Doing linux upgrade sometimes is quite dangous, especially from very old kernel to latest,
for example, from 3.x to 5.x, or even from 4.x to 5.x.
the mdadm soft raid arary may not work correctly after upgrade, it may shows:
"Invalid Argument" / "does not have a valid v1.2 superblock"
first we should make sure the array is still correct, by running:
mdadm --examine /dev/sd[abcd]1
make sure the disks in array are all valid.
sencod we could try if the array create in old kernel are using different layout, by running:
mdadm --stop /dev/md0
mdadm --assemble -U layout-alternate /dev/md0 /dev/sda1 /dev/sda2
https://wiki.ubuntu.com/Kernel/Raid0LayoutMigration
if sencond step is not working, last step we could try will be using “–assume-clean” to recreate the array:
mdadm -v --create /dev/md0 --readonly --bitmap=none --chunk=4M --level=6 --raid-device=6 --assume-clean --data-offset 99328 /dev/sdd2 /dev/sde2 /dev/sdg2 /dev/sdc2 /dev/sdb2 /dev/sdf2
be sure to using correct raid level/chunk size/data-offset from the output of the first step.
https://www.spinics.net/lists/raid/msg50760.html