recovering corrupt file system

[Boylan, Ross]

Any recommendations for tools to diagnose and recover problems on an ext4 file system?

In particular:
root at jessie01:~# mount -o ro /dev/markov02/root /mnt/markov02
mount: wrong fs type, bad option, bad superblock on /dev/mapper/markov02-root,
       missing codepage or helper program, or other error

       In some cases useful info is found in syslog - try
       dmesg | tail or so.
and e2fsck says
root at jessie01:~# e2fsck /dev/markov02/root
e2fsck 1.42.12 (29-Aug-2014)
/dev/markov02/root: recovering journal
Superblock needs_recovery flag is clear, but journal has data.

markov02/root is an LVM volume, built on partitions from 2 disks in a virtual machine.  The initial symptom was that the VM the disks were in originally would only get as far as busybox when it started.  However, I think the filesystem was OK even after that, since it was visible in busybox and in another VM.  I think virt-manager might have overwritten on of the disks because I left "allocate entire disk now" checked when I moved one of the disks between machines.

I'm making copies of the virtual disks now.
Ross Boylan