Storage Device Computer Component : Storage Device (External Memory, Secondary Memory, Secondary Storage) Storage Types Magnetic Types Optical Types

Storage Types ๏ Technology Types Magnetic (Disk & Tape) Optical (Disk) ๏ Access Method Types Sequential Access (Magnetic Type) Direct Access (Magnetic & Optical Disk) ๏ Shape Types Disk (Fixed, Removable) Tape (Cartridges, Cassette)

Magnetic Types ๏ Magnetic Disk Definition Data Organization, Formatting and Disk Layout Method Physical Characteristic Disk Performance Parameters ๏ Magnetic Tape Definition Data Organization and Formatting

Magnetic Disk Definition A Disk –Circular platter constructed of metal or of plastic coated with a magnetizable material. –Data are recorded on and retrieved from the disk via a conducting coil, head. –To write, electricity flows through a coil produces a magnetic field then magnetic patterns (positive or negative) are recorded. –To read, magnetic field moves relative to a coil produces an electrical current of the same polarity as it recorded in the coil

Magnetic Disk Data Organization Tracks - the organization of data on the platter in a concentric set of rings, each track is the same width as the head. Data are transferred to and from a disk in blocks Sectors - data are stored in these block- size regions that maybe either fixed or variable length. Adjacent tracks or sectors are separated by gaps Density - bits per inch, increases from outer track to inner track Clusters - groups of sectors that use to store a file Cylinders - tracks in the same position of each side in multiple platter

Magnetic Disk Formatting A process that prepare a disk for use Create tracks and sectors on disk Record information of disk number of byte/sector and sector/track File Allocation Table (FAT) Root folder (directory) A bit near the center of a rotating disk travels past a fixed point (e.g.. read- write head) slower than a bit on the outside.

Magnetic Disk Disk Layout Methods Multiple Zone Recording devide disk into a number of zones (~16) within a zone, the number of bits per track is constant zones farther from the center contain more bits (more sectors) than the closer. the time to move a head for reads and writes vary from one zone to another. Pro : increasing capacity Constant Angular Velocity (CAV) increasing space between bits bits can be scanned at the same rate devide disk into a number of pie-shaped sectors and a series of concentric tracks Pro : block can be directly address by track and sector Con : long outer track can store data in the same amount as short inner tracks

Magnetic Disk Physical Characteristics Head Motion –Fixed head (one per track) –Movable head (one per surface) Disk Portability –Nonremovable disk –Removable Sides –Single sided –Double sided Platters –Single platter –Multiple platters Head Mechanisms –Contact (Floppy) –Fixed gap –Aerodynamic gap (Winchester)

Magnetic Disk Disk Performance Parameters Average Access Time ( Ta ) –Seek time ( Ts ) –Latency time ( r - Rotational delay ) –Transfer time ( T ) Ta = Ts T 2r

Magnetic Disk Disk Performance Parameters Seek Time ( Ts ) –The time required to move the head to the required track –A movable-head system moves the head –A fixed-head system electrically selects one head m = constant depends on the disk drive n = number of tracks traversed s = startup time Ts = m x n + s

Magnetic Disk Disk Performance Parameters Latency Time ( r - Rotational delay ) –The time required to move the beginning of the sector to the head –Floppy disk rotates rpm, average delay will be ms –Other disks rotate rpm (1 revolution per 16.7 ms to 1 revolution per 4 ms ), average delay will be 8.3 ms

Magnetic Disk Disk Performance Parameters Transfer Time ( T ) –The time to transfer data to or from the disk –Depend on the rotation speed of the disk b = number of bytes to be transferred N = number of bytes on a track r = rotation speed, in revolutions per second T = b rN

Ts = 20 ms, transfer rate = 1 MB/s, 512 bytes/sector, 32 sectors/track, a file consists of 256 sectors for 128 KB Magnetic Disk A Timing Comparison A sequential organization (8 adjacent tracks x 32 sector) Average seek 20.0 ms Rotational delay 8.3 ms Read 32 sectors 16.7 ms 45 ms Read 1 track = 25 ms Ta = x 25 = 220 ms A random organization Average seek 20.0 ms Rotational delay 8.3 ms Read 1 sector 0.5 ms 28.8 ms Ta = 256 x 28.8 = 7373 ms

