Comment 22 for bug 1852747

I installed the update on 4 basically identical systems (note to self: hostnames starting with g, k, r, w):

I enabled -proposed and installed the package:

sudo vi /etc/apt/sources.list.d/ubuntu-proposed.list
sudo apt update
sudo apt install mdadm=4.1-5ubuntu1.1

I tested the scrub on one system (hostname starts with k):

# In another terminal:
watch cat /proc/mdstat

sudo systemctl start mdcheck_start.service &
# This started the scrub.

In ~20 minutes, the scrub completed and the service stopped ~1 min thereafter. (This makes sense given the "sleep 120" that the script uses.)

Logs:
2020-09-24T12:28:56.517769-05:00 k... root: mdcheck start checking /dev/md0
2020-09-24T12:50:56.665042-05:00 k... root: mdcheck finished checking /dev/md0

I tested the continue script on one other system (hostname starts with r):

# I changed the time from 6 hours to 3 minutes with a drop-in unit file.
sudo mkdir /etc/systemd/system/mdcheck_start.service.d
sudo vi /etc/systemd/system/mdcheck_start.service.d/time.conf
sudo systemctl daemon-reload

# In another terminal:
watch cat /proc/mdstat

sudo systemctl start mdcheck_start.service &
# This started the scrub.

watch systemctl status mdcheck_start.service
# Again, the script uses a "sleep 120" (two minutes), so at the 4 minute mark
# the service stopped, as did the scrub.

sudo systemctl start mdcheck_continue.service &
watch systemctl status mdcheck_start.service
# The scrub started where it left off.
# The time on this was still the default of 6 hours.
# After another ~18 minutes, the scrub completed.

sudo rm /etc/systemd/system/mdcheck_start.service.d/time.conf
sudo rmdir /etc/systemd/system/mdcheck_start.service.d
sudo systemctl daemon-reload

sudo systemctl start mdcheck_start.service &
# This started the scrub.

watch systemctl status mdcheck_start.service
# After ~20 minutes, the scrub completed and the service stopped.

Logs:
2020-09-24T12:14:56.204254-05:00 r... root: mdcheck start checking /dev/md0
2020-09-24T12:17:37.912431-05:00 r... root: mdcheck start checking /dev/md0
2020-09-24T12:21:38.282462-05:00 r... root: pause checking /dev/md0 at 95207168
2020-09-24T12:21:50.636301-05:00 r... root: mdcheck continue checking /dev/md0 from 95207168
2020-09-24T12:39:50.737671-05:00 r... root: mdcheck finished checking /dev/md0
2020-09-24T12:41:03.127050-05:00 r... root: mdcheck start checking /dev/md0
2020-09-24T13:03:03.243179-05:00 r... root: mdcheck finished checking /dev/md0

I have NOT marked this verification-done, as I believe you wanted to see a "natural" start of the service on October 4 before calling the testing complete. I will leave these systems as is. That will give us 4 examples of it, two of which were not touched in any special way at all (other than installing the update from -proposed, of course). However, if you want to mark this verification-done now, or ask me to do it, I am not opposed to that either.