Raid rebuild performance impact

In my experience the system stays quite usable during the rebuild. Of course, this depends on your hardware and workload. Is it mostly read or is there usually lots of write activity? On the top of my hat I would say the rebuild slows things down around 20-30 percent.

You can tune the rebuild rate with dev.raid.speed_limit_{min,max} sysctl tunables. By default the minimum rebuild rate is 1000 kilobytes per second and the maximum 100 000 kilobytes per second. To tune them, use for example

sysctl -w dev.raid.speed_limit_max=10000

or to make changes permanent, use

echo dev.raid.speed_limit_max=10000 >>/etc/sysctl.conf and activate the changes with

sysctl -p

It can impact it a lot of rebuild priority is high, 'reasonable' is subjective, it'll do best-efforts if set to high whereas if rebuild is set to low then it's the rebuild that gets best-efforts.

As for changing these priorities look at /proc/sys/dev/raid/speed_limit_max & /proc/sys/dev/raid/speed_limit_min.