[virt-tools-list] sparse files and ls

[Richard W.M. Jones]

On Tue, Jun 14, 2011 at 11:53:32AM +0100, Paul Mansfield wrote:
> On 08/06/11 17:53, Gary Scarborough wrote:
> > I guess the difference is that on the qcow2 files, the full(maximum)
> > size is not showing up.
> > 
> > qemu-img info junk.img shows:
> > 
> > image: junk.img
> > file format: qcow2
> > virtual size: 10G (10737418240 bytes)
> > disk size: 140K
> > cluster_size: 65536
> 
> I would guess the "disk size" is the contained file system, not
> including the qcow2 "wrapper" overhead
> 
> > ls -lash junk.img  shows:
> > 
> > 140K -rw-r--r-- 1 root root 193K Jun  8 11:11 junk.img
> 
> so the overhead of qcow2 container is ?53K. does this grow as the
> virtual disk fills up?

qcow2 uses a two-level directory.  The second level is not allocated
until required unless you selected the "preallocation" option when
creating the disk.  So if you didn't use "-o preallocation" then yes
this overhead can grow as more disk blocks get allocated.  By the way,
we found that preallocation is faster.

Mark wrote extensive documentation on qcow2 here:

http://people.gnome.org/~markmc/qcow-image-format.html

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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