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| + ⎻⎻⎻⎻⎻⎻⎻⎻⎻⎻ ⎻⎻⎻⎻⎻⎻⎻⎻⎻⎻ + + 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' + -- 2.11.0 -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
