MEMORY HIERARCHY

In the memory hierarchy, holographic memory lies somewhere between RAM and magnetic storage in terms of data transfer rates, storage capacity, and data access times. The theoretical limit of the number of pixels that can be stored using volume holography is V2/3/l2 where V is the volume of the recording medium and l is the wavelength of the reference beam. For green light, the maximum theoretical storage capacity is 0.4 Gbits/cm2 for a page size of 1 cm x 1 cm. Also, holographic memory has an access time near 2.4 ms, a recording rate of 31 kB/s, and a readout rate of 10 GB/s. Modern magnetic disks have data transfer rates in the neighborhood of 5 to 20 MB/s. Typical DRAM today has an access time close to 10 – 40 ns, and a recording rate of 10 GB/s.


Storage
Medium

Access Time
Data Transfer
Rate
Storage Capacity
Holographic Memory
2.4 ms
10 GB/s
400 Mbits/cm2
Main Memory (RAM)
10 – 40 ns
5 MB/s
4.0 Mbits/cm2
Magnetic Disk
8.3 ms
5 – 20 MB/s
100 Mbits/cm2


Table 1: This table shows the comparison of access time, data transfer rates (readout), and storage capacity (storage density) for three types of memory; holographic, RAM, and magnetic disk.



Holographic memory has an access time somewhere between main memory and magnetic disk, a data transfer rate that is an order of magnitude better than both main memory and magnetic disk, and a storage capacity that is higher than both main memory and magnetic disk. Certainly if the issues of hologram decay and interference are resolved, then holographic memory could become a part of the memory hierarchy, or take the place of magnetic disk much as magnetic disk has displaced magnetic tape for most applications.