Adding a component to an array which has changed size

[Chris Webb <chris@arachsys.com>]

The amount of space reserved in each component for an internal bitmap
depends on the size of the component. If I have (say) a raid5 arrray of size
50GB that used to be a raid5 array of size 1GB, and I fail one of the slots
and add in a new replacement, the new slot needs to be bigger than all the
existing ones because a larger space is reserved by mdadm for the internal
bitmap.

  3# mdadm --version && uname -sv
  mdadm - v2.6.7 - 6th June 2008
  Linux 2.6.27.7 #2 SMP Wed Dec 3 12:00:03 GMT 2008

  3# mdadm --fail /dev/md127 /dev/mapper/slot.51000.1 
  mdadm: set /dev/mapper/slot.51000.1 faulty in /dev/md127
  3# mdadm --remove /dev/md127 /dev/mapper/slot.51000.1 
  mdadm: hot removed /dev/mapper/slot.51000.1
 
  3# # wipe superblock so this is a new slot rather than a re-add...
  3# dd if=/dev/zero of=/dev/mapper/slot.51000.1 bs=1M count=1
  1+0 records in
  1+0 records out
  3# mdadm --add /dev/md127 /dev/mapper/slot.51000.1
  mdadm: /dev/mapper/slot.51000.1 not large enough to join array

  3# mdadm --examine /dev/mapper/slot.51000.0 
  /dev/mapper/slot.51000.0:
            Magic : a92b4efc
          Version : 1.1
      Feature Map : 0x0
       Array UUID : 61a6b9ac:4b38148f:c0c0837e:1c34783f
             Name : 3:shelf.51000
    Creation Time : Thu Dec  4 11:21:49 2008
       Raid Level : raid1
     Raid Devices : 2

   Avail Dev Size : 104857584 (50.00 GiB 53.69 GB)
       Array Size : 104857584 (50.00 GiB 53.69 GB)
      Data Offset : 16 sectors
     Super Offset : 0 sectors
            State : active
      Device UUID : f8c6dcec:add24452:267858ec:617382cd

      Update Time : Thu Dec  4 12:07:22 2008
         Checksum : 1e90683 - correct
           Events : 19


      Array Slot : 0 (0, failed)
     Array State : U_ 1 failed


Is there any way either to persuade mdadm to take the bitmap size for from the
existing components, or ideally to override the bitmap size when creating
arrays and adding components so I can just choose a constant size for all our
automatically generated arrays, given that the arrays will vary in size over
their lifetimes?

Best wishes,

Chris.