Asymmetric Memory Hierarchies
 A study is presented of some of the system
implications of memory hierarchies in which the 
backing or secondary store has a very small read time,
relative of both the time required for writing 
and to the read time of conventional backing storage
devices. Several analytic models are introduced, 
and it is shown that such hierarchies may operate in
ways which differ from those of more conventional 
hierarchies.  In particular, it is shown that it may not
be necessary to multiprogram in such a situation. 
 In the past, backing storage devices have been roughly
symmetric with respect to their read and write 
times.  This situation may not continue, as several
devices are currently under development which may 
have a very small read-time/write-time ratio.  This study
places particular emphasis on one such system-the 
RCA read/write holographic optical memory.
CACM April, 1973
Williams, J.G.
