Lecture 12
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The document discusses various types of storage media including magnetic tape, hard disks, optical disks, solid state drives, and discusses their characteristics. Magnetic tape provides sequential access and is inexpensive but has slow access times. Hard disks allow direct access via platters, tracks, sectors and cylinders but have mechanical components. Optical disks like CDs and DVDs use lasers to read and write high capacity data in a non-volatile format. Newer solid state drives have no moving parts and provide faster access times than hard disks. The document also discusses file allocation tables, storage hierarchies and defragmentation.
Lecture 12
- 3. Magnetic Tape 17-Sep-17 3 Characteristics No direct access, but very fast sequential access. Resistant to different environmental conditions. Easy to transport, store, cheaper than disk. Before it was widely used to store application data; nowadays, it’s mostly used for backups or archives.
- 4. Magnetic Tape 17-Sep-17 4 Characteristics A sequence of bits are stored on magnetic tape. For storage, the tape is wound on a reel. To access the data, the tape is unwound from one reel to another. As the tape passes the head, bits of data are read from or written onto the tape.
- 6. Tape Organization 17-Sep-17 6 Typically data on tape is stored in 9 separate bit streams, or tracks. Each data block is a sequence of contiguous records. A record is the unit of data that a user’s program deals with. The tape drive reads an entire block of records at once. Unlike a disk, a tape starts and stops. When stopped, the read/write head is over an inter-block gap.
- 7. Magnetic Tap 17-Sep-17 7 Advantages: Relatively inexpensive storage medium Large storage capacity. Compact and can easily be stored on library racks Old record can be erased and can be used again and again Reel of tape is also convenient way of carrying information from one place to another
- 8. Magnetic Tap 17-Sep-17 8 Disadvantages: Record can be accessed only serially Information cannot be edited Dust and uncontrolled humidity or temperature levels can cause tape-reading errors Data transmission is slow compared to disks Too much operator’s time would be required to load and unload tapes
- 9. Magnetic (Hard) Disk 17-Sep-17 9 A hard disk drive (HDD) is a non-volatile, random access device for digital data. The disk is mounted on a vertical shaft which rotates at a high and constant speed. Both surfaces of the disk are available for storage and each surface has a read/write head. An access mechanism moves the read/write head to the desired record to provide direct access.
- 10. Magnetic (Hard) Disk Continue 17-Sep-17 10 Disk contains concentric tracks. Tracks are divided into sectors A sector is the smallest addressable unit in a disk.
- 11. 17-Sep-17 11 The arm assembly is moved in or out to position a head on a desired track. Tracks under heads make a cylinder (imaginary!). Only one head reads/writes at any one time Block size is a multiple of sector size (which is often fixed). Magnetic (Hard) Disk Continue Platters Spindle Disk head Arm movement Arm assembly Tracks Sector
- 12. Continue 17-Sep-17 12 A cylinder is the set of tracks at a given radius of a disk pack. A cylinder is the set of tracks that can be accessed without moving the disk arm. All the information on a cylinder can be accessed without moving the read/write arm.
- 13. Estimating Capacities 17-Sep-17 13 Track capacity = # of sectors/track * bytes/sector Cylinder capacity = # of tracks/cylinder * track capacity Drive capacity = # of cylinders * cylinder capacity Number of cylinders = # of tracks in a surface
- 14. The Cost of a Disk Access 17-Sep-17 14 Average seek time: time required to move from one track to another Latency: time required for disk to rotate to beginning of correct sector Transfer time: time required to transfer a block of data to the disk controller buffer
- 15. Seek Time 17-Sep-17 15 The time required by the access arm to reach the specified cylinder is known as the seek time. The seek time depends upon the position where the arm assembly was at the time read/write command was received by the controller. The common average seek time is 20-30 ms. The maximum seek time is the time taken by the head assembly to reach the innermost cylinder from the outermost cylinder and vice versa
- 16. Latency Time 17-Sep-17 16 The time required for the rotating disk pack is to bring the correct sector to a position under the read/write head. This time depends on the speed with which the disk is rotating which is usually 3600 rpm. The average latency time is 8-15 ms. The sum of the average latency and seek time is known as the average access time. Average latency (r) = time for ½ disk revolution
- 17. Transfer Time 17-Sep-17 17 Transfer time is the time for the read/write head to pass over a block. The transfer time is given by the formula: number of sectors Transfer time = ------------------------------------------------------------- x rotation time track capacity in number of sectors The transfer time depends only on the speed at which the spindle rotates, and the number of sectors that must be read.
