[Fedora-xen] Guest Image File Size

[Daniel P. Berrange]

On Sat, Dec 01, 2007 at 02:03:06PM -0500, Mark Nielsen wrote:
> I think you'll get the same sort of thing if you gzip the file. I've 
> gzip'd some 20G xen images down to around 2G.

AFAIR, this is just because sparse files appear as long sequences of
zeros & thus compress wel - gzip isn't actually optimizing for sparseness.
The trouble is that when you extract the file gunzip will fully allocate
it filling with zeros.

My original 1M file takes 4k

# ls -lhs foo 
4.0K -rw-r--r-- 1 root root 1.0M 2007-12-01 14:18 foo

#gzip  foo
# ls -lhs foo.gz 
8.0K -rw-r--r-- 1 root root 1.1K 2007-12-01 14:18 foo.gz

Now when it uncompress it it takes up the full 1 MB :-(

# gunzip foo.gz 
# ls -lhs foo
1.1M -rw-r--r-- 1 root root 1.0M 2007-12-01 14:18 foo


You *always* want to use tar to preserve sparseness, and then gzip the tar 
file so you get optimal resource usage both on archiving & extracting.

Dan.
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