- Primary storage of a computer system has limited capacity and is volatile hence most computer systems use additional memory called auxiliary memory or secondary storage
- Secondary storage of a computer system is non-volatile and has low cost per bit stored but operates generally at speeds far slower than of primary storage
- A sequential- access storage device is one in which the arrival at a desired location is preceded by sequencing through other locations so that access time varies according to location on the other hand a random-access storage device is one in which we can reach and access any storage location at random and approximately equal access time is required for accessing each location
- Magnetic tape is the most popular storage medium for large data sets which applications need to access and process sequentially magnetic tape medium is a plastic ribbon usually ¼-inch or ¼-inch wide its surface has a coating (such as iron oxide or chromium dioxide) that can record data by magnetization tape ribbon itself is stored in reels or a small cartridge or cassette
- The four commonly used types of magnetic tapes are ½-inch tape reel, ½-inch tape cartridge ¼-inch streamer tap and 4-mm digital audio tape
- Magnetic disk is the most popular direct-access secondary storage device a magnetic disk is a thin circular plate/platter made of metal or plastic and coated on both sides with a recording material (such as iron oxide) which can be magnetized the disk itself is stored in a specially designed protective envelope or cartridge or several of them are stacked together in a sealed contamination-free container
- All types of magnetic disks are classified broadly into two types floppy disks and hard disks floppy disks are made of flexible plastic, whereas hard disk are made of rigid metal depending on the type of packaging hard disks are classified further into zip/Bernoulli disks disk packs and Winchester disks
- An optical-disk storage system consists of a metallic pr plastic disk coated with a highly reflective material it uses laser beam technology for reading/writing of data from to disk surface
- Commonly used types of optical disks are CD-ROM WORM (CD-R), CD-RW, and DVD. CD-ROM disk is a read-only storage medium WORM (CD-R) disk is a write-once read many storage medium CD-RW disk allows erasing of previous content and writing on it multiple times DVD is a standard format for distribution and interchange of digital content in both computer based and consumer electronics products the format enables storage of various types of large multimedia data on a new and improved version of CD, which works on optical disk technology
- Memory storage devices use flash memory technology for secondary storage devices two popular memory storage devices are flash drive (pen drive) and memory card (SD/MMC)
- Mass storage devices use multiple units of a storage media to create a single secondary storage device commonly used types of mass storage devices are disk array (also known as RAID), automated tape library and CD-ROM jukebox
- Data backup is the process of creating a copy of some data from an on-line storage device to a secondary storage backup device so that if the original data on the on-line storage device is corrupted or lost by accident it can be retrieved from the backup device and stored back on the on-line storage device
- Two basic types of backup are-full backup-an incremental backup full backup method copies all data from an on-line secondary storage device to a backup device at the time of backup whereas incremental backup method copies only newly created files or files changed since the last backup
- Yet another classification of secondary storage devices is on-line near-line and off-line storage devices an on-line storage device is under direct control of the processing unit of the computer to which it belongs whereas an off-line storage device is not under direct control of any computer a near-line storage device is slower than on-line storage but data can still be accessed from it without any human intervention (off-line storage requires human intervention)
- As a single type of storage is not superior in all aspect (speed of access capacity and cost ), most computer systems build their storage solutions by making use of a hierarchy of storage technologies such a storage solution called hierarchical storage system ( H S S ), is a cost effective way of designing computer systems with very large storage capacities a typical storage hierarchy pyramid includes cache memory main memory on-line secondary storage devices near-line mass storage devices and off-line storage devices
