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I have a DD image from a 4GB SD card that has two partitions, these two partitions are only using up about 800 MB and as such I wish to reduce the size of the img fie.

Does anyone know of a way to remove the "free space" from the img file?

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you may want to add what the desired outcome is. ie: image back to the card, to store on CD. –  Wayne Jun 22 '13 at 16:41

2 Answers 2

up vote 1 down vote accepted

First make sure the free space is actually empty, and doesn't contain leftovers of deleted files. The easiest way to achieve this is to create a huge file on the disk, containing only null bytes, then delete it.

# losetup --find --partscan foo.img
# lsblk
NAME      MAJ:MIN RM    SIZE RO TYPE MOUNTPOINT
loop0       7:0    0   4096M  0 loop 
├─loop0p1 259:0    0   2048M  0 loop 
└─loop0p2 259:1    0   2048M  0 loop 
# for part in /dev/loop0p*; do
    mount $part /mnt
    dd if=/dev/zero of=/mnt/filler conv=fsync bs=1M
    rm /mnt/filler
    umount /mnt
  done
dd: error writing ‘/mnt/filler’: No space left on device
dd: error writing ‘/mnt/filler’: No space left on device
# losetup --detach /dev/loop0

Then compress it with a tool like gzip or xz. Even at lowest compression levels, a long series of zeros will compress well:

# ls -s
4096M foo.img
# gzip foo.img
# ls -s
11M foo.img.gz

Note that you must uncompress the image when writing it back to disk. But this can be done without needing any space for the full image:

# cat foo.img.gz | gunzip | dd of=/dev/sda

An alternative method, if you want to keep using the image – e.g. with a virtual machine – is to convert the raw image to one of the image formats used by virtualization software; e.g. qcow2 for Qemu, VDI for VirtualBox, or VMDK for VMware.

Note that this still requires you to prepare the image by cleaning the free space using the above method.

# qemu-img convert -f raw -O qcow2 foo.img foo.qcow

# qemu-img convert -f raw -O vmdk foo.img foo.vmdk

But if it's going to be written to a real disk again, you have to convert it back to a raw image.

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how can you be sure that all of the files that are currently stored on the partitions aren't physically scattered around on the disk? –  Valerio Santinelli Jun 18 '14 at 13:38
    
@ValerioSantinelli: They usually are, but why should I care about that? –  grawity Jun 18 '14 at 15:26
    
because I suppose that files aren't written sequentially on the disk and if a file is at the end of the storage, when I cut the size down, I'll lose the content of that file. –  Valerio Santinelli Jun 18 '14 at 17:35
1  
@ValerioSantinelli: How is this at all related to my post? –  grawity Jun 19 '14 at 16:56
    
Because the OP is asking how to shrink a disk image without losing data. If you truncate an image at a certain size you can't be sure the data will be preserved. –  Valerio Santinelli Jun 19 '14 at 17:05

much much easier

resize2fs -M xxx.img

(you will be asked to e2fsck first - so e2fsck -f -y xxx.img

(image must NOT be mounted!)

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