Advanced use of disk images

7 minute read

Filling empty space with zeros

To improve compression of image files, the remaining free space of a partision can be filled with zeros before the image is created. Run this on the system to be backed up:

dd if=/dev/zero of=file; rm file

This command uses the low-level dd copy command to write zeros to a file, and when the disk is full and the command exits, removes the file.

Check if the space is actually freed again (file is actually deleted).

df -h

Create an Image from removable media

If the Motorcortex system is installed on a removable drive an image can also be made by attaching the drive to your PC and creating an image directly. Many Linux distributions already have convenient graphical tools to create an image if a disk.

In Ubuntu you can use the Disks tool to create an image file.

Then from the menu select create Disk Image:

Mounting an image

If you need to inspect or modify a partition of an image after the image has been created you can mount it on a Linux System.

First check the partition table of the image file:

fdisk -l image.img

In this case we would like to mount the second partition, which starts at block 1026048. We first need a mount point; and empty folder that will then be the location where the image will be mounted. In this case we use /mnt/image. If that does not exist, create a folder somewhere. Then mount the image:

sudo mount -o loop,offset=$((512*1026048)) image.img /mnt/image

When you are finished inspecting the contents of the image, unmount it.

sudo umount /mnt/image

Compressing an image

This command compresses the image using all available cpus:

xz -v \--threads=0 image.img

The result will be an image.img.xz file, which in Ubuntu can be restored with the Disks tool. if you want to keep the original file:

xz -v -k \--threads=0 image.img

An typical 8 GB image file should compress to about 400 MB if the empty space was first zeroed out.

Uncompressing an image file

Some operations on image files, such as mounting an image to make modifications to it or converting the image to a VirtalBox image require the xz compressed image file to be uncompressed. In Linux this is accomplished by executing the shell command:

unxz -k image.img.xz

the -k option keeps the original compressed file; without this option the compressed file is removed after uncompression.

Restoring an image to a removable disk

You can restore an image to a removable disk (for instance a CFAST card that is attached to your laptop with a card-reader) in Ubuntu by right-clicking the image and choosing “Open With Disk Image Writer” from the context menu:

This also works with xz compressed images directly.

Truncating (resizing) an image

Orignal Article:

When creating images from existing ISOs you often need to allocate a number of MB for the image to at least fit the files that are in the ISO. Predicting the exact size of the image is hard, even for a program. In this case you will create an image that is larger than actually needed: the image is much larger than the files on the image are combined.

This post will show how to shrink an existing image to a more optimal size. We will do this on Linux, since all required tools are available there: GParted, fdisk, gdisk (for GPT), and truncate.


  • A Linux PC
  • Some knowledge how the terminal works will helps
  • The unoptimal image (myimage.img in this example)

Creating loopback device

GParted is a great application that can handle partition tables and filesystems quite well. In this tutorial we will use GParted to shrink the filesystem (and its accompaning partition in the partition table).

GParted operates on devices, not simple files like images. This is why we first need to create a device for the image. We do this using the loopback-functionality of Linux.

First we will enable loopback if it wasn’t already enabled:

$ sudo modprobe loop

Now we can request a new (free) loopback device:

$ sudo losetup -f

This will return the path to a free loopback device. In this example this is /dev/loop0.

Next we create a device of the image:

$ sudo losetup /dev/loop0 myimage.img

Now we have a device /dev/loop0 that represents myimage.img. We want to access the partitions that are on the image, so we need to ask the kernel to load those too:

$ sudo partprobe /dev/loop0

This should give us the device /dev/loop0p1, which represents the first partition in myimage.img. We do not need this device directly, but GParted requires it.

Resize partition using GParted

Next we can load the device using GParted:

$ sudo gparted /dev/loop0

This should show a window similar to the following:

Now notice a few things:

  • In this example, there is only one partition. There may also be a boot partion preceding the main partition.
  • The partition allocates the entire disk/device/image.
  • The partition is filled only partly.

We want to resize this partition so that is fits it content, but not more than that.

Select the partition and click Resize/Move. A window similar to the following will popup:


Drag the right bar to the left as much as possible.

Note that sometimes GParted will need a few MB extra to place some filesystem-related data. You can press the up-arrow at the New size-box a few times to do so. For example, I pressed it 10 times (=10MiB) for FAT32 to work. For NTFS you might not need to at all.

Finally press Resize/Move. You will return to the GParted window. This time it will look similar to the following:


Notice that there is a part of the disk unallocated. This part of the disk will not be used by the partition, so we can shave this part off of the image later. GParted is a tool for disks, so it doesn’t shrink images, only partitions, we have to do the shrinking of the image ourselves.

Press Apply in GParted. It will now move files and finally shrink the partition, so it can take a minute or two, most of the time it finishes quickly. Afterwards close GParted.

Now we don’t need the loopback-device anymore, so unload it:

$ sudo losetup -d /dev/loop0

Shaving the image

Now that we have all the important data at the beginning of the image it is time to shave of that unallocated part. We will first need to know where our partition ends and where the unallocated part begins. We do this using fdisk:

$ fdisk -l myimage.img

Here we will see an output similar to the following:

Disk myimage.img: 6144 MB, 6144000000 bytes, 12000000 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000ea37d

      Device Boot      Start         End      Blocks   Id  System
myimage.img1            2048     9181183     4589568    b  W95 FAT32

Note two things in the output:

  • The last partition (in this example there is only one partition) ends on block 9181183 (shown under End)
  • The block-size is 512 bytes (shown as sectors of 1 * 512)

We will use these numbers in the rest of the example. The block-size (512) is often the same, but the ending block (9181183) will differ for you. The numbers mean that the parition ends on byte 9181183512 of the file. After that byte comes the unallocated-part. Only the first 9181183512 bytes will be useful for our image.

Next we shrink the image-file to a size that can just contain the partition. For this we will use the truncate command (thanks uggla!). With the truncate command need to supply the size of the file in bytes. The last block was 9181183 and block-numbers start at 0. That means we need (9181183+1)*512 bytes. This is important, else the partition will not fit the image. So now we use truncate with the calculations:

Additionally (for a GPT) we will leave some free space (say 1Mb = 1048576 (= 1024**2) bytes) at the end of the image for the GPT backup header which we need to fix after truncation.

$ truncate --size=$(((9181183+1)*512+1024**2)) myimage.img

To fix the GPT backup header we use gdisk

(echo r; echo d; echo w; echo y) | gdisk ./myimage.img

Meaning of letters in gdisk:

r	recovery and transformation options (experts only)
d	use main GPT header (rebuilding backup)
w	write table to disk and exit
y	yes to confirm

Converting an image to VirtualBox

If you have a compressed image you first need to uncompress it:

unxz -k -v 2019-03-18-image.img.xz

Then convert it to a VDI image:

VBoxManage convertfromraw 2019-03-18-Vectioneer-v1.0.5-test.img

For using the Virtual Machine see the Virtual Machine user guide.

Last modified March 23, 2021: Restructured GRID (44d0658)