Hello,
running "/usr/sbin/hardlink" on the CentOS 5.5 and 5.6 directory saves 2 GB of disk space. Not huge, but if that is done on the main server once, all rsync mirrors would pick this up.
Reducing the overall mirror space needed and the size of new data for a CentOS Update release should be important.
best regards,
Florian La Roche