1dm-switch 2========= 3 4The device-mapper switch target creates a device that supports an 5arbitrary mapping of fixed-size regions of I/O across a fixed set of 6paths. The path used for any specific region can be switched 7dynamically by sending the target a message. 8 9It maps I/O to underlying block devices efficiently when there is a large 10number of fixed-sized address regions but there is no simple pattern 11that would allow for a compact representation of the mapping such as 12dm-stripe. 13 14Background 15---------- 16 17Dell EqualLogic and some other iSCSI storage arrays use a distributed 18frameless architecture. In this architecture, the storage group 19consists of a number of distinct storage arrays ("members") each having 20independent controllers, disk storage and network adapters. When a LUN 21is created it is spread across multiple members. The details of the 22spreading are hidden from initiators connected to this storage system. 23The storage group exposes a single target discovery portal, no matter 24how many members are being used. When iSCSI sessions are created, each 25session is connected to an eth port on a single member. Data to a LUN 26can be sent on any iSCSI session, and if the blocks being accessed are 27stored on another member the I/O will be forwarded as required. This 28forwarding is invisible to the initiator. The storage layout is also 29dynamic, and the blocks stored on disk may be moved from member to 30member as needed to balance the load. 31 32This architecture simplifies the management and configuration of both 33the storage group and initiators. In a multipathing configuration, it 34is possible to set up multiple iSCSI sessions to use multiple network 35interfaces on both the host and target to take advantage of the 36increased network bandwidth. An initiator could use a simple round 37robin algorithm to send I/O across all paths and let the storage array 38members forward it as necessary, but there is a performance advantage to 39sending data directly to the correct member. 40 41A device-mapper table already lets you map different regions of a 42device onto different targets. However in this architecture the LUN is 43spread with an address region size on the order of 10s of MBs, which 44means the resulting table could have more than a million entries and 45consume far too much memory. 46 47Using this device-mapper switch target we can now build a two-layer 48device hierarchy: 49 50 Upper Tier - Determine which array member the I/O should be sent to. 51 Lower Tier - Load balance amongst paths to a particular member. 52 53The lower tier consists of a single dm multipath device for each member. 54Each of these multipath devices contains the set of paths directly to 55the array member in one priority group, and leverages existing path 56selectors to load balance amongst these paths. We also build a 57non-preferred priority group containing paths to other array members for 58failover reasons. 59 60The upper tier consists of a single dm-switch device. This device uses 61a bitmap to look up the location of the I/O and choose the appropriate 62lower tier device to route the I/O. By using a bitmap we are able to 63use 4 bits for each address range in a 16 member group (which is very 64large for us). This is a much denser representation than the dm table 65b-tree can achieve. 66 67Construction Parameters 68======================= 69 70 <num_paths> <region_size> <num_optional_args> [<optional_args>...] 71 [<dev_path> <offset>]+ 72 73<num_paths> 74 The number of paths across which to distribute the I/O. 75 76<region_size> 77 The number of 512-byte sectors in a region. Each region can be redirected 78 to any of the available paths. 79 80<num_optional_args> 81 The number of optional arguments. Currently, no optional arguments 82 are supported and so this must be zero. 83 84<dev_path> 85 The block device that represents a specific path to the device. 86 87<offset> 88 The offset of the start of data on the specific <dev_path> (in units 89 of 512-byte sectors). This number is added to the sector number when 90 forwarding the request to the specific path. Typically it is zero. 91 92Messages 93======== 94 95set_region_mappings <index>:<path_nr> [<index>]:<path_nr> [<index>]:<path_nr>... 96 97Modify the region table by specifying which regions are redirected to 98which paths. 99 100<index> 101 The region number (region size was specified in constructor parameters). 102 If index is omitted, the next region (previous index + 1) is used. 103 Expressed in hexadecimal (WITHOUT any prefix like 0x). 104 105<path_nr> 106 The path number in the range 0 ... (<num_paths> - 1). 107 Expressed in hexadecimal (WITHOUT any prefix like 0x). 108 109R<n>,<m> 110 This parameter allows repetitive patterns to be loaded quickly. <n> and <m> 111 are hexadecimal numbers. The last <n> mappings are repeated in the next <m> 112 slots. 113 114Status 115====== 116 117No status line is reported. 118 119Example 120======= 121 122Assume that you have volumes vg1/switch0 vg1/switch1 vg1/switch2 with 123the same size. 124 125Create a switch device with 64kB region size: 126 dmsetup create switch --table "0 `blockdev --getsize /dev/vg1/switch0` 127 switch 3 128 0 /dev/vg1/switch0 0 /dev/vg1/switch1 0 /dev/vg1/switch2 0" 128 129Set mappings for the first 7 entries to point to devices switch0, switch1, 130switch2, switch0, switch1, switch2, switch1: 131 dmsetup message switch 0 set_region_mappings 0:0 :1 :2 :0 :1 :2 :1 132 133Set repetitive mapping. This command: 134 dmsetup message switch 0 set_region_mappings 1000:1 :2 R2,10 135is equivalent to: 136 dmsetup message switch 0 set_region_mappings 1000:1 :2 :1 :2 :1 :2 :1 :2 \ 137 :1 :2 :1 :2 :1 :2 :1 :2 :1 :2 138 139