mdadm checking RAID 5 array after each restart

Solution 1:

UPDATE 18/06/2021

for svc in mdcheck_start.timer mdcheck_continue.timer; do sudo systemctl stop ${svc}; sudo systemctl disable ${svc}; done

Taken from: https://a20.net/bert/2020/11/02/disable-periodic-raid-check-on-ubuntu-20-04-systemd/

UPDATE 04/05/2021 begin.
My previous answer seems to be not helpful.
The check occurred again despite the change in /etc/default/mdadm.

I've found something else to investigate.
mdcheck_start.service
mdcheck_start.timer
mdcheck_continue.service
mdcheck_continue.timer
/etc/systemd/system/mdmonitor.service.wants/mdcheck_start.timer
/etc/systemd/system/mdmonitor.service.wants/mdcheck_continue.timer
/etc/systemd/system/mdmonitor.service.wants/mdmonitor-oneshot.timer

systemctl status mdcheck_start.service
● mdcheck_start.service - MD array scrubbing
     Loaded: loaded (/lib/systemd/system/mdcheck_start.service; static; vendor preset: enabled)
     Active: inactive (dead)
TriggeredBy: ● mdcheck_start.timer

systemctl status mdcheck_start.timer
● mdcheck_start.timer - MD array scrubbing
     Loaded: loaded (/lib/systemd/system/mdcheck_start.timer; enabled; vendor preset: enabled)
     Active: active (waiting) since Sun 2021-05-02 19:40:50 CEST; 1 day 14h ago
    Trigger: Sun 2021-06-06 22:36:42 CEST; 1 months 3 days left
   Triggers: ● mdcheck_start.service

May 02 19:40:50 xxx systemd[1]: Started MD array scrubbing.

systemctl status mdcheck_continue.service
● mdcheck_continue.service - MD array scrubbing - continuation
     Loaded: loaded (/lib/systemd/system/mdcheck_continue.service; static; vendor preset: enabled)
     Active: inactive (dead)
TriggeredBy: ● mdcheck_continue.timer
  Condition: start condition failed at Tue 2021-05-04 06:38:39 CEST; 3h 26min ago
             └─ ConditionPathExistsGlob=/var/lib/mdcheck/MD_UUID_* was not met

systemctl status mdcheck_continue.timer
● mdcheck_continue.timer - MD array scrubbing - continuation
     Loaded: loaded (/lib/systemd/system/mdcheck_continue.timer; enabled; vendor preset: enabled)
     Active: active (waiting) since Sun 2021-05-02 19:40:50 CEST; 1 day 14h ago
    Trigger: Wed 2021-05-05 00:35:53 CEST; 14h left
   Triggers: ● mdcheck_continue.service

May 02 19:40:50 xxx systemd[1]: Started MD array scrubbing - continuation.

sudo cat /etc/systemd/system/mdmonitor.service.wants/mdcheck_start.timer
#  This file is part of mdadm.
#
#  mdadm is free software; you can redistribute it and/or modify it
#  under the terms of the GNU General Public License as published by
#  the Free Software Foundation; either version 2 of the License, or
#  (at your option) any later version.

[Unit]
Description=MD array scrubbing

[Timer]
OnCalendar=Sun *-*-1..7 1:00:00
RandomizedDelaySec=24h
Persistent=true

[Install]
WantedBy=mdmonitor.service
Also=mdcheck_continue.timer

sudo cat /etc/systemd/system/mdmonitor.service.wants/mdcheck_continue.timer 
#  This file is part of mdadm.
#
#  mdadm is free software; you can redistribute it and/or modify it
#  under the terms of the GNU General Public License as published by
#  the Free Software Foundation; either version 2 of the License, or
#  (at your option) any later version.

[Unit]
Description=MD array scrubbing - continuation

[Timer]
OnCalendar=daily
RandomizedDelaySec=12h
Persistent=true

[Install]
WantedBy=mdmonitor.service

sudo cat /etc/systemd/system/mdmonitor.service.wants/mdmonitor-oneshot.timer 
#  This file is part of mdadm.
#
#  mdadm is free software; you can redistribute it and/or modify it
#  under the terms of the GNU General Public License as published by
#  the Free Software Foundation; either version 2 of the License, or
#  (at your option) any later version.

[Unit]
Description=Reminder for degraded MD arrays

[Timer]
OnCalendar=daily
RandomizedDelaySec=24h
Persistent=true

[Install]
WantedBy= mdmonitor.service

UPDATE 04/05/2021 end.

Try sudo dpkg-reconfigure mdadm.

Please note that I am not sure if the above tip would help.
I have the same problem with my raid5 on 20.04.

First I've tried editing the /etc/default/mdadm manually and changed AUTOCHECK=true into AUTOCHECK=false. It did not help.

Today I've used dpkg-reconfigure mdadm. Now the /etc/default/mdadm file looks the same (AUTOCHECK=false) but as a part of dpkg-reconfigure mdadm there was also an update-initramfs call. I hope it will help.

... update-initramfs: deferring update (trigger activated) ...

The extended log:

sudo dpkg-reconfigure mdadm
update-initramfs: deferring update (trigger activated)
Sourcing file `/etc/default/grub'
Sourcing file `/etc/default/grub.d/50-curtin-settings.cfg'
Sourcing file `/etc/default/grub.d/init-select.cfg'
Generating grub configuration file ...
File descriptor 3 (pipe:[897059]) leaked on vgs invocation. Parent PID 655310: /usr/sbin/grub-probe
File descriptor 3 (pipe:[897059]) leaked on vgs invocation. Parent PID 655310: /usr/sbin/grub-probe
Found linux image: /boot/vmlinuz-5.4.0-72-generic
Found initrd image: /boot/initrd.img-5.4.0-72-generic
Found linux image: /boot/vmlinuz-5.4.0-71-generic
Found initrd image: /boot/initrd.img-5.4.0-71-generic
Found linux image: /boot/vmlinuz-5.4.0-70-generic
Found initrd image: /boot/initrd.img-5.4.0-70-generic
File descriptor 3 (pipe:[897059]) leaked on vgs invocation. Parent PID 655841: /usr/sbin/grub-probe
File descriptor 3 (pipe:[897059]) leaked on vgs invocation. Parent PID 655841: /usr/sbin/grub-probe
done
Processing triggers for initramfs-tools (0.136ubuntu6.4) ...
update-initramfs: Generating /boot/initrd.img-5.4.0-72-generic

The full /etc/default/mdadm file:

cat /etc/default/mdadm 
# mdadm Debian configuration
#
# You can run 'dpkg-reconfigure mdadm' to modify the values in this file, if
# you want. You can also change the values here and changes will be preserved.
# Do note that only the values are preserved; the rest of the file is
# rewritten.
#

# AUTOCHECK:
#   should mdadm run periodic redundancy checks over your arrays? See
#   /etc/cron.d/mdadm.
AUTOCHECK=false

# AUTOSCAN:
#   should mdadm check once a day for degraded arrays? See
#   /etc/cron.daily/mdadm.
AUTOSCAN=true

# START_DAEMON:
#   should mdadm start the MD monitoring daemon during boot?
START_DAEMON=true

# DAEMON_OPTIONS:
#   additional options to pass to the daemon.
DAEMON_OPTIONS="--syslog"

# VERBOSE:
#   if this variable is set to true, mdadm will be a little more verbose e.g.
#   when creating the initramfs.
VERBOSE=false