On Fri, 11 Jun 2010 13:10:23 +0200
John Hendrikx [off-list ref] wrote:

quoted

I would be really interested to find out if my assumption of small numbers
of bits set is valid.   You can find out the number of bits set at any
instant with  "mdadm -X" run on some component of the array.
  

I was interested as well, so I ran this command:

 > mdadm -X /dev/md2

and this is the result(??):

        Filename : /dev/md2
           Magic : d747992c
mdadm: invalid bitmap magic 0xd747992c, the bitmap file appears to be 
corrupted
         Version : 1132474982
mdadm: unknown bitmap version 1132474982, either the bitmap file is 
corrupted or you need to upgrade your tools

I stumbled in the same way initially, but then re-read more closely and
noticed that Neil said to run it "on some component of the array",
e.g. /dev/sdxN, not the array itself -- and that way it worked fine. However as
my array sees almost no write load at the moment, I have no useful results to
report.

-- 
With respect,
Roman