=========dm-switch========= The device-mapper switch target creates a device that supports anarbitrary mapping of fixed-size regions of I/O across a fixed set ofpaths.  The path used for any specific region can be switcheddynamically by sending the target a message. It maps I/O to underlying block devices efficiently when there is a largenumber of fixed-sized address regions but there is no simple patternthat would allow for a compact representation of the mapping such asdm-stripe. Background---------- Dell EqualLogic and some other iSCSI storage arrays use a distributedframeless architecture.  In this architecture, the storage groupconsists of a number of distinct storage arrays ("members") each havingindependent controllers, disk storage and network adapters.  When a LUNis created it is spread across multiple members.  The details of thespreading are hidden from initiators connected to this storage system.The storage group exposes a single target discovery portal, no matterhow many members are being used.  When iSCSI sessions are created, eachsession is connected to an eth port on a single member.  Data to a LUNcan be sent on any iSCSI session, and if the blocks being accessed arestored on another member the I/O will be forwarded as required.  Thisforwarding is invisible to the initiator.  The storage layout is alsodynamic, and the blocks stored on disk may be moved from member tomember as needed to balance the load. This architecture simplifies the management and configuration of boththe storage group and initiators.  In a multipathing configuration, itis possible to set up multiple iSCSI sessions to use multiple networkinterfaces on both the host and target to take advantage of theincreased network bandwidth.  An initiator could use a simple roundrobin algorithm to send I/O across all paths and let the storage arraymembers forward it as necessary, but there is a performance advantage tosending data directly to the correct member. A device-mapper table already lets you map different regions of adevice onto different targets.  However in this architecture the LUN isspread with an address region size on the order of 10s of MBs, whichmeans the resulting table could have more than a million entries andconsume far too much memory. Using this device-mapper switch target we can now build a two-layerdevice hierarchy:     Upper Tier - Determine which array member the I/O should be sent to.    Lower Tier - Load balance amongst paths to a particular member. The lower tier consists of a single dm multipath device for each member.Each of these multipath devices contains the set of paths directly tothe array member in one priority group, and leverages existing pathselectors to load balance amongst these paths.  We also build anon-preferred priority group containing paths to other array members forfailover reasons. The upper tier consists of a single dm-switch device.  This device usesa bitmap to look up the location of the I/O and choose the appropriatelower tier device to route the I/O.  By using a bitmap we are able touse 4 bits for each address range in a 16 member group (which is verylarge for us).  This is a much denser representation than the dm tableb-tree can achieve. Construction Parameters=======================     <num_paths> <region_size> <num_optional_args> [<optional_args>...] [<dev_path> <offset>]+	<num_paths>	    The number of paths across which to distribute the I/O. 	<region_size>	    The number of 512-byte sectors in a region. Each region can be redirected	    to any of the available paths. 	<num_optional_args>	    The number of optional arguments. Currently, no optional arguments	    are supported and so this must be zero. 	<dev_path>	    The block device that represents a specific path to the device. 	<offset>	    The offset of the start of data on the specific <dev_path> (in units	    of 512-byte sectors). This number is added to the sector number when	    forwarding the request to the specific path. Typically it is zero. Messages======== set_region_mappings <index>:<path_nr> [<index>]:<path_nr> [<index>]:<path_nr>... Modify the region table by specifying which regions are redirected towhich paths. <index>    The region number (region size was specified in constructor parameters).    If index is omitted, the next region (previous index + 1) is used.    Expressed in hexadecimal (WITHOUT any prefix like 0x). <path_nr>    The path number in the range 0 ... (<num_paths> - 1).    Expressed in hexadecimal (WITHOUT any prefix like 0x). R<n>,<m>    This parameter allows repetitive patterns to be loaded quickly. <n> and <m>    are hexadecimal numbers. The last <n> mappings are repeated in the next <m>    slots. Status====== No status line is reported. Example======= Assume that you have volumes vg1/switch0 vg1/switch1 vg1/switch2 withthe same size. Create a switch device with 64kB region size::     dmsetup create switch --table "0 `blockdev --getsz /dev/vg1/switch0`	switch 3 128 0 /dev/vg1/switch0 0 /dev/vg1/switch1 0 /dev/vg1/switch2 0" Set mappings for the first 7 entries to point to devices switch0, switch1,switch2, switch0, switch1, switch2, switch1::     dmsetup message switch 0 set_region_mappings 0:0 :1 :2 :0 :1 :2 :1 Set repetitive mapping. This command::     dmsetup message switch 0 set_region_mappings 1000:1 :2 R2,10 is equivalent to::     dmsetup message switch 0 set_region_mappings 1000:1 :2 :1 :2 :1 :2 :1 :2 \	:1 :2 :1 :2 :1 :2 :1 :2 :1 :2