[Fedora-xen] Guest Image File Size

[Daniel P. Berrange]

On Sat, Dec 01, 2007 at 12:42:40PM -0500, Aaron Metzger wrote:
> Sadique Puthen wrote:
> 
> >
> >Basically it if you don't pre-allocate the entire image, virt-manager 
> >creates a sparse file to store the guest data which only occupies disk 
> >blocks while you write contents to that sparse file . When checking the 
> >size of the pre-allocated and sparse images, you should use "du", not 
> >"ls -lh".
> >
> 
> Thank you for your insight.  You are correct.
> 
> I have a guest image stored in a file called "subversion".
> 
> "du" shows the actual size being used is smaller than what "ls" thinks.
> 
> du -a subversion
> 2158768 subversion
> 
> ls -ld subversion
> -rwxr-xr-x 1 root root 64424509441 2007-11-27 18:16 subversion

You don't need to use  'du' - just use the '-s' flag to ls

$ ls -lsh dan1.img 
16K -rwxr-xr-x 1 root root 2.1G 2007-11-29 17:51 dan1.img

Shows its physical size is 16k, while its logical size is 2.1 GB.

> I have a follow up question though.  How do you cleanly backup and 
> restore these files using the smaller amount of space?  Any simplistic 
> program that I try (e.g. "tar") also thinks my file is 60 Gig not 2 Gig 
> and creates a real file that uses 60 Gig of actual disk space.

Use the --sparse option with tar.

       -S, --sparse
              handle sparse files efficiently

> Can anyone share the simple steps with "dd" or other programs that let 
> you cleanly store a backup of the guest image which uses the smaller 
> amount of space and also correctly restores from that backup in a way 
> that preserves the orginal idea of a 60 Gig max guest disk size?

'cp' will also try to detect sparse files & handle them properly, but
if it gets it wrong you can also use  '--sparse=always'

Dan
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