On Sat, 8 Oct 2016 15:54:26 +0300
Dark Penguin [off-list ref] wrote:

So if I were to place it on a tmpfs, I could eliminate this problem only 
at the expense of being unable to re-add drives after a reboot, right?.. 

If you don't need that ability, you can just remove bitmap entirely, it's not
mandatory. Run:

  mdadm --grow --bitmap=none /dev/mdX

However I'd say being able to re-add drives is very useful, so first consider
switching to a higher bitmap granularity, 

  mdadm --grow --bitmap=none /dev/mdX
  mdadm --grow --bitmap=internal --bitmap-chunk=131072 /dev/mdX

(or even 262144, 524288) as that will reduce the performance impact of the
bitmap.

-- 
With respect,
Roman