Tuesday, April 26, 2016

HOWTO - Resize partitions in raw disk (dd) image files (.img)

If we want to modify partitions and file systems in our disk image file, we will need to mount the image as a so-called loop device in linux.
Not only will we need to mount the image as a loop device, but we also need to make sure that our loop device will auto create additional device entries for the partitions detected on our image.

On Ubuntu, this function is baked into the linux kernel, and this functionality can be enabled in the following way:

edit /etc/default/grub
and added the code max_loop=64 just after the GRUB_CMDLINE_LINUX_DEFAULT=""then execute update-grub2, reboot and verify with ls /dev/loop*

for ubuntu 16.04 LTS
sudo losetup -Pf disk_image.raw

You can list the partition table of an image file with the following command:

fdisk -l my-raw-imagefile.img

We gather from the partition table that we have two available partitions in the image file.

Before we can make any modification to the partitions or filesystems in our image, we will first need to mount the image as a loop device, which will install the required dev entries to address our partitions with other software as gparted.

To mount the image as a loop device:
sudo losetup --find --show my-raw-imagefile.img

This will mount /dev/loop0 as a 'virtual raw disk' and it's containing partitions accordingly as /dev/loop0p1 and /dev/loop0p2

Now that we have set-up our raw imagefile as a loop device, we can run filesystem tools on it.  We can use for example gparted to resize the file systems contained in the partitions of our image:

gparted /dev/loop0

Do any resize operation that you would like to have for your partitions and apply them, in my case I needed to trim down my image size as small as possible.
Now that the resize operations have been done in our raw disk image, we will dump the partitions to a smaller image.

First remove our loop device for the image, as it will interfere with the dump process:
sudo losetup -d /dev/loop0

Now list our partition table so we have an idea what to dump:

In our partition table we can see that our blocks are 512 bytes long, and we have a total number of 2811903 blocks to dump ... doing the actual dump is done with dd which just reads raw blocks of data and dumps them somwhere.

dd bs=512 count=2811904 if=my-original-raw-imagefile.img of=my-resized-raw-imagefile.img

bs = the blocksize to read
count = the amount of blocks to dump
if = input file to read from
of = output file to write to

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