1 Chapter 6 Storage and Multimedia: The Facts and More

2 Benefits of Secondary Storage Space Reliability Convenience Economy

3 Magnetic Disk Storage Data represented in magnetic spots Each spot is one bit Magnetized spot = 1 Non-magnetized spot = 0 Hard disks, zip disks, and floppy disks are magnetic storage

4 Optical Disk Storage Data written and read with a laser Write-once, read-many (WORM) commonly used for backup Some forms of optical storage allow the user to write multiple times

5 Compact Disks (Optical) Compact Disk Read-Only Memory (CD-ROM) 700 MB – about 450 floppies worth of storage Compact Disk-Recordable (CD-R) Write (record) once Compact Disk-Rewritable (CD-RW) Record multiple times Many PC’s now come with CD-R or CD-RW drives

6 Digital Versatile Disk (Optical) Digital Versatile Disk Read-Only Memory (DVD-ROM) 4.7 GB – about the same as 7 CDs common storage format for movies replaces VHS tape format Writable version of Digital Versatile Disk (DVD-RAM)

7 Magnetic Tape Storage Data represented in magnetic spots Primarily for backup Density cpi = characters per inch bpi = bits per inch Inexpensive Sequential access to data Slow

8 Diskette Flexible plastic with metallic coating 3 ½ inch 1.44 MB Low capacity – small files Hard plastic jacket Portable