PHYSICAL STORAGE MEDIA – A BRIEF INTRODUCTION

The physical storage media are divided based on few basic features as follows:

  1. Data access speed- The speed at which each data in the storage medium is accessed.
  2. Cost- This is calculated per unit data.
  3. Reliability- The system must be capable of withstanding power failures, system crashes and even the physical disk failures.
  4. Types of storage- The physical data storage can be commonly divided into two types namely volatile and non-volatile storage. In volatile storage, the data is temporary and gets lost as soon as the power is turned off. In non-volatile storage, the data exists even after the power gets turned off. This is a form of permanent memory.

Important Definitions:

Cache:

  • Cache is the fastest and quickly accessible data storage centre nearer to the primary memory.
  • It is costly and volatile.
  • Cache memory is managed by hardware of the computer system.

Main memory:

  • The speed of main memory is in the order of nanoseconds- 10 to 100 nanoseconds are needed to fetch a particular data. One nanosecond is 10–9 seconds.
  • It is not possible to store entire database in the primary memory as it is very small and also highly costly.
  • In recent times, the primary memory capacity is up to gigabytes.
  • There are risks taken to increase the primary storage capacity and also to decrease the costs per byte. For every 2 or 3 years, it is made possible by a factor of 2.
  • Volatile memory — The content of the primary memory are lost when power is turned off or during physical failures like system crash.

 Flash memory:

  • Flash memory is capable of withstanding power failures and it is non-volatile. The content of flash memory is not lost when power is turned off.
  • It provides only write-once possibilities, that is the data written at a particular location can be deleted (erased) and written again in the same location but never rewritten anywhere else.
  • Flash memory supports limited write and erase cycles of 10K 1M.
  • If you want to erase memory, you have to deal with the entire chunks.
  • Read operations at flash memory are very fast as primary memory.
  • Write operations may take few microseconds are comparatively slower. This is mainly due to the reason that data erasing operation is much slower in flash memory.

Types of Flash memory

  • There are two main kinds of flash memory namely NOR and NAND flash.

NOR Flash:

  •  It offers quick read operations but has very low capacity with low erase operation.
  • NAND flash is opt for storing programs in embedded systems.

NAND Flash:

  • This provide page by page read (only one page at a time) but multiple page erasing at once.
  • It has capacities of gigabytes- high capacity compared to NOR flash.
  • In majority of the portable devices like pendrives, laptops- NAND flash is used for data storage.

Magnetic-disk storage

  • In magnetic disks, the data storage is on spin disks and read/write operations occur magnetically.
  • Magnetic disks are usually preferred storage mechanisms for long-term data storages.
  • Magnetic disks have large memory to hold the entire database contents.
  • When you want to access a particular data, it must be moved to primary memory and again it can be written back to the magnetic disk.
  • Magnetic disks provide random and direct data access unlike magnetic tape storage.
  • It is a non-volatile memory which is capable of withstanding system and power failures.
  • Only when disk is destroyed, data gets lost, though disk failure is very rare.

Optical storage media

  • Optical disks are non-volatile kind of data storage where data is optically read from disk spins with the help of LASER.
  • The most popular kinds of optical storage media are CD-ROMs and DVDs. CD-ROMs of 640 MB and DVDs of range 4.7 to 17 gigabytes are widely used.
  • It offers write-once and read-many (WORM) types of optical media for archiving CDs and DVDs.
  • Many kinds of write versions are available in optical disks namely CD-RW, DVD+RW, DVD-RW and DVD-RAM).
  • Compared to magnetic disks, both read and write operations are slower.
  • For storing huge data volumes, Juke boxes containing removable disks, loading/ unloading mechanisms and drives are available.

Tape storage media

  • Tape storage is of non-volatile type and is preferred mainly for data backup in cases of disk failures and also for data archives.
  • Since it provides sequential data access, it is slower compared to magnetic disks- magnetic disks offer direct, random access of data.
  • Capacity of magnetic tapes is high of 40 to 300 gigabytes.
  • Storage costs of tapes are cheaper than magnetic disks. Tapes can be separately removed from tape drives as drives are costly.
  • For storing large data volumes, juke boxes are available. They are capable of storing terabytes and even perabytes of data.