On Wed, 3 Feb 2010 20:12:32 +0100 (CET)
Mikael Abrahamsson [off-list ref] wrote:

Any reason this doesn't work with a bitmap, and if it doesn't, then 
perhaps this should be documented somewhere or at least mdadm should throw 
an error message?

It doesn't work with a bitmap because md is not able to resize the bitmap
which might be needed when resizing an array.
The next release of mdadm should provide a more helpful message.
I'll look in to ways of getting md to resize the bitmap.

NeilBrown