“DegradedArray event”

fdisk is the wrong tool for disks >2TB. Use parted or gdisk instead.

It appears that /dev/sdc1 and /dev/sdd1 are 2TB partitions, so that's what limits your array size. For the other disks, they have GPT so I assume they are 3TB already, but you should check.

Basically you have to stop the array, enlarge each partition to 3TB (without changing the starting offset), then start it again and follow it up with a grow:

mdadm --grow /dev/md0 --size=max

If you can't stop the array, you'll have to fail each 2TB partition individually, repartition and re-add it. This might go faster if you add a write-intent bitmap first.

mdadm --grow /dev/md0 --bitmap=internal

Then for each disk individually,

mdadm /dev/md0 --fail /dev/disk1 # check mdstat for [UUUU] first
mdadm /dev/md0 --remove /dev/disk1
parted /dev/disk -- mklabel gpt mkpart primary 1mib -1mib
mdadm /dev/md0 --re-add /dev/disk1
mdadm --wait /dev/md0 # must wait for sync

Once that's done you can remove the bitmap again (keeping it may harm performance).

mdadm --grow /dev/md0 --bitmap=none
mdadm --grow /dev/md0 --size=max

Finally do your resize2fs or whatever.

Since you've partitioned your RAID as if it was a single disk, you can ignore the RAID altogether in this case. So it's merely a problem of resizing / shifting partitions.

So for example, you could shrink the www partition, delete the swap and then shift the root partition to the left in order to grow it.

Or, if that seems to complicated and you don't strictly need separate partitions, you could merge the root partition into your www partition since that's already large enough to hold both root and www. That's kind of what I would do.

# mount stuff
mkdir /mnt/root /mnt/www
mount /dev/md0p5 /mnt/root
mount /dev/md0p2 /mnt/www

# since /mnt/www will be the new root, move www files to /var/www
mkdir -p /mnt/www/var/www
mv /mnt/www/* /mnt/var/www/

# copy the root files
rsync -avAHSX /mnt/root/. /mnt/www/.

# comment out old root partition in fstab
# change /var/www to / in fstab

# update bootloader and reboot

This approach also has the advantage that if anything goes wrong, the original root partition is still intact, so you can revert the operation.

Once everything is working fine with the merged root+www partition, you can delete the old root partition and grow it to the full disk size.

Or you could decide that you want to stick with separate partitions after all and move the www files to the old root partition, if you think that's going to be large enough for your www in the foreseeable future.

Or you could shrink the www partition to make room for a new one.

Endless possibilities...