[mapfs-user] how to make a read-only fs writable?

[Albrecht G. <alb...@un...>]

Hi,

reading the announcement I tought it should be possible to make
a readonly mounted path (eg. part of a mounted cdrom) writable by adding
a COW layer on top of it (as other layer filesystems like unionfs,
minifo_fs, mulafs do ...).

I tried the following

cd /tmp
dd if=3D/dev/zero of=3D/tmp/ro.img bs=3D1M count=3D2
mkfs.ext2 -F /tmp/ro.img
mkdir /tmp/ro-tree
mount -o loop /tmp/ro.img /tmp/ro-tree
cat > /tmp/ro-tree/test.txt <<EOT
test
EOT
mkdir /tmp/ro-tree/dir
cp /tmp/ro-tree/test.txt /tmp/ro-tree/dir
sync
umount /tmp/ro-tree
mount -o loop,ro /tmp/ro.img /tmp/ro-tree

Now I have i readonly filesystem mounted at /tmp/ro-tree with some files
in it:

# ls /tmp/ro-tree
dir  lost+found  test.txt
# touch /tmp/ro-tree/xxx
touch: cannot touch `/tmp/ro-tree/xxx': Read-only file system

Now I want to mount this path with mapfs to /tmp/rw-tree where I would
like to see the files from /tmp/ro-tree an would like to be able to
modify/add/delete files in the /tmp/rw-tree (virtually including the
files from /tmp/ro-tree).

The rw directories:

mkdir /tmp/rw-tree
mkdir -p /tmp/mapfs/view
mkdir -p /tmp/mapfs/overlay

I tried

mount -t mapfs -o \
root=3D/tmp/ro-tree,\
view=3D/tmp/mapfs/view,\
overlay=3D/tmp/mapfs/overlay,\
cow_base=3D/overlay none /tmp/rw-tree

Does not work, the test.txt files from /tmp/ro-path are not visible
in /tmp/rw-tree (Tough I can create and edit files in /tmp/rw-tree
resulting in new files in the view and overlay location)

# ls -l /tmp/rw-tree
total 0


I even tried to move the (rw) view and overlay directories with mount
--bind into the ro-tree:

umount /tmp/rw-tree
umount /tmp/ro-tree
mount -o loop /tmp/ro.img /tmp/ro-tree
mkdir /tmp/ro-tree/mapfs
sync
umount /tmp/ro-tree
mount -o loop,ro /tmp/ro.img /tmp/ro-tree
mount --bind /tmp/mapfs /tmp/ro-tree/mapfs

and now:

mount -t mapfs -o \
root=3D/tmp/ro-tree,\
view=3D/tmp/ro-tree/mapfs/view,\
overlay=3D/tmp/ro-tree/mapfs/overlay,\
cow_base=3D/mapfs/overlay none /tmp/rw-tree

Still /tmp/rw-tree is empty.

Do I miss something?

Thanks

Albrecht


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// Albrecht Gebhardt          Tel.: (++43 463) 2700/3118
// Institut fuer Mathematik   Fax : (++43 463) 2700/3198
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