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2 dm-switch
5 The device-mapper switch target creates a device that supports an
6 arbitrary mapping of fixed-size regions of I/O across a fixed set of
10 It maps I/O to underlying block devices efficiently when there is a large
11 number of fixed-sized address regions but there is no simple pattern
13 dm-stripe.
16 ----------
23 spreading are hidden from initiators connected to this storage system.
26 session is connected to an eth port on a single member.  Data to a LUN
29 forwarding is invisible to the initiator.  The storage layout is also
30 dynamic, and the blocks stored on disk may be moved from member to
31 member as needed to balance the load.
35 is possible to set up multiple iSCSI sessions to use multiple network
36 interfaces on both the host and target to take advantage of the
38 robin algorithm to send I/O across all paths and let the storage array
39 members forward it as necessary, but there is a performance advantage to
40 sending data directly to the correct member.
42 A device-mapper table already lets you map different regions of a
48 Using this device-mapper switch target we can now build a two-layer
51     Upper Tier - Determine which array member the I/O should be sent to.
52     Lower Tier - Load balance amongst paths to a particular member.
55 Each of these multipath devices contains the set of paths directly to
57 selectors to load balance amongst these paths.  We also build a
58 non-preferred priority group containing paths to other array members for
61 The upper tier consists of a single dm-switch device.  This device uses
62 a bitmap to look up the location of the I/O and choose the appropriate
63 lower tier device to route the I/O.  By using a bitmap we are able to
66 b-tree can achieve.
73 	    The number of paths across which to distribute the I/O.
76 	    The number of 512-byte sectors in a region. Each region can be redirected
77 	    to any of the available paths.
84 	    The block device that represents a specific path to the device.
88 	    of 512-byte sectors). This number is added to the sector number when
89 	    forwarding the request to the specific path. Typically it is zero.
96 Modify the region table by specifying which regions are redirected to
101     If index is omitted, the next region (previous index + 1) is used.
105     The path number in the range 0 ... (<num_paths> - 1).
109     This parameter allows repetitive patterns to be loaded quickly. <n> and <m>
126     dmsetup create switch --table "0 `blockdev --getsz /dev/vg1/switch0`
129 Set mappings for the first 7 entries to point to devices switch0, switch1,
132     dmsetup message switch 0 set_region_mappings 0:0 :1 :2 :0 :1 :2 :1
136     dmsetup message switch 0 set_region_mappings 1000:1 :2 R2,10
138 is equivalent to::
140     dmsetup message switch 0 set_region_mappings 1000:1 :2 :1 :2 :1 :2 :1 :2 \
141 	:1 :2 :1 :2 :1 :2 :1 :2 :1 :2