1 =========
2 dm-switch
3 =========
4
5 The device-mapper switch target creates a devi
6 arbitrary mapping of fixed-size regions of I/O
7 paths. The path used for any specific region
8 dynamically by sending the target a message.
9
10 It maps I/O to underlying block devices effici
11 number of fixed-sized address regions but ther
12 that would allow for a compact representation
13 dm-stripe.
14
15 Background
16 ----------
17
18 Dell EqualLogic and some other iSCSI storage a
19 frameless architecture. In this architecture,
20 consists of a number of distinct storage array
21 independent controllers, disk storage and netw
22 is created it is spread across multiple member
23 spreading are hidden from initiators connected
24 The storage group exposes a single target disc
25 how many members are being used. When iSCSI s
26 session is connected to an eth port on a singl
27 can be sent on any iSCSI session, and if the b
28 stored on another member the I/O will be forwa
29 forwarding is invisible to the initiator. The
30 dynamic, and the blocks stored on disk may be
31 member as needed to balance the load.
32
33 This architecture simplifies the management an
34 the storage group and initiators. In a multip
35 is possible to set up multiple iSCSI sessions
36 interfaces on both the host and target to take
37 increased network bandwidth. An initiator cou
38 robin algorithm to send I/O across all paths a
39 members forward it as necessary, but there is
40 sending data directly to the correct member.
41
42 A device-mapper table already lets you map dif
43 device onto different targets. However in thi
44 spread with an address region size on the orde
45 means the resulting table could have more than
46 consume far too much memory.
47
48 Using this device-mapper switch target we can
49 device hierarchy:
50
51 Upper Tier - Determine which array member
52 Lower Tier - Load balance amongst paths to
53
54 The lower tier consists of a single dm multipa
55 Each of these multipath devices contains the s
56 the array member in one priority group, and le
57 selectors to load balance amongst these paths.
58 non-preferred priority group containing paths
59 failover reasons.
60
61 The upper tier consists of a single dm-switch
62 a bitmap to look up the location of the I/O an
63 lower tier device to route the I/O. By using
64 use 4 bits for each address range in a 16 memb
65 large for us). This is a much denser represen
66 b-tree can achieve.
67
68 Construction Parameters
69 =======================
70
71 <num_paths> <region_size> <num_optional_ar
72 <num_paths>
73 The number of paths across which t
74
75 <region_size>
76 The number of 512-byte sectors in
77 to any of the available paths.
78
79 <num_optional_args>
80 The number of optional arguments.
81 are supported and so this must be
82
83 <dev_path>
84 The block device that represents a
85
86 <offset>
87 The offset of the start of data on
88 of 512-byte sectors). This number
89 forwarding the request to the spec
90
91 Messages
92 ========
93
94 set_region_mappings <index>:<path_nr> [<index>
95
96 Modify the region table by specifying which re
97 which paths.
98
99 <index>
100 The region number (region size was specifi
101 If index is omitted, the next region (prev
102 Expressed in hexadecimal (WITHOUT any pref
103
104 <path_nr>
105 The path number in the range 0 ... (<num_p
106 Expressed in hexadecimal (WITHOUT any pref
107
108 R<n>,<m>
109 This parameter allows repetitive patterns
110 are hexadecimal numbers. The last <n> mapp
111 slots.
112
113 Status
114 ======
115
116 No status line is reported.
117
118 Example
119 =======
120
121 Assume that you have volumes vg1/switch0 vg1/s
122 the same size.
123
124 Create a switch device with 64kB region size::
125
126 dmsetup create switch --table "0 `blockdev
127 switch 3 128 0 /dev/vg1/switch0 0 /dev
128
129 Set mappings for the first 7 entries to point
130 switch2, switch0, switch1, switch2, switch1::
131
132 dmsetup message switch 0 set_region_mappin
133
134 Set repetitive mapping. This command::
135
136 dmsetup message switch 0 set_region_mappin
137
138 is equivalent to::
139
140 dmsetup message switch 0 set_region_mappin
141 :1 :2 :1 :2 :1 :2 :1 :2 :1 :2
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