- 18. Optical Disk 17-Sep-17 18 Latest development in secondary storage is the optical disk It consists of a rotating disk which is coated with highly reflective material The data are written by focusing high power laser beam on the surface of the spinning disk Storage capacity is tremendous in comparison to magnetic disk Storage cost is low Two common types of optical disk: CD, DVD
- 19. Compact Disks 17-Sep-17 19 Optical format From 650 MB to 1 GB capacity Rotation speeds vary Types Read only: CD-ROM Write once: CD-R Rewriteable: CD-RW Picture CDs and Photo CDs
- 20. Digital Versatile Disc 17-Sep-17 20 Digital Versatile Disk or Digital Video Disk (DVD) Similar to CDs, but can store more data Types Read only Write once Rewritable
- 21. Magnetic vs. Optical Storage 17-Sep-17 21 Magnetic Storage Optical Storage Stores data in magnetic form. Stores data optically & used laser to read/write. It is affected by magnetic field. It is not affected by magnetic field. It has high storage capacity. It has less storage than hard disk. It doesn't use laser to read/write data. Data accessing is high as compared to floppy. Magnetic storage devices are ; Hard disk , Floppy disk, Magnetic tape etc. Optical storage devices are ; CD- ROM,CD-R, CD-RW, DVD etc.
- 22. Storage Hierarchy 17-Sep-17 22 Trade-off among the three key characteristics of storage devices: cost capacity access time. Smaller access time, greater cost per bit. Greater capacity, smaller cost per bit. Greater capacity, greater access time.
- 24. File Allocation Table (FAT) 17-Sep-17 24 A file allocation table (FAT) is a file system developed for hard drives that originally used 12 or 16 bits for each cluster entry into the file allocation table. It is used by the operating system (OS) to manage files on hard drives and other computer systems. It is often also found on in flash memory, digital cameras and portable devices. It is used to store file information and extend the life of a hard drive. The FAT file system was designed to reduce the amount of seeking and thus minimize the wear and tear on the hard disc. FAT16 was introduced in 1983 by IBM with the simultaneous releases of IBM's personal computer AT (PC AT) and Microsoft’s MS-DOS (disk operating system) 3.0 software.
- 25. New Technology File System (NFTS) 17-Sep-17 25 NTFS (NT file system; sometimes New Technology File System) is the file system that the Windows NT operating system uses for storing and retrieving files on a hard disk. NTFS is the Windows NT equivalent of the Windows 95 file allocation table (FAT) and the OS/2 High Performance File System (HPFS). However, NTFS offers a number of improvements over FAT and HPFS in terms of performance, extendibility, and security.
- 26. Solid-State Storage Devices 17-Sep-17 26 Solid-state storage (SSS) is a type of computer storage media made from silicon microchips. SSS stores data electronically instead of magnetically, as spinning hard disk drives (HDDs) or magnetic tape do. Solid-state storage can be found in three form factors: solid-state drives (SSD), solid-state cards (SSC) and solid-state modules (SSM). An important advantage of solid-state storage is that it contains no mechanical parts, allowing data transfer to and from storage media to take place at a much higher speed and providing a more predictable lifespan for the storage media. There are no moving parts, SSDs produce far less heat than HDDs.
- 27. Solid-State Drives (SSD) 17-Sep-17 27 A solid-state drive (SSD) is a nonvolatile storage device that stores persistent data on solid-state flash memory. Solid-state drives actually aren't hard drives in the traditional sense of the term, as there are no moving parts involved. A traditional hard disk drive (HDD) consists of a spinning disk with a read/write head on a mechanical arm called an actuator. An SSD, on the other hand, has an array of semiconductor memory organized as a disk drive, using integrated circuits (ICs) rather than magnetic or optical storage media. An SSD may also be referred to as a solid-state disk.
- 28. Defragmentation 17-Sep-17 28 A process that reduces the amount of fragmentation. Fragments are simply pieces of files that aren't placed next to each other on the drive. Defragmentation is the process of locating the noncontiguous fragments of data into which a computer file may be divided as it is stored on a hard disk, and rearranging the fragments and restoring them into fewer fragments or into the whole file. Defragmentation reduces data access time and allows storage to be used more efficiently. Some operating systems automatically defragment storage periodically; others require that the user occasionally use a special utility for this purpose. Windows 98 comes with a built-in defragmenter as a "system tool" that the user can run