Betsy - One would not use caching if the recall pattern did not make
        its use worthwhile, to eliminate some overhead.  In most
shops, an initial disk in a backup storage pool hierarchy basically
serves as a buffer rather than a repository: it's usually "write-only"
with minimal prospect of retrieval.  In such situations this just
presents ADSM with more work to do in maintaining the cache, including
rapidly having to dismiss cache images when more space is called for.
Caching makes a lot more sense for Archive and HSM storage pool disk,
where recall of files is more probable.
   Richard Sims, BU