OS-RAM and Optical Disk

RAM DISKS:

A RAM disk is a disk device simulated in conventional random access memory. It completely eliminates delays suffered in conventional disks because of the mechanical motions inherent in seeks and in spinning a disks. RAM disks are especially useful in high-performance applications.

Caching incurs a certain amount of CPU overhead in maintaining the contents of the cache and in searching for data in the cache before attempting to read the data from disk. If the record reference patterns is not seen in the cache, then the disk cache hit ratio will be small and the CPU’s efforts in managing the cache will be waster, possibly resulting in poor performance.

RAM disks are much faster than conventional disks because they involve no mechanical motion. They are separate from main memory so they do not occupy space needed by the operating system or applications. Reference times to individual data items are uniform rather than widely variable as with conventional disks.

RAM disks are much more expensive than regular disks. Most forms of RAM in use today are volatile ie., they lose their contents when power is turned off or when the power supply is interrupted. Thus RAM disk users should perform frequent backups to conventional disks. As memory prices continue decreasing, and as capacities continue increasing it is anticipated that RAM disks will become increasingly popular.

OPTICAL DISKS:

Various recording techniques are used. In one technique, intense laser heat is used to burn microscopic holes in a metal coating. In another technique, the laser heat causes raised blisters on the surface. In a third technique, the reflectivity of the surface is altered.

The first optical disks were write-once-read-many(WORM) devices. This is not useful for applications that require regular updating. Several rewritable optical disk products have appeared on the market recently. Each person could have a disk with the sum total of human knowledge and this disk could be updated regularly. Some estimates of capacities are so huge that researchers feel it will be possible to store 10^21 bits on a single optical disk.
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