I like to create an image backup the first time I'm backing up a system. After this first time I use rsync to do incremental backups.

My usual image backup is as follows:

  • Mount and zero out the empty space:

    dd if=/dev/zero of=temp.dd bs=1M
    rm temp.dd
  • Umount and dd the drive while compressing it

    dd if=/dev/hda conv=sync,noerror bs=64K | gzip -c  > /mnt/sda1/hda.ddimg.gz
  • To put the system back to normal, I will usually do a

    gzip -dc /mnt/sda1/hda.img.gz | dd of=/dev/hda conv=sync,noerror bs=64K

This is really straightforward and allows me to save the 'whole drive' but really just save the used space.

Here is the problem. Lets say I do the above but not on a clean system and don't get the rsync backups going soon enough and there are files that I want to access that are on the image. Let's say I don't have the storage space to actually unzip and dd the image to a drive but want to mount the image to get individual files off of it.... Is this possible?

Normally, one wouldn't compress the dd image, which will allow you to just mount the image using -o loop... but this isn't my case...

Any suggestions for mounting the compressed img on the fly?

Would using AVFS to 'mount' the gz file then mounting the internal dd.img work (I don't think so... but would need verification...)?

  • You should use SquashFS for this kind of things. It also de-dupes duplicated files.
    – Avio
    Commented Oct 24, 2012 at 8:56
  • 1
    It looks like this fellow is doing what you are asking about: blogs.gnome.org/muelli/2012/10/…
    – Joshua
    Commented Jan 2, 2013 at 20:47
  • I second Avio's suggestion. The only thing squashfs doesn't archive is acls. It archives xattrs, so selinux attributes, etc. If you don't use acls, then squashfs is the way to go IMHO. I've recently had to archive "just in case" some old drives that have already been migrated to new storage, and squashfs was perfect for the job. Commented Feb 24, 2016 at 19:41
  • Also, this is possible without restrictions for .vhd(x) images. These are quite common in the Windows world and the full solution for mounting can be found here: how2shout.com/linux/…
    – Cadoiz
    Commented Jan 20, 2021 at 0:38
  • You can also consider this short anser: askubuntu.com/a/252719/830570 and this Q/A: superuser.com/a/1097391/910769
    – Cadoiz
    Commented Jan 20, 2021 at 1:02

7 Answers 7


It depends on whether the disk image is a full disk image or just a partition.

Washing the partition(s)

If the disk is in good working condition, you will get better compression if you wash the empty space on the disk with zeros. If the disk is failing, skip this step.

If you're imaging an entire disk, then you will want to wash each of the partitions on the disk.

CAUTION: Be careful, you want to set the of to a file in the mounted partition, NOT THE PARTITION ITSELF!

mkdir image_source
sudo mount /dev/sda1 image_source
dd if=/dev/zero of=image_source/wash.tmp bs=4M
rm image_source/wash.tmp
sudo umount image_source

Making a Partition Image

mkdir image
sudo dd if=/dev/sda1 of=image/sda1_backup.img bs=4M

Where sda is the name of the device, and 1 is the partition number. Adjust accordingly for your system if you want to image a different device or partition.

Making a Whole Disk Image

mkdir image
sudo dd if=/dev/sda of=image/sda_backup.img bs=4M

Where sda is the name of the device. Adjust accordingly for your system if you want to image a different device.


Make a "squashfs" image that contains the full, uncompressed image.

sudo apt-get install squashfs-tools
mksquashfs image squash.img

Streaming Compression

To avoid making a separate temporary file the full size of the disk, you can stream into a squashfs image.

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

Mounting a compressed partition image

  • First mount the squashfs image, then mount the partition image stored in the mounted squashfs image.
    mkdir squash_mount
    sudo mount squash.img squash_mount
  • Now you have the compressed image mounted, mount the image itself (that is inside the squashfs image)
    mkdir compressed_image
    sudo mount squash_mount/sda1_backup.img compressed_image
  • Now your image is mounted under compressed_image.

EDIT: If you wanted to simply restore the disk image onto a partition at this point (instead of mounting it to browse/read the contents), just dd the image at squash_mount/sda1_backup.img onto the destination instead of doing mount.

Mounting a compressed full disk image

This requires you to use a package called kpartx. kpartx allows you to mount individual partitions in a full disk image.

sudo apt-get install kpartx
  • First, mount your squashed partition that contains the full disk image

    mkdir compressed_image
    sudo mount squash.img compressed_image
  • Now you need to create devices for each of the partitions in the full disk image:

    sudo kpartx -a compressed_image/sda_backup.img

    This will create devices for the partitions in the full disk image at /dev/mapper/loopNpP, where N is the number assigned for the loopback device, and P is the partition number, e.g., /dev/mapper/loop0p1.  You can find this number N in the output of losetup --list.  The most recently created loopback device should have the largest N number.

