grub2 - Mounting the persistence partition (casper-rw) read-only

I'm trying to create a fresh ISO image for use on a USB stick. I've partitioned my USB stick into 2 partitions, one vfat and one ext4. The vfat partition has the ISO and GRUB2 installed to it, such that GRUB2 boots the ISO.

The ext4 partition has the volume label casper-rw and is used as the persistent data store for the Live USB. I've modified the grub.cfg such that I get two options at boot, one to boot the ISO in persistent mode and one to boot normally.

However, when booting in "normal" mode, the casper-rw partition is mounted as a normal drive and changes made in persistent mode don't stay.

What I would like is the casper-rw partition to be mounted overlaid on the / filesystem, but when I ask Ubuntu to boot normally (i.e., without the persistent option in GRUB2), I'd like that partition to be mounted read-only so that changes made in the persistent mode persist.

What is the best way to achieve this?

What i did is I modify casper script inside initrd.lz.

The script will mount casper-rw as read-only and then on overlay it on top of /.
It will then mount tmpfs on top of the overlay.

I use mint btw, not sure about ubuntu, hope it works for you.

Here are more details:

1. Extract initrd using instruction from here.



2. Edit scripts/casper, modify the setup_unionfs() function.
   You find the modified function here.

   
   
   

   ...
   p_ro_mount=""
   if [ -n "${PERSISTENT}" ]; then
         cowprobe=$(find_cow_device "${root_persistence}")
         if [ -b "${cowprobe}" ]; then
   
               p_ro_device=${cowprobe}
               p_ro_fstype=$(get_fstype "${cowprobe}")
               p_ro_mountopt="ro,noatime"
               p_ro_mount="/persistent"
               mkdir -p $p_ro_mount
               mount -t ${p_ro_fstype} -o ${p_ro_mountopt} \   
                 ${p_ro_device} \
                 ${p_ro_mount} || panic "Can not mount"
   ...
   case ${UNIONFS} in
   
   ...
      overlayfs)
          mounts=""
          for mount in /cow $p_ro_mount $rofslist
   ...
   




3. Replace initrd.lz, or in my case, I just created a new one.

   
   
   

   find . | cpio --quiet -o -H newc | lzma -7 > ../ro-initrd.lz
   





4. Add more boot option to syslinux.cfg

   
   
   

   append initrd=/casper/ro-initrd.lz file=/cdrom/preseed/linuxmint.seed boot=casper  splash -- persistent
   

Note: I use Mint 16 (x86) and unetbootin.