On Sat, 02 Feb 2013 15:52:12 -0600
Stan Hoeppner [off-list ref] wrote:

So under heavy read load, in this case we'd probably want the slow 3TB drive
to contribute to the workload.

No, it should behave as documented.

       -W, --write-mostly
              subsequent devices listed in a --build, --create, or --add  com‐
              mand will be flagged as 'write-mostly'.  This is valid for RAID1
              only and means that the 'md'  driver  will  avoid  reading  from
              these devices if at all possible. 

if at all possible = if there is any other mirror still alive, do not read from
this one.

BTW I also run an SSD+HDD array, and after a brief testing today I did notice
some reads from the HDD on reading from the array showing up in iostat. So I
can confirm the bug.

-- 
With respect,
Roman