PFC wrote:

    When rebuilding md1, it does not realize accesses to md0 wait for
the  same disks. Thus reconstruction of md1 runs happily at full speed,
and the  machine is dog slow, because the OS and everything is on md0.
    (I cat /dev/zero to a file on md1 to slow the rebuild so it would
let me  start a web browser so I don't get bored to death)

echo "10000" > /proc/sys/dev/raid/speed-limit-max (or similar?)

You can do that in /etc/rc.local or something to make sure it sticks,
then you'll be able to use your machine while any array rebuilds.

I guess the feature you're asking for is for md to guess that accessing
any partition component on a disk that has a partition being rebuilt
should throttle the rebuild, right?

Can that heuristic be successful at all times? I think it might.

Does md have enough information to do that? I don't know...

-Mike