diff options
Diffstat (limited to 'Documentation/device-mapper/switch.txt')
| -rw-r--r-- | Documentation/device-mapper/switch.txt | 138 |
1 files changed, 0 insertions, 138 deletions
diff --git a/Documentation/device-mapper/switch.txt b/Documentation/device-mapper/switch.txt deleted file mode 100644 index 5bd4831db4a8..000000000000 --- a/Documentation/device-mapper/switch.txt +++ /dev/null @@ -1,138 +0,0 @@ -dm-switch -========= - -The device-mapper switch target creates a device that supports an -arbitrary mapping of fixed-size regions of I/O across a fixed set of -paths. The path used for any specific region can be switched -dynamically by sending the target a message. - -It maps I/O to underlying block devices efficiently when there is a large -number of fixed-sized address regions but there is no simple pattern -that would allow for a compact representation of the mapping such as -dm-stripe. - -Background ----------- - -Dell EqualLogic and some other iSCSI storage arrays use a distributed -frameless architecture. In this architecture, the storage group -consists of a number of distinct storage arrays ("members") each having -independent controllers, disk storage and network adapters. When a LUN -is created it is spread across multiple members. The details of the -spreading are hidden from initiators connected to this storage system. -The storage group exposes a single target discovery portal, no matter -how many members are being used. When iSCSI sessions are created, each -session is connected to an eth port on a single member. Data to a LUN -can be sent on any iSCSI session, and if the blocks being accessed are -stored on another member the I/O will be forwarded as required. This -forwarding is invisible to the initiator. The storage layout is also -dynamic, and the blocks stored on disk may be moved from member to -member as needed to balance the load. - -This architecture simplifies the management and configuration of both -the storage group and initiators. In a multipathing configuration, it -is possible to set up multiple iSCSI sessions to use multiple network -interfaces on both the host and target to take advantage of the -increased network bandwidth. An initiator could use a simple round -robin algorithm to send I/O across all paths and let the storage array -members forward it as necessary, but there is a performance advantage to -sending data directly to the correct member. - -A device-mapper table already lets you map different regions of a -device onto different targets. However in this architecture the LUN is -spread with an address region size on the order of 10s of MBs, which -means the resulting table could have more than a million entries and -consume far too much memory. - -Using this device-mapper switch target we can now build a two-layer -device 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 to -the array member in one priority group, and leverages existing path -selectors to load balance amongst these paths. We also build a -non-preferred priority group containing paths to other array members for -failover reasons. - -The upper tier consists of a single dm-switch device. This device uses -a bitmap to look up the location of the I/O and choose the appropriate -lower tier device to route the I/O. By using a bitmap we are able to -use 4 bits for each address range in a 16 member group (which is very -large for us). This is a much denser representation than the dm table -b-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 to -which 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 with -the 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 - |