Tag Archives: disk image

Tip: compress raw disk images using qcow2

$ qemu-img convert -c -f raw -O qcow2 win.img winq.img
$ ls -lh win*
-rw-r--r--. 1 root   root    10G May 18 14:34 win.img
-rw-r--r--. 1 rjones rjones 6.5G May 18 14:59 winq.img

Of course the degree of compression you get depends on the amount of zeroed free space in the image, and the amount by which qcow2 is able to compress the other blocks containing data.

qcow2 uses zlib for compression, so the compression won’t be that spectacular. It’s better to keep the filesystems “sparse” in the first place, by ensuring unused disk blocks are zeroed.

For ext2/3 filesystems, Fedora ships a utility called zerofree, which you can either run inside the guest, or run offline from guestfish. This turns unused filesystem blocks into zeroes, which will make outside compression eg with qcow2 much more efficient. For other filesystems, the usual trick is to create a large file of all zeroes until you fill up the free space, then delete it.

qcow2 files are completely interchangeable with raw disk images:

$ virt-df -h win.img
Filesystem                                Size       Used  Available  Use%
win.img:/dev/vda1                       100.0M      24.1M      75.9M   25%
win.img:/dev/vda2                         9.9G       7.4G       2.5G   75%
$ virt-df -h winq.img
Filesystem                                Size       Used  Available  Use%
winq.img:/dev/vda1                      100.0M      24.1M      75.9M   25%
winq.img:/dev/vda2                        9.9G       7.4G       2.5G   75%


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Virt-rescue is a new tool proposed for inclusion in libguestfs. It lets you get a rescue shell on your virtual machine, good for making quick, interactive, unstructured fixes:

$ virt-rescue F11x64

Welcome to virt-rescue, the libguestfs rescue shell.

Note: The contents of / are the rescue appliance.
You have to mount the guest's partitions under /sysroot
before you will be able to examine them.

><rescue> /sbin/e2fsck /dev/vg_f11x64/lv_root
><rescue> mount /dev/vg_f11x64/lv_root /sysroot
><rescue> ls /sysroot/
bin   dev  home  lib64       media  opt   root  selinux  sys  usr
boot  etc  lib   lost+found  mnt    proc  sbin  srv      tmp  var
><rescue> sync
><rescue> umount /sysroot
><rescue> exit

Of course we encourage you to continue using libguestfs and guestfish for making properly structured changes through a stable, programmable API!

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