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diff --git a/Documentation/device-mapper/unstriped.txt b/Documentation/device-mapper/unstriped.txt new file mode 100644 index 000000000..0b2a306c5 --- /dev/null +++ b/Documentation/device-mapper/unstriped.txt @@ -0,0 +1,124 @@ +Introduction +============ + +The device-mapper "unstriped" target provides a transparent mechanism to +unstripe a device-mapper "striped" target to access the underlying disks +without having to touch the true backing block-device. It can also be +used to unstripe a hardware RAID-0 to access backing disks. + +Parameters: +<number of stripes> <chunk size> <stripe #> <dev_path> <offset> + +<number of stripes> + The number of stripes in the RAID 0. + +<chunk size> + The amount of 512B sectors in the chunk striping. + +<dev_path> + The block device you wish to unstripe. + +<stripe #> + The stripe number within the device that corresponds to physical + drive you wish to unstripe. This must be 0 indexed. + + +Why use this module? +==================== + +An example of undoing an existing dm-stripe +------------------------------------------- + +This small bash script will setup 4 loop devices and use the existing +striped target to combine the 4 devices into one. It then will use +the unstriped target ontop of the striped device to access the +individual backing loop devices. We write data to the newly exposed +unstriped devices and verify the data written matches the correct +underlying device on the striped array. + +#!/bin/bash + +MEMBER_SIZE=$((128 * 1024 * 1024)) +NUM=4 +SEQ_END=$((${NUM}-1)) +CHUNK=256 +BS=4096 + +RAID_SIZE=$((${MEMBER_SIZE}*${NUM}/512)) +DM_PARMS="0 ${RAID_SIZE} striped ${NUM} ${CHUNK}" +COUNT=$((${MEMBER_SIZE} / ${BS})) + +for i in $(seq 0 ${SEQ_END}); do + dd if=/dev/zero of=member-${i} bs=${MEMBER_SIZE} count=1 oflag=direct + losetup /dev/loop${i} member-${i} + DM_PARMS+=" /dev/loop${i} 0" +done + +echo $DM_PARMS | dmsetup create raid0 +for i in $(seq 0 ${SEQ_END}); do + echo "0 1 unstriped ${NUM} ${CHUNK} ${i} /dev/mapper/raid0 0" | dmsetup create set-${i} +done; + +for i in $(seq 0 ${SEQ_END}); do + dd if=/dev/urandom of=/dev/mapper/set-${i} bs=${BS} count=${COUNT} oflag=direct + diff /dev/mapper/set-${i} member-${i} +done; + +for i in $(seq 0 ${SEQ_END}); do + dmsetup remove set-${i} +done + +dmsetup remove raid0 + +for i in $(seq 0 ${SEQ_END}); do + losetup -d /dev/loop${i} + rm -f member-${i} +done + +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| + ---------- ---------- + +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 dm-unstriped target we're able to segregate an 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 read latency using this device mapper target. + + +Example dmsetup usage +===================== + +unstriped ontop of Intel NVMe device that has 2 cores +----------------------------------------------------- +dmsetup create nvmset0 --table '0 512 unstriped 2 256 0 /dev/nvme0n1 0' +dmsetup create nvmset1 --table '0 512 unstriped 2 256 1 /dev/nvme0n1 0' + +There will now be two devices that expose Intel NVMe core 0 and 1 +respectively: +/dev/mapper/nvmset0 +/dev/mapper/nvmset1 + +unstriped ontop of striped with 4 drives using 128K chunk size +-------------------------------------------------------------- +dmsetup create raid_disk0 --table '0 512 unstriped 4 256 0 /dev/mapper/striped 0' +dmsetup create raid_disk1 --table '0 512 unstriped 4 256 1 /dev/mapper/striped 0' +dmsetup create raid_disk2 --table '0 512 unstriped 4 256 2 /dev/mapper/striped 0' +dmsetup create raid_disk3 --table '0 512 unstriped 4 256 3 /dev/mapper/striped 0' |