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