David Greaves wrote:

How do I interpret:
      bitmap: 0/117 pages [0KB], 1024KB chunk
in the mdstat output

what does it mean when it's, eg: 23/117

This refers to the in-memory bitmap (basically a cache of what's in the 
on-disk bitmap -- it allows bitmap operations to be more efficient).

If it's 23/117 that means there are 23 of 117 pages allocated in the 
in-memory bitmap. The pages are allocated on demand, and get freed when 
they're empty (all zeroes). The in-memory bitmap uses 16 bits for each 
bitmap chunk to count all ongoing writes to the chunk, so it's actually 
up to 16 times larger than the on-disk bitmap.

--
Paul