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

[libvirt_users at skagitattic.com]

Hello Rich,

Thanks for the reply and information.

> What is file.img?

Its simply a file I made while testing sparse files "truncate -s 512M
file.img".

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

Indeed, I expected those to be the same, just figured I would show them
both for completeness.

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

So you are saying when I do the virt-sparsify its converting the image
from raw to cow2?

I studied the man page for virt-sparsify and tried again with
the flag "--format raw" (output2.cow2).  This output files looks as I
first expected.

# ls -lrh
total 4.7G
-rw-r--r-- 1 root root 790M Jun 14 22:36 output.qcow2
-rw-r--r-- 1 root root  51G Jun 21 18:34 output2.qcow2
-rw-r--r-- 1 root root  51G Jun 14 22:30 input.qcow2
-rw-r--r-- 1 root root 512M Jun 14 22:30 file.img
# du -sh *
0       file.img
2.4G    input.qcow2
1.6G    output2.qcow2
790M    output.qcow2

So I guess it was detecting the source wrong and doing a conversion?
(As it says "If this is not specified, then the input format is used."
in the man page)

Also seems odd that it is 1.6G rather then the 790M of the output in
qcow2 format.  Does qcow2 do some compression or something?


Is there some way I can see if a file is raw or cow2?  When I did
"qemu-img info" it reported both of them as cow2.

I ran the initial install like the following and it ended up
with the sparse file that reports full size with ls: "virt-install
--name test-install --memory 512 -l
'http://ftp.nl.debian.org/debian/dists/jessie/main/installer-amd64/'
--disk size=50 --nographics -x "console=ttyS0"" 

I tried again and got the same result as the initial input file.  So
why is it naming it qcow2 if its actually raw?  Or is there two
way's of doing formatting inside the qcow2 file, one raw and the other
called something else?

Is there any reason to use raw vs non-raw?  From some reading online it
appears performance is better with raw files.  Is there a downside to
using raw?


Thanks!
Keelan



On Fri, 17 Jun 2016 09:11:04 +0100
"Richard W.M. Jones" <rjones at redhat.com> wrote:

> 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.
>