  • Now you have a way to mount the individual partitions in the full disk image:

    mkdir fulldisk_part1
    sudo mount /dev/mapper/loop0p1 fulldisk_part1
  • interesting take on this problem (squashfs instead of gzip). I am pretty unfamiliar with squashfs tools... can you pipe the output of dd to create a squash partition on the fly as you can with the gzip partition? what is the compression ratios (gzip is okay/good, esp given the fact that I am clearing 'empty space with zeros')?
    – g19fanatic
    Commented May 13, 2013 at 3:12
  • 2
    @g19fanatic The uncompressed disk image is "inside" the squashfs image. You mount the squashfs image, then dd the image inside it to the destination disk.
    – doug65536
    Commented May 13, 2013 at 5:27
  • 3
    @g19fanatic You can stream into squashfs using the -p or -pf flags to pass it a pseudo-file. A pseudo file can be used for things like making device nodes which you can't otherwise do without root (useful for building images as part of a build process) or for streaming the output of some command into the image. One of the examples given in the docs (/usr/share/doc/squashfs-tools/examples/pseudo-file.example on Debian/Ubuntu) is input f 444 root root dd if=/dev/sda1 bs=1024 count=10 to copy the first 10K from a disk image into a file named "input" in the squashfs image. Commented Jul 22, 2014 at 3:45
  • 1
    A good explanation of the mksquashfs command arguments can be found here: askubuntu.com/questions/836217/…
    – HackerBoss
    Commented Mar 19, 2020 at 15:34
  • 1
    mksquashfs procedure worked great for streaming compression for a partition. For unknown reason (got a permission denied error), I needed to specify read-only to mount the inner partition. I.e., using the example above, I needed to use sudo mount -r squash_mount/sda1_backup.img compressed_image.
    – rickhg12hs
    Commented Oct 1, 2020 at 11:47

Try archivemount

root@srv1:/backup# archivemount windows-2003-S.gz /target/
Unrecognized archive format

root@srv1:/backup# archivemount -o formatraw windows-2003-S.gz /target/
Calculating uncompressed file size. Please wait.

root@srv1:/backup# ls /target/

root@srv1:/backup# file /target/data
/target/data: DOS/MBR boot sector; partition 1 : ID=0x7, start-CHS (0x0,1,1), end-CHS (0x3ff,254,63), startsector 63, 58717512 sectors, extended partition table (last)

archivemount is a FUSE-based file system for Unix variants, including Linux. Its purpose is to mount archives (i.e. tar, tar.gz, etc.) to a mount point where it can be read from or written to as with any other file system. This makes accessing the contents of the archive, which may be compressed, transparent to other programs, without decompressing them.


After mounting archive you can use it contents like regular file. Maybe get partition table, or convert, mount image with qemu tools.

squashfs useful for booting from image, but much complex for backuping.

  • 1
    Perfect! This is the most easy and elegant solution so far. I wonder why there are no votes here. Commented Apr 28, 2015 at 20:54
  • 1
    Of course, but still the original question doesn't talk about a tar.gz. My point was to tell @TranslucentCloud this answer has no votes because it doesn't actually answer the question, it tells "you should have created a tar.gz instead"
    – p91paul
    Commented May 22, 2015 at 8:41
  • 2
    This answer is very interesting too. I believe squashfs gets more love because it has more awareness. I instantly recognized the name but have never heard of archivemount. I will have to give it a shot too!
    – g19fanatic
    Commented Nov 16, 2015 at 13:37
  • 6
    archivemount does not allow to mount an image created by command dd if=/dev/hda conv=sync,noerror bs=64K | gzip -c > /mnt/sda1/hda.ddimg.gz Commented Nov 6, 2016 at 15:58
  • 3
    Agreed - at time of writing, archivemount supports tar archives that are gzipped, but not plain gzipped files.
    – mwfearnley
    Commented Apr 22, 2017 at 12:34

If the image is read-only you can also use nbdkit (man page) and its xz filter (xz should provide better compression and random access times than gzip). If you need temporarily write access, the cow (Copy On Write) filter might be useful.

Create the compressed partition image

dd if=/dev/sda1 bs=16M | xz -9 --block-size=16MiB > sda1.img.xz

A --block-size option of 16 MiB should provide good random access performance.

Note: you may use alternative xz compression programs such as pixz which provides parallel compression, just make sure it splits the output in multiple small blocks, otherwise nbdkit has to decompress a lot of data. For example as of September 2015, pxz does not support this.

