Mounting a disk image split up into two files

disk-imagemount

If I had a four gigabyte disk image, and I copied the first, say, two gigs, into a file at one path, and the remaining two gigs into a file at a different path, could I mount it as one disk image on one mount point despite half being in one file and the other half being in another file? Or if that's not possible if I had one disk image with one, say, ext4 partition and another with the same partition table and ext4 partition, would I be able to mount them on the same mount point? Methods that require FUSE will work for me.

Best Answer

You can mount two filesystems on a single mount point and there are actually a few ways of doing this and almost all live CD distros use them. But first you have to decide how you want to handle writes. If you need both filesystems to be merged and appear as one with all the files from both systems available but the writes going to only one of the filesystems then you can use overlayfs https://en.m.wikipedia.org/wiki/OverlayFS, unionfs or aufs

If however you need all of the filesystems to be read-write enabled you can have a look at mhddfs http://svn.uvw.ru/mhddfs/trunk/README

Solution

With apt-get-able tools the following kludge is possible:

mkdir mnt
xmount --in dd --out vdi disk.img mnt

mkdir mnt2
vdfuse -f mnt/disk.vdi 

mkdir mnt3
fuseext2 -o "rw" mnt2/Partition1 mnt3

Explanation

The basic idea is that fuse can be used to separate a full disk image in place into files that point to it's partitions. vdfuse does this, but is a VirtualBox tool and requires a VDI or VMDK file to work. xmount uses fuse to make a raw disk image appear as a VDI file.

Finally once the partition file is available via vdfuse, it can be mounted via an ext2/3/4 tool fuseext2.

It's ugly but it works completely in userspace.

Update

vdfuse should be able to mount a raw image without the help of xmount, but there is a bug which ignores the RAW option.

I tracked down and fixed the bug with a patch here:

https://bugs.launchpad.net/ubuntu/+source/virtualbox-ose/+bug/1019075

How to split a ddrescue disk image and how to use it again

To answer the second part of your question. How to mount a FS stored in two files (a and b) Two options I can think of:

Using device-mapper and loop devices:

losetup /dev/loop1 a
losetup /dev/loop2 b
s() { blockdev --getsize "$1"; }
dmsetup create merge << EOF
0 $(s /dev/loop1) linear /dev/loop1 0
$(s /dev/loop1) $(s /dev/loop2) linear /dev/loop2 0
EOF
mount /dev/mapper/merge /mnt

The idea being to do a linear device-mapper device which is just the concatenation of the two loop devices.

Using nbd-client + nbd-server

ln -s a part.0
ln -s b part.1
nbd-server 127.1@12345 "$PWD/part" -m
nbd-client 127.1 12345 /dev/nbd0
mount /dev/nbd0 /mnt

(easier but less efficient)

Here, we're using the "multi-part" mode of nbd-server that expects the parts to be named as part.0, part.1... Unfortunately, contrary to qemu-nbd, nbd-server/client can't work with Unix domain sockets which mean we have to have the TCP overhead and qemu-nbd doesn't have such a multi-part mode.