Is it safe to interrupt a mdadm --grow operation?
Solution 1:
On the first part you may be a victim of MD's default slow rebuild speed.
A good article covering the options is at:
http://www.cyberciti.biz/tips/linux-raid-increase-resync-rebuild-speed.html
But in short:
sysctl -w dev.raid.speed_limit_min=1000
sysctl -w dev.raid.speed_limit_max=100000
To guarantee between 1 and 100MB available for rebuilds, if the server is active upping the min is a good way to speed things up, but at the cost of some responsiveness.