17G debian5x64.img.orig
The original image has about 4.7 GB of data, plus a large swap partition, according to virt-df:
$ virt-df -a debian5x64.img.orig -h
Filesystem Size Used Available Use%
debian5x64.img.orig:/dev/sda1 322M 66M 239M 21%
debian5x64.img.orig:/dev/debian5x64.home.annexia.org/home
3.4G 359M 2.9G 11%
debian5x64.img.orig:/dev/debian5x64.home.annexia.org/root
320M 301M 2.4M 95%
debian5x64.img.orig:/dev/debian5x64.home.annexia.org/tmp
300M 8.2M 276M 3%
debian5x64.img.orig:/dev/debian5x64.home.annexia.org/usr
3.4G 2.0G 1.2G 60%
debian5x64.img.orig:/dev/debian5x64.home.annexia.org/var
2.6G 2.0G 536M 76%
Using virt-sparsify, all unused space in the image is made sparse. The most recent version can sparsify swap partitions too:
4.6G debian5x64.img
xz --best -T 0 reduces the final image to under a gigabyte:
971M debian5x64.img.xz
Richard WM Jones 
Would it still be able to compress the image so well, if you had done
dd if=/dev/random of=/t bs=1000 count=1000000
rm -f /t
?
If you run virt-sparsify afterwards, then yes. virt-sparsify turns all unused space sparse, even if it’s filled with random data.
So, that method works for storage… and of course distribution too… so that if you want to share a disk image with others, they have less to download.
Let’s pretend that is the scenerio. Could you please give the opposite set of instructions whereby one would get that image and try to make it as performant as possible… where perforance is more important that disk space used? I would assume one would turn it back into a raw partition?
Some rules of thumb below.
Use the fastest disk format possible: It used to be a logical volume or partition containing a raw guest image, but … (a) filesystems have got faster, so now you don’t necessarily need to use LVs, and (b) qcow2 got a lot faster (thanks kwolf and friends!) so qcow2 is now a reasonable alternative to raw. If you do use qcow2, preallocate metadata.
Don’t use a sparse format on the host (since it slows down writes).
Make sure the partitions in the guest are aligned: virt-alignment-scan, possibly followed by virt-resize.
Take usual steps to ensure your host disks are fast, perhaps using RAID 10 or a SAN.
Test everything to see what works and what doesn’t.
(And those are just for the storage)
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