dm: add unstriped target

This device mapper "unstriped" target remaps and unstripes I/O so it
is issued solely on a single drive in a HW RAID0 or dm-striped target.

In a 4 drive HW RAID0 the striped target exposes 1/4th of the LBA range
as a virtual drive.  Each I/O to that virtual drive will only be issued
to the 1 drive that was selected of the 4 drives in the HW RAID0.

This unstriped target is most useful for Intel NVMe drives that have
multiple cores but that do not have firmware control to pin separate LBA
ranges to each discrete cpu core.

Signed-off-by: Scott Bauer <scott.bauer@intel.com>
Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Acked-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

1 files changed, 124 insertions, 0 deletions

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+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'