Magnetic Tape Definition The medium is flexible polyester tape coated with magnetizable material. The tape and tape drive are analogous to a home tape recorder system. Use the same reading and recording techniques as disk system. Tapes used to be packaged as open reels that have to be threaded through a second spindle for use. Today all tapes are housed in cartridge

Magnetic Tape Data Organization & Formatting Structured as a number of parallel tracks running length-wise. Serial recording - data are laid out as a sequence of bits along each track. Data are read and written in contiguous blocks, physical records. Block are separated by interrecord gaps. The tape is formatted to assist in locating physical records. Data are recorded serially along individual tracks. Blocks in sequence are stored on adjacent tracks.

Magnetic Tape Characteristics A tape drive is a sequential-access device. Read all of the front records sequentially to get the desired one. Tape is in motion only during read or write operation. It is the first kind of secondary memory. Usually use for backup data from the system. Lowest-cost, slowest-speed memory

Optical Types ๏ Compact Disk (CD) CD-ROM CD Recordable CD Rewritable ๏ Digital Versatile Disk (DVD)

Compact Disk CD-ROM CD digital audio system was introduced in 1983 as a nonerasable disk that can store more than 60 mins of audio information. Audio CD and CD-ROM share a similar technology. CD-ROM player are more rugged and have error correction device. The disk is formed form a resin. Digitally recorded information is imprinted as a series of microscopic pits on the surface and then coated with a highly reflective surface (aluminum or goal) and then coated with acrylic.

Compact Disk CD-ROM Layout Method CD and CD-ROM contain a single spiral track, begin near the center and spiraling out to the outer edge of the disk. Sectors near the outside are the same length as those near the inside. The information is packed in segments of the same size and these are scanned at the same rate by rotating disk at a variable speed, known as Constant Linear Velocity (CLV). The capacity of a track and the rotational delay increase for positions nearer the outer edge of disk.

Compact Disk CD-ROM data Organization Data are organized as a sequence of block A typical block consists of –Sync : identifies the beginning of a block (12 bytes). –Header : contain block address and mode byte (4 bytes). Mode 0 - a blank data field Mode 1 - the use of an error-correcting code and 2048 byte of data Mode bytes of data with no error- correcting code –Data : User data –Auxiliary : Additional user data in mode 2. In mode 1, 288-byte error-correcting code (ECC).

Compact Disk CD-ROM Pros & Cons Pros: The optical disk can be mass replicated inexpensively. The optical disk is removable, allowing the disk itself to be used for archival storage. Cons: It is read-only and cannot be updated. It has an access time much longer than that of a magnetic disk.

Compact Disk CD Recordable (CD-R) A disk is prepared in the way that it can be subsequently written once with a laser beam with a more expensive disk controller than for CD-ROM The medium is similar to (not identical) that of CD or CD-ROM. Information is recorded by the the pitting of the surface, which changes reflectivity. The disk can be read on a CD-R or CD- ROM drive. It provides a permanent record of large volumes of user data.

Compact Disk CD Rewritable (CD-RW) The disk can be repeatedly written and overwritten. Current materials can be used for 500,000-1,000,000 erase cycles. It can be rewritten and used as a true secondary storage. It is higher reliability and longer life than magnetic disks.

Digital Versatile Disk The industry has a found an acceptable replacement for the analog VHS video tape and replace the CD-ROM in PCs and servers. It can be randomly accessed like audio CDs. It can store data with vivid quality, currently 7 times of CD-ROM. The greater capacity is due to : –Bits are packed more closely on a DVD (4.7 GB). –It employs a dual-layer (8.5 GB), double sided disk (17 GB). DVDs come in recordable, rewritable as well as read-only version.