Ubuntu – How to mount a compressed disk image

backupdddisk-imagemountpartitioning

If I make a disk image and compress it with gzip/xz/etc is there a way to mount it directly without first uncompressing it?

Say I've used 

sudo dd if=/dev/sdc1 | gzip -9 > image1.dd.gz

how can I mount the original image, without creating an uncompressed copy first?

Or I've used

sudo dd if=/dev/sdc | gzip -9 > wholedisk.dd.gz

and the disk has multiple partitions, would that make it any harder?

With an uncompressed image of a whole disk then using kpartx or newer versions of losetup with it's -P flag should create a loop for each partition.

But is there a way to mount/losetup/read the compressed image?

If it won't work for gzip/xz, is there any compression method this would work for?

Note on duplicate Q

The currently suggested duplicate

DOES NOT USE COMPRESSION, and IS NOT A DUPLICATE.

mount will not mount a compressed image by itself.

Best Answer

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.

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