[Libguestfs] [libvirt-users] virt-sparsify changing the apparent-size of files

[Richard W.M. Jones]

On Wed, Jun 15, 2016 at 10:20:54AM -0500, libvirt_users at skagitattic.com wrote:
> Hello, 
> 
> I am curious why when using virt-sparsify the apparent-size of the file
> gets reduced to the actual file size?  Is there a way to get the
> apparent-size left as the full size?
> 
> In the man page for virt-sparsify it has a section on
> "IMPORTANT NOTE ABOUT SPARSE OUTPUT IMAGES" and its output seems to
> indicate what I would expect where "ls -lh" shows the full apparent size
> and "du -sh" shows the actual size.  But my actual results from testing
> are different.
> 
> In the following example input.qcow2 is before sparsify and output.qcow2
> is after.
> 
> Doing ls reports full size for input but a small size for
> the output:
> root at testingbox:/tmp/test # ls -lh
> total 3.1G
> -rw-r--r-- 1 root root 512M Jun 14 22:30 file.img
> -rw-r--r-- 1 root root  51G Jun 14 22:30 input.qcow2
> -rw-r--r-- 1 root root 790M Jun 14 22:36 output.qcow2

What is file.img?

> Same when asking du for apparent-size:
> root at testingbox:/tmp/test # du -h --apparent-size *
> 512M    file.img
> 51G     input.qcow2
> 790M    output.qcow2

'du --apparent-size' and 'ls -l' are the same thing, so it's not
really surprising that you see the same numbers here.

> A normal du will show the actual size of the files:
> root at testingbox:/tmp/test # du -h *
> 0       file.img
> 2.4G    input.qcow2
> 790M    output.qcow2
> 
> The only way I have found to check the full virtual size of the file
> after virt-sparsify is with qemu-img:
> root at testingbox:/tmp/test # qemu-img info input.qcow2
> image: input.qcow2
> file format: qcow2
> virtual size: 50G (53687091200 bytes)
> disk size: 2.3G
> cluster_size: 65536
> Format specific information:
>     compat: 1.1
>     lazy refcounts: true
> root at testingbox:/tmp/test # qemu-img info output.qcow2
> image: output.qcow2
> file format: qcow2
> virtual size: 50G (53687091200 bytes)
> disk size: 789M
> cluster_size: 65536
> Format specific information:
>     compat: 1.1
>     lazy refcounts: false

If I understand your question correctly, then everything is working as
it should.

For qcow2 files, the virtual size is not related to the "apparent"
size, as you can easily prove:

$ qemu-img create -f qcow2 huge.qcow2 1T
Formatting 'huge.qcow2', fmt=qcow2 size=1099511627776 encryption=off cluster_size=65536 lazy_refcounts=off refcount_bits=16
$ ls -lh huge.qcow2
-rw-r--r--. 1 rjones rjones 208K Jun 17 09:09 huge.qcow2

If you use raw files, then the file apparent size and virtual size are
the same.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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