How to mount a compressed disk image?

You can use squashfs to compress disk images and then mount them.

Create the disk image

If you haven't got a disk image yet use dd to create one:

dd if=/dev/sda1 of=/tmp/sda1.img bs=4M

Compress the image with squashfs

Install squashfs:

apt-get install squashfs-tools

Compress the image:

mksquashfs /tmp/sda1.img /tmp/sda1.squash.img

Or Stream the compression (don't need a temporary dd file)

mkdir empty-dir
mksquashfs empty-dir squash.img -p 'sda_backup.img f 444 root root dd if=/dev/sda bs=4M'

Compliments to terminator14 at UbuntuForums.org. Definitions/Explanations:

• empty-dir - "source" dir. Basically in our case, just an empty dir to satisfy mksquashfs' input arg format
• squash.img - the destination and filename of the output squashfs file
• sda_backup.img - the name of the dd backup INSIDE the squashfs file
• f - specifies that sda_backup.img is a regular file (as opposed to a directory, block device, or char device)
• 444 - permissions of the sda_backup.img file inside the squashfs image
• root root - UID and GID for the sda_backup.img file inside the squashfs image. Can be specified by decimal numbers, or by name
• dd if=/dev/sda bs=4M - the dd command used to read the device we want backed up

Mount the image

First mount the squashfs image:

mkdir /mnt/squash
mount /tmp/sda1.squash.img /mnt/squash

This will present the un-compressed disk image for you to mount:

mkdir /mnt/sda1
mount /mnt/squash/sda1.img /mnt/sda1

Or if it's a full drive image (partitioned) you could use losetup to attach the dd image to a loop device (possibly optional) and then kpartx -a or partprobe to find & separate the partitions to separate devices, or even vgscan / vgchange -ay if there's LVM.

I was able to get an NBD server to "serve up" an un-compressed version of a .gz or .xz file. I'm not sure if it has to read the entire compressed file very often (only tested so far with small files in ram) but at least it works without having to extract a whole un-compressed copy.

Viewing a gz / xz Compressed dd image "on-the-fly"

1. Install nbdkit & client

   apt install nbdkit nbd-client
   

2. Create demo "dd" style file - an empty 50M file with an ext filesystem & zip it (Optional for testing only)

   fallocate -l 50M 50m
   mkfs.ext3 -v 50m
   

   Then compress with either gzip or xz

   gzip -vk9 50m
   xz -v9k 50m
   

   Note: An xz option of --block-size=16MiB should give better random access performance, but at the expense of less compression (maybe 1%?). See the nbdkit's xz plugin description.

3. Run server on COMPRESSED image

   nbdkit -v --no-fork -i 127.0.0.1 /usr/lib/i386-linux-gnu/nbdkit/plugins/nbdkit-gzip-plugin.so file=~/Downloads/50m.gz
   

   or for xz

   nbdkit -v --no-fork -i 127.0.0.1 /usr/lib/i386-linux-gnu/nbdkit/plugins/nbdkit-xz-plugin.so file=~/Downloads/50m.xz
   

   The --no-fork tells the server "Don't fork into the background" so seeing possible error messages & killing it with CTRL-C is a little easier.

   -v may / will show too many status messages, especially when reading.

4. Run client (in another terminal/window)

   sudo nbd-client -nofork 127.0.0.1 10809 /dev/nbd0
   

   The -nofork is similar to -no-fork above, but I could not kill the client with CTRL-C, kill <pid>, or sudo kill <pid>. Ending the server with CTRL-C stopped both server & client.

5. Observe that /dev/nbd0 is now identical to UN-COMPRESSED image

   $ sudo diff -s /dev/nbd0 ~/Downloads/50m
   Files /dev/nbd0 and /home/user/Downloads/50m are identical
   

   /dev/nbd0 can now be used just like the uncompressed image; mounted, read/copy with dd, or use kpartx, partprobe, vgscan / vgchange, etc!

_{Thanks to Cristian Ciupitu on Unix & Linux, though it took a little more digging to find the plugin & get everything going together.}