Nikolay Borisov]

On 11.12.2017 18:00, Scott Bauer wrote:
> Signed-off-by: Scott Bauer <scott.bauer@xxxxxxxxx>
> ---
>  Documentation/device-mapper/dm-unstripe.txt | 82 +++++++++++++++++++++++++++++
>  1 file changed, 82 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..4e1a0a39a689
> --- /dev/null
> +++ b/Documentation/device-mapper/dm-unstripe.txt
> @@ -0,0 +1,82 @@
> +Device-Mapper Unstripe
> +=====================
> +
> +The device-mapper Unstripe (dm-unstripe) target provides a transparent
> +mechanism to unstripe a RAID 0 striping to access segregated disks.
> +
> +This module should be used by users who understand what the underlying
> +disks look like behind the software/hardware RAID.
> +
> +Parameters:
> +<drive (ex: /dev/nvme0n1)> <drive #> <# of drives> <stripe sectors>
> +
> +
> +<drive>
> +	The block device you wish to unstripe.
> +
> +<drive #>
> +        The physical drive you wish to expose via this "virtual" device
> +	mapper target. This must be 0 indexed.
> +
> +<# of drives>
> +        The number of drives in the RAID 0.
> +
> +<stripe sectors>
> +	The amount of 512B sectors in the raid striping, or zero, if you
> +	wish you use max_hw_sector_size.
> +
> +
> +Why use this module?
> +=====================
> +
> +As a use case:
> +
> +
> +    As an 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 stripe across the two cores:
> +
> +       Core 0:                Core 1:
> +      __________            __________
> +      | LBA 511|            | LBA 768|
> +      | LBA 0  |            | LBA 256|
> +      ââââââââââ            ââââââââââ

If it's 128k stripe shouldn't it be LBAs 0/256 on core0 and LBAs 128/511
on core1?

> +
> +    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. With the striping concurrent
> +    reads and writes and I/O on opposite cores have lower completion times,
> +    and better tail latencies.
> +
> +    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.
> +
> +
> +    One could use the module to Logical de-pop a HDD if you have sufficient
> +    geometry information regarding the drive.
> +
> +
> +Example scripts:
> +====================
> +
> +dmsetup create nvmset1 --table '0 1 dm-unstripe /dev/nvme0n1 1 2 0'
> +dmsetup create nvmset0 --table '0 1 dm-unstripe /dev/nvme0n1 0 2 0'
> +
> +There will now be two mappers:
> +/dev/mapper/nvmset1
> +/dev/mapper/nvmset0
> +
> +that will expose core 0 and core 1.
> +
> +
> +In a Raid 0 with 4 drives of stripe size 128K:
> +dmsetup create raid_disk0 --table '0 1 dm-unstripe /dev/nvme0n1 0 4 256'
> +dmsetup create raid_disk1 --table '0 1 dm-unstripe /dev/nvme0n1 1 4 256'
> +dmsetup create raid_disk2 --table '0 1 dm-unstripe /dev/nvme0n1 2 4 256'
> +dmsetup create raid_disk3 --table '0 1 dm-unstripe /dev/nvme0n1 3 4 256'
> +
>