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      1 Let's say we have an LV, made up of three segments of different PV's,
      2 I've also added in the device major:minor as this will be useful
      3 later:
      4 
      5 +-----------------------------+
      6 |  PV1     |   PV2   |   PV3  | 254:3
      7 +----------+---------+--------+
      8 
      9 
     10 Now our hero decides to PV move PV2 to PV4:
     11 
     12 1. Suspend our LV (254:3), this starts queueing all io, and flushes
     13    all pending io.  Once the suspend has completed we are free to change
     14    the mapping table.
     15 
     16 2. Set up *another* (254:4) device with the mapping table of our LV.
     17 
     18 3. Load a new mapping table into (254:3) that has identity targets for
     19    parts that aren't moving, and a mirror target for parts that are.
     20 
     21 4. Unsuspend (254:3)
     22 
     23 So now we have:
     24                            destination of copy
     25                +--------------------->--------------+
     26                |                                    |
     27 +-----------------------------+               + -----------+
     28 | Identity | mirror  | Ident. | 254:3         |    PV4     |
     29 +----------+---------+--------+               +------------+
     30      |         |         |
     31      \/        \/        \/
     32 +-----------------------------+
     33 |  PV1     |   PV2   |   PV3  | 254:4
     34 +----------+---------+--------+
     35 
     36 Any writes to segment2 of the LV get intercepted by the mirror target
     37 who checks that that chunk has been copied to the new destination, if
     38 it hasn't it queues the initial copy and defers the current io until
     39 it has finished.  Then the current io is written to *both* PV2 and the
     40 PV4.
     41 
     42 5. When the copying has completed 254:3 is suspended/pending flushed.
     43 
     44 6. 254:4 is taken down
     45 
     46 7. metadata is updated on disk
     47 
     48 8. 254:3 has new mapping table loaded:
     49 
     50 +-----------------------------+
     51 |  PV1     |   PV4   |   PV3  | 254:3
     52 +----------+---------+--------+
     53