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====== Redimensionner un fichier image d'un disque ======

===== Context of the problem =====

Having a myimage.img bigger then the hardware support (if it is smaller there should be no problem; however,
using the same strategy, you can better fit the image in the hardware support).

The secret is to use standard Linux tools and instruments: GParted, fdisk and truncate.

===== Requirements =====

  * A Linux PC
  * The .img you want to shrink (myimage.img in this example)

===== Creating loopback device =====

GParted is an application typically used to manage partition tables and filesystems.
In order to shrink the image, GParted is going to be used along the first part of the answer.

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.

==== Let's enable enable the loopback ====

  sudo modprobe loop

==== Let's request a new (free) loopback device ====

  sudo losetup -f

The command returns the path to a free loopback device:

  /dev/loop0

==== Let's create a device of the image ====

  sudo losetup /dev/loop0 myimage.img

The device /dev/loop0 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 ====

Let's load the new device using GParted

  sudo gparted /dev/loop0

Now notice a few things:

  * The partition allocates the entire disk/device/image.
  * The partition is filled partly.

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

Select the partition and click Resize/Move.

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:

unallocated space on right

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, but 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 off 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 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 partition 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:

  truncate --size=$[(9181183+1)*512] myimage.img