how to know execution time of a background task in Linux

Question

I am working with software raid in Linux. When I start the recovery of a raid array after a disk failure this task happens in background. /proc/mdstat should show the estimated time of recovery but it is not correct. In my case /proc/mdstat was showing estimated 15 hours but the recovery was done in around 10 hours. Is there a way to know exact time of execution of recovery daemon?

Answer 1

You can run a simple script in the background that checks for the "rebuilding status" and dumps the time it's been waiting:

tick=$(date +%s)
while [ -n "$(cat /proc/mdstat | grep 'rebuilding')" ]; do
    sleep 10
done
tock=$(date +%s)
echo $(($tock-$tick))

mdadm also has a --monitor option that will sendmail when events are raised. I've never used it, but it may be worth looking into since a completed rebuild may qualify as a noteworthy event.