Reducing the actual size of the XEN disk image. Is it possible to?

Question

Xen

Colleagues, good afternoon.
Two cases:

1. I make an hdd image using dd, transfer it in a compressed state via nfs to another server:
   

   mount -o ro,remount /
   dd if=/dev/sda bs=64K | gzip -c > /share/test.img.gz

   
   after gunzip I get a 20GB (for example) file finished with zeros at the end:
   

   du -h /md/share/test.img
   21G /md/share/test.img
   

   
   
2. I create an empty image through the same dd:
   

   dd if=/dev/zero of=/md/share/test2.img bs=1024k seek=20480 count=0

   
   And I get the file:
   

   du -h /md/share/test2.img
   0 /md/share/test2.img
   ls -lah /md/share/test2.img
   -rw-r--r-- 1 root root 20G Aug 2 2:46 pm / md/share/test2.img
   

   
   After that, having mounted it, I can easily write anything to the VM, the file size will be equal to the actual content (but not more than 20GB).
   

Question! how to reduce file size for 1 case?

Answer 1

Use qcow format which works in thin provisioning mode.
But the performance drops noticeably.

Answer 2

Answered on Serverfault:
serverfault.com/questions/710299/reduce-xen-image-...
In short, add conv=sparse to dd and pack with tar --sparse .
Only here is sparse for Centos 5.11 dd no) https://bugzilla.redhat.com/show_bug.cgi?id=908980