Create A Disk Image Without Enough Free Space


Recently I purchased a new laptop. Typically my first move is to ditch Windows or Mac (yes I’ve done both) and install Linux. This time, just in case I ever want to completely restore the system (recovery partition and all), I decided to make a disk image of the entire hard drive.

Being that the capacity of the hard drive is close to 500 GB it actually turned out that I didn’t have enough free space to store the entire image. However, since it was a brand new computer I knew there was plenty of zeroes on the disk and thus plenty of opportunity to reduce the actual size of the disk image by converting those zeroes into holes in a sparse file .

Now usually when creating a disk image I would just use dd, but dd doesn’t have any options that are sparse file aware. Luckily I was able to find a page that addressed my particular predicament.

The secret sauce here is the --sparse=always option of cp. By piping the output of dd to cp, I could then copy my disk image! The next step was to fire up a live cd, connect an external hard drive and run the following command to create the disk image:

dd if=/dev/sda | cp --sparse=always /dev/stdin lenovo.img

Just to be careful I decided to md5sum both the disk image and the block device to make sure they had the same sums:

dustymabe@media: >sudo md5sum lenovo.img
fbd07bc0fc46622e0bfc1ac2b44915e5  lenovo.img

ubuntu@ubuntu:~$ sudo md5sum /dev/sda 
fbd07bc0fc46622e0bfc1ac2b44915e5  /dev/sda

Success! So how much space did I actually save by make the image a sparse image? We can see by using the du command:

dustymabe@media: mnt>du -sh lenovo.img
38G     lenovo.img
dustymabe@media: mnt>du -sh --apparent-size lenovo.img
466G    lenovo.img

This means that the disk image only actually took 38 GB of disk space but would have taken 466 GB if I had not made it a sparse disk image… Mission accomplished!!

Until next time…

Dusty

References:
http://serverfault.com/questions/93667/create-image-from-hard-disk-without-free-space-linux