136 lines
4.2 KiB
ReStructuredText
136 lines
4.2 KiB
ReStructuredText
================================
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Device-mapper "unstriped" target
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================================
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Introduction
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============
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The device-mapper "unstriped" target provides a transparent mechanism to
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unstripe a device-mapper "striped" target to access the underlying disks
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without having to touch the true backing block-device. It can also be
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used to unstripe a hardware RAID-0 to access backing disks.
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Parameters:
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<number of stripes> <chunk size> <stripe #> <dev_path> <offset>
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<number of stripes>
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The number of stripes in the RAID 0.
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<chunk size>
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The amount of 512B sectors in the chunk striping.
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<dev_path>
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The block device you wish to unstripe.
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<stripe #>
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The stripe number within the device that corresponds to physical
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drive you wish to unstripe. This must be 0 indexed.
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Why use this module?
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====================
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An example of undoing an existing dm-stripe
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-------------------------------------------
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This small bash script will setup 4 loop devices and use the existing
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striped target to combine the 4 devices into one. It then will use
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the unstriped target on top of the striped device to access the
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individual backing loop devices. We write data to the newly exposed
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unstriped devices and verify the data written matches the correct
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underlying device on the striped array::
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#!/bin/bash
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MEMBER_SIZE=$((128 * 1024 * 1024))
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NUM=4
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SEQ_END=$((${NUM}-1))
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CHUNK=256
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BS=4096
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RAID_SIZE=$((${MEMBER_SIZE}*${NUM}/512))
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DM_PARMS="0 ${RAID_SIZE} striped ${NUM} ${CHUNK}"
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COUNT=$((${MEMBER_SIZE} / ${BS}))
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for i in $(seq 0 ${SEQ_END}); do
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dd if=/dev/zero of=member-${i} bs=${MEMBER_SIZE} count=1 oflag=direct
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losetup /dev/loop${i} member-${i}
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DM_PARMS+=" /dev/loop${i} 0"
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done
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echo $DM_PARMS | dmsetup create raid0
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for i in $(seq 0 ${SEQ_END}); do
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echo "0 1 unstriped ${NUM} ${CHUNK} ${i} /dev/mapper/raid0 0" | dmsetup create set-${i}
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done;
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for i in $(seq 0 ${SEQ_END}); do
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dd if=/dev/urandom of=/dev/mapper/set-${i} bs=${BS} count=${COUNT} oflag=direct
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diff /dev/mapper/set-${i} member-${i}
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done;
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for i in $(seq 0 ${SEQ_END}); do
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dmsetup remove set-${i}
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done
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dmsetup remove raid0
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for i in $(seq 0 ${SEQ_END}); do
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losetup -d /dev/loop${i}
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rm -f member-${i}
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done
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Another example
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---------------
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Intel NVMe drives contain two cores on the physical device.
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Each core of the drive has segregated access to its LBA range.
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The current LBA model has a RAID 0 128k chunk on each core, resulting
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in a 256k stripe across the two cores::
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Core 0: Core 1:
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__________ __________
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| LBA 512| | LBA 768|
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| LBA 0 | | LBA 256|
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---------- ----------
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The purpose of this unstriping is to provide better QoS in noisy
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neighbor environments. When two partitions are created on the
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aggregate drive without this unstriping, reads on one partition
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can affect writes on another partition. This is because the partitions
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are striped across the two cores. When we unstripe this hardware RAID 0
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and make partitions on each new exposed device the two partitions are now
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physically separated.
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With the dm-unstriped target we're able to segregate an fio script that
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has read and write jobs that are independent of each other. Compared to
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when we run the test on a combined drive with partitions, we were able
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to get a 92% reduction in read latency using this device mapper target.
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Example dmsetup usage
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=====================
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unstriped on top of Intel NVMe device that has 2 cores
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------------------------------------------------------
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::
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dmsetup create nvmset0 --table '0 512 unstriped 2 256 0 /dev/nvme0n1 0'
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dmsetup create nvmset1 --table '0 512 unstriped 2 256 1 /dev/nvme0n1 0'
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There will now be two devices that expose Intel NVMe core 0 and 1
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respectively::
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/dev/mapper/nvmset0
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/dev/mapper/nvmset1
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unstriped on top of striped with 4 drives using 128K chunk size
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---------------------------------------------------------------
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::
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dmsetup create raid_disk0 --table '0 512 unstriped 4 256 0 /dev/mapper/striped 0'
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dmsetup create raid_disk1 --table '0 512 unstriped 4 256 1 /dev/mapper/striped 0'
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dmsetup create raid_disk2 --table '0 512 unstriped 4 256 2 /dev/mapper/striped 0'
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dmsetup create raid_disk3 --table '0 512 unstriped 4 256 3 /dev/mapper/striped 0'
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