Libguestfs - Jan 2014 - IRC question: virt-builder & size of qcow2 output files

This question was asked on IRC:

  $ virt-builder centos-6 --output box.img --format qcow2 --size 40G # ...
  $ qemu-img convert -O qcow2 box.img box-qcow2.img
  $ ls -lAhs
  1.6G -rw-r--r--. 1 james  41G Jan  2 21:53 box.img
  1.1G -rw-r--r--. 1 james 1.1G Jan  3 18:54 box-qcow2.img
  $ file box*
  box.img:       QEMU QCOW Image (v2), 42949672960 bytes
  box-qcow2.img: QEMU QCOW Image (v2), 42949672960 bytes

I can reproduce this as well.  It turns out to be because virt-builder
runs the following command to create the output container:

  qemu-img create -f qcow2 -o preallocation=metadata box.img 42949672960

which you can see produces an apparently huge, but *sparse* file:

  $ ls -lh box.img 
  -rw-r--r--. 1 rjones rjones 41G Jan  4 12:56 box.img
  $ du -sh box.img 
  6.4M			    box.img

Note that the output file from virt-builder is also sparse:

  $ ls -lh box.img 
  -rw-r--r--. 1 rjones rjones 41G Jan  4 12:55 box.img
  $ du -sh box.img 
  935M			    box.img

so I don't think this counts as a bug, but I'm surprised that qemu-img
+ -o preallocation=metadata causes such an (apparently) huge file to
be created.  It's not taking up as much disk space as it appears to.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Fedora Windows cross-compiler. Compile Windows programs, test, and
build Windows installers. Over 100 libraries supported.
http://fedoraproject.org/wiki/MinGW