Read the wiki - the section on badblocks will be - enlightening - shall
we say.

https://raid.wiki.kernel.org/index.php/The_Badblocks_controversy

Yeah, I had read that already. Seems like a strange limbo state to be
in with no clear way of fixing bad blocks....

Anyway, I think the bad blocks may be related to the problem I'm
having now. At first everything looks good:

bill@bill-desk:~$ cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4] [linear] [multipath] [raid0]
[raid1] [raid10]
md0 : active raid5 sdk1[1] sdm1[4] sdj1[0] sdl1[2]
      23441679360 blocks super 1.2 level 5, 64k chunk, algorithm 2 [4/4] [UUUU]
      bitmap: 0/59 pages [0KB], 65536KB chunk
unused devices: <none>

bill@bill-desk:~$ sudo fsck /dev/md0
fsck from util-linux 2.34
e2fsck 1.45.3 (14-Jul-2019)
/dev/md0: clean, 11274/366276608 files, 5285497435/5860419840 blocks

bill@bill-desk:~$ sudo lsblk -f
NAME    FSTYPE            LABEL       UUID
    FSAVAIL FSUSE% MOUNTPOINT
.
.
.
sdj
└─sdj1  linux_raid_member bill-desk:0
06ad8de5-3a7a-15ad-8811-6f44fcdee150
  └─md0 ext4
ceef50e9-afdd-4903-899d-1ad05a0780e0
sdk
└─sdk1  linux_raid_member bill-desk:0
06ad8de5-3a7a-15ad-8811-6f44fcdee150
  └─md0 ext4
ceef50e9-afdd-4903-899d-1ad05a0780e0
sdl
└─sdl1  linux_raid_member bill-desk:0
06ad8de5-3a7a-15ad-8811-6f44fcdee150
  └─md0 ext4
ceef50e9-afdd-4903-899d-1ad05a0780e0
sdm
└─sdm1  linux_raid_member bill-desk:0
06ad8de5-3a7a-15ad-8811-6f44fcdee150
  └─md0 ext4                          ceef50e9-afdd-4903-899d-1ad05a0780e0

But then I get confused about the UUIDs. I'm trying to automount the
array using fstab (no unusual settings in there, just defaults), but
I'm not sure which of the two UUIDs above to use. So I look at mdadm
for help:

bill@bill-desk:~$ sudo mdadm --examine --scan
ARRAY /dev/md/0  metadata=1.2 UUID=06ad8de5:3a7a15ad:88116f44:fcdee150
name=bill-desk:0

However, if I use this UUID starting with "06ad", then I get an error:

bill@bill-desk:~$ sudo mount -all
mount: /media/bill/STUFF: mount(2) system call failed: Structure needs cleaning.

But I don't know how to clean it if fsck says it's OK.

On the other hand, if I use the UUID above starting with "ceef", then
it mounts and everything seems OK.

Basically, I don't understand why lsblk lists two UUIDs for the array,
and why mdadm gives the wrong one in terms of mounting. This is where
I was confused before about the UUID changing. Any insight here?

Cheers,
Bill