The caching mechanism used by RHN Proxy Server is the Squid HTTP proxy, which saves significant bandwidth for the clients. It should have a reasonable amount of space available. The cached packages are stored in /var/spool/squid. The required free space allotment is 1.5 GB (including updates and source packages).
If the RHN Proxy Server is configured to distribute custom, or local packages, make sure that the /var mount point on the system storing local packages has sufficient disk space to hold all of the custom packages, which are stored in /var/spool/rhn-proxy. The required disk space for local packages depends on the number of custom packages deployed.