DBMS-Memory Hierarchies and Storage Devices
In a modern computer system data resides and is transported throughout a hierarchy of storage media. The highest-speed memory is the most expensive and is therefore available with the least capacity. The lowest-speed memory is tape storage, which is essentially available in indefinite storage capacity.At the primary storage level, the memory hierarchy includes at the most expensive end cache memory, which is a static RAM (Random Access Memory). Cache memory is typically used by the CPU to speed up execution of programs. The next level of primary storage is DRAM (Dynamic RAM), which provides the main work area for the CPU for keeping programs and data and is popularly called main memory. The advantage of DRAM is its low cost, which continues to decrease; the drawback is its volatility (Note 1) and lower speed compared with static RAM. At the secondary storage level, the hierarchy includes magnetic disks, as well as mass storage in the form of CD-ROM (Compact Disk–Read-Only Memory) devices, and finally tapes at the least expensive end of the hierarchy. The storagecapacity is measured in kilobytes (Kbyte or 1000 bytes), megabytes (Mbyte or 1 million bytes), gigabytes (Gbyte or 1 billion bytes), and even terabytes (1000 Gbytes).
Programs reside and execute in DRAM. Generally, large permanent databases reside on secondary storage, and portions of the database are read into and written from buffers in main memory as needed. Now that personal computers and workstations have tens of megabytes of data in DRAM, it is becoming possible to load a large fraction of the database into main memory. In some cases, entire databases can be kept in main memory (with a backup copy on magnetic disk), leading to main memory databases; these are particularly useful in real-time applications that require extremely fast response times. An example is telephone switching applications, which store databases that contain routing and line information in main memory.
Between DRAM and magnetic disk storage, another form of memory, flash memory, is becoming common, particularly because it is nonvolatile. Flash memories are high-density, high-performance memories using EEPROM (Electrically Erasable Programmable Read-Only Memory) technology. The advantage of flash memory is the fast access speed; the disadvantage is that an entire block must be erased and written over at a time.
CD-ROM disks store data optically and are read by a laser. CD-ROMs contain prerecorded data that cannot be overwritten. WORM (Write-Once-Read-Many) disks are a form of optical storage used for archiving data; they allow data to be written once and read any number of times without the possibility of erasing. They hold about half a gigabyte of data per disk and last much longer than magnetic disks. Optical juke box memories use an array of CD-ROM platters, which are loaded onto drives on demand. Although optical juke boxes have capacities in the hundreds of gigabytes, their retrieval times are in the hundreds of milliseconds, quite a bit slower than magnetic disks (Note 3). This type of storage has not become as popular as it was expected to be because of the rapid decrease in cost and increase in capacities of magnetic disks. The DVD (Digital Video Disk) is a recent standard for optical disks allowing four to fifteen gigabytes of storage per disk.
Finally, magnetic tapes are used for archiving and backup storage of data. Tape jukeboxes—which contain a bank of tapes that are catalogued and can be automatically loaded onto tape drives—are becoming popular as tertiary storage to hold terabytes of data. For example, NASA’s EOS (Earth Observation Satellite) system stores archived databases in this fashion.
It is anticipated that many large organizations will find it normal to have terabytesized databases in a few years. The term very large database cannot be defined precisely any more because disk storage capacities are on the rise and costs are declining. It may very soon be reserved for databases containing tens of terabytes.
