1 ================================
2 Device-mapper "unstriped" target
3 ================================
4
5 Introduction
6 ============
7
8 The device-mapper "unstriped" target provides
9 unstripe a device-mapper "striped" target to a
10 without having to touch the true backing block
11 used to unstripe a hardware RAID-0 to access b
12
13 Parameters:
14 <number of stripes> <chunk size> <stripe #> <d
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 chun
21
22 <dev_path>
23 The block device you wish to unstripe.
24
25 <stripe #>
26 The stripe number within the device th
27 drive you wish to unstripe. This must
28
29
30 Why use this module?
31 ====================
32
33 An example of undoing an existing dm-stripe
34 -------------------------------------------
35
36 This small bash script will setup 4 loop devic
37 striped target to combine the 4 devices into o
38 the unstriped target on top of the striped dev
39 individual backing loop devices. We write dat
40 unstriped devices and verify the data written
41 underlying 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} ${CH
53 COUNT=$((${MEMBER_SIZE} / ${BS}))
54
55 for i in $(seq 0 ${SEQ_END}); do
56 dd if=/dev/zero of=member-${i} bs=${MEMBER
57 losetup /dev/loop${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} /
64 done;
65
66 for i in $(seq 0 ${SEQ_END}); do
67 dd if=/dev/urandom of=/dev/mapper/set-${i}
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
82 Another example
83 ---------------
84
85 Intel NVMe drives contain two cores on the phy
86 Each core of the drive has segregated access t
87 The current LBA model has a RAID 0 128k chunk
88 in 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
96 The purpose of this unstriping is to provide b
97 neighbor environments. When two partitions are
98 aggregate drive without this unstriping, reads
99 can affect writes on another partition. This
100 are striped across the two cores. When we uns
101 and make partitions on each new exposed device
102 physically separated.
103
104 With the dm-unstriped target we're able to seg
105 has read and write jobs that are independent o
106 when we run the test on a combined drive with
107 to get a 92% reduction in read latency using t
108
109
110 Example dmsetup usage
111 =====================
112
113 unstriped on top of Intel NVMe device that has
114 ----------------------------------------------
115
116 ::
117
118 dmsetup create nvmset0 --table '0 512 unstri
119 dmsetup create nvmset1 --table '0 512 unstri
120
121 There will now be two devices that expose Inte
122 respectively::
123
124 /dev/mapper/nvmset0
125 /dev/mapper/nvmset1
126
127 unstriped on top of striped with 4 drives usin
128 ----------------------------------------------
129
130 ::
131
132 dmsetup create raid_disk0 --table '0 512 uns
133 dmsetup create raid_disk1 --table '0 512 uns
134 dmsetup create raid_disk2 --table '0 512 uns
135 dmsetup create raid_disk3 --table '0 512 uns
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