1================================
2Device-mapper "unstriped" target
3================================
4
5Introduction
6============
7
8The device-mapper "unstriped" target provides a transparent mechanism to
9unstripe a device-mapper "striped" target to access the underlying disks
10without having to touch the true backing block-device.  It can also be
11used to unstripe a hardware RAID-0 to access backing disks.
12
13Parameters:
14<number of stripes> <chunk size> <stripe #> <dev_path> <offset>
15
16<number of stripes>
17        The number of stripes in the RAID 0.
18
19<chunk size>
20	The amount of 512B sectors in the chunk striping.
21
22<dev_path>
23	The block device you wish to unstripe.
24
25<stripe #>
26        The stripe number within the device that corresponds to physical
27        drive you wish to unstripe.  This must be 0 indexed.
28
29
30Why use this module?
31====================
32
33An example of undoing an existing dm-stripe
34-------------------------------------------
35
36This small bash script will setup 4 loop devices and use the existing
37striped target to combine the 4 devices into one.  It then will use
38the unstriped target ontop of the striped device to access the
39individual backing loop devices.  We write data to the newly exposed
40unstriped devices and verify the data written matches the correct
41underlying device on the striped array::
42
43  #!/bin/bash
44
45  MEMBER_SIZE=$((128 * 1024 * 1024))
46  NUM=4
47  SEQ_END=$((${NUM}-1))
48  CHUNK=256
49  BS=4096
50
51  RAID_SIZE=$((${MEMBER_SIZE}*${NUM}/512))
52  DM_PARMS="0 ${RAID_SIZE} striped ${NUM} ${CHUNK}"
53  COUNT=$((${MEMBER_SIZE} / ${BS}))
54
55  for i in $(seq 0 ${SEQ_END}); do
56    dd if=/dev/zero of=member-${i} bs=${MEMBER_SIZE} count=1 oflag=direct
57    losetup /dev/loop${i} member-${i}
58    DM_PARMS+=" /dev/loop${i} 0"
59  done
60
61  echo $DM_PARMS | dmsetup create raid0
62  for i in $(seq 0 ${SEQ_END}); do
63    echo "0 1 unstriped ${NUM} ${CHUNK} ${i} /dev/mapper/raid0 0" | dmsetup create set-${i}
64  done;
65
66  for i in $(seq 0 ${SEQ_END}); do
67    dd if=/dev/urandom of=/dev/mapper/set-${i} bs=${BS} count=${COUNT} oflag=direct
68    diff /dev/mapper/set-${i} member-${i}
69  done;
70
71  for i in $(seq 0 ${SEQ_END}); do
72    dmsetup remove set-${i}
73  done
74
75  dmsetup remove raid0
76
77  for i in $(seq 0 ${SEQ_END}); do
78    losetup -d /dev/loop${i}
79    rm -f member-${i}
80  done
81
82Another example
83---------------
84
85Intel NVMe drives contain two cores on the physical device.
86Each core of the drive has segregated access to its LBA range.
87The current LBA model has a RAID 0 128k chunk on each core, resulting
88in a 256k stripe across the two cores::
89
90   Core 0:       Core 1:
91  __________    __________
92  | LBA 512|    | LBA 768|
93  | LBA 0  |    | LBA 256|
94  ----------    ----------
95
96The purpose of this unstriping is to provide better QoS in noisy
97neighbor environments. When two partitions are created on the
98aggregate drive without this unstriping, reads on one partition
99can affect writes on another partition.  This is because the partitions
100are striped across the two cores.  When we unstripe this hardware RAID 0
101and make partitions on each new exposed device the two partitions are now
102physically separated.
103
104With the dm-unstriped target we're able to segregate an fio script that
105has read and write jobs that are independent of each other.  Compared to
106when we run the test on a combined drive with partitions, we were able
107to get a 92% reduction in read latency using this device mapper target.
108
109
110Example dmsetup usage
111=====================
112
113unstriped ontop of Intel NVMe device that has 2 cores
114-----------------------------------------------------
115
116::
117
118  dmsetup create nvmset0 --table '0 512 unstriped 2 256 0 /dev/nvme0n1 0'
119  dmsetup create nvmset1 --table '0 512 unstriped 2 256 1 /dev/nvme0n1 0'
120
121There will now be two devices that expose Intel NVMe core 0 and 1
122respectively::
123
124  /dev/mapper/nvmset0
125  /dev/mapper/nvmset1
126
127unstriped ontop of striped with 4 drives using 128K chunk size
128--------------------------------------------------------------
129
130::
131
132  dmsetup create raid_disk0 --table '0 512 unstriped 4 256 0 /dev/mapper/striped 0'
133  dmsetup create raid_disk1 --table '0 512 unstriped 4 256 1 /dev/mapper/striped 0'
134  dmsetup create raid_disk2 --table '0 512 unstriped 4 256 2 /dev/mapper/striped 0'
135  dmsetup create raid_disk3 --table '0 512 unstriped 4 256 3 /dev/mapper/striped 0'
136