Serve it with nbdkit

nbdkit --no-fork --user nobody --group nobody -i \
       --filter xz file sda1.img.xz

Connect to the NBD server

nbd-client 10809 /dev/nbd0 -nofork

Mount it read-only

mount -o ro /dev/nbd0 sda1

When done

umount /dev/nbd0
nbd-client -d /dev/nbd0

Stop the nbdkit server by pressing Ctrl+C (or with kill).


This answer complements Cristian Ciupitu's answer. If you use xz compression with a reasonable block size, you can access the disk image using guestfish or other libguestfs tools like this:

nbdkit xz file=disk.img.xz --run 'guestfish --format=raw -a $nbd -i'

UPDATE: Since xz is not a plugin anymore, but has become a filter, the command is now:

nbdkit file disk.img.xz --filter xz --run 'guestfish --format=raw -a $nbd -i'
  • 1
    See original answer is better: unix.stackexchange.com/questions/31669/… Commented Dec 25, 2019 at 14:17
  • Guestfish is nice and can also be used for .vhd(x) files without restrictions, they are pretty common in the Windows world. For the full solution on mounting look here: how2shout.com/linux/… (I expect this to also help here as guestfish is discussed in detail)
    – Cadoiz
    Commented Jan 20, 2021 at 1:05

Not really. You can't really seek to a specific block in the compressed file without decompressing the whole thing first, which makes it difficult to use the compressed image as a block device.

You could use something like dump and restore (or tar, really), all of which use a streaming format...so you can access invidividual files by effectively scanning through the uncompressed stream. It means if the file you want is at the end of the compressed archive you may have a long time to wait, but it doesn't require you to actually decompress everything onto disk.

Using tar for backups may seem a bit old fashioned, but you get a lot of flexability.

  • 1
    The problem lies in the fact that I do not even know if the file of interest is actually on this compressed backup... Do you know of a file explorer that will go through the whole .gz'd image, keep the file/dir structure in memory, provide a simple view of the structure and allow you to 'pick' files (now that it knows where they exist) to extract? Its a very niche specification... but I could see tons of uses for something like this... if it exists.
    – g19fanatic
    Commented Feb 15, 2012 at 12:09
  • 1
    If it doesn't, would you be able to point me towards some instruction on how to pull the structure from the gz'd image? I would be able to create such a program (program for a living...) but am blind on the topic of decompressing image data and the specifics of different filesystems.
    – g19fanatic
    Commented Feb 15, 2012 at 12:11
  • I suspect that building your own tool is going to be a larger project than you really want to undertake. However...assuming that you have an ext[234] filesystem, I would suggest the e2fsprogs package, or maybe something like fuse-ext2. Both provide user-space tools for interacting with ext[234] filesystems.
    – larsks
    Commented Feb 15, 2012 at 13:59
  • Also note that what you have doesn't appear to be a filesystem image, it's a whole disk image, which means you'll first have to parse out the partition table and locate the appropriate partition.
    – larsks
    Commented Feb 15, 2012 at 14:00
  • I mistyped in the above question and will fix it. I usually do a partition based dd image and save a copy of the partition table. I used to do whole disk copies but hated needing to mount with options to get to the proper location.
    – g19fanatic
    Commented Feb 15, 2012 at 14:51

Another addendum to Cristian Ciupitu's answer:

If you use nbdkit to mount a full disk image (vs. a partition image), you might need to specify the block size (sector size of the disk) when connecting to the NBD server, as it defaults to 1024 bytes. To use 512 bytes instead:

nbd-client /dev/nbd0 -b 512 -n

After that, the disk will appear as /dev/nbd0, and you should be able to view the partition table using fdisk -l. However, the partitions are not yet mountable - Use kpartx (from doug65536's answer) to create devices for the partitions, e.g.:

kpartx -arv /dev/nbd0

Finally, the partitions will appear in /dev/mapper/, and you can mount them as usual. Make sure to use readonly mode (-o ro), as the xz plugin only supports reads:

mount -o ro /dev/mapper/nbd0p3 /mnt

Yes, AVFS would work.

First you create a "portal" over your filesystem where all operations performed on your files are intercepted through the AVFS FUSE system:

~ $ mkdir avfs                # the AVFS mount-point
~ $ avfsd -o allow_root avfs  # Needs `user_allow_other` option in /etc/fuse.conf
~ $ cd avfs
~/avfs $ ls  # your root filesystem listed

Then attach a loopback device with partition table on the transparently uncompressed disk-image located in the ~/avfs mount-point:

ℹ️ Note: the # character suffix is not a comment but part of the AVFS-syntax that uncompresses the file.

~/avfs $ sudo losetup -Pf ./absolute/path/to/full-disk.img.gz#

The command above selected and printed the first unused loop-device.

The kernel has now created additional loop-devices for all the partitions contained in the full-disk.img.gz. You can mount them the usual way:

$ sudo mount /dev/loop0p1 /some/mount/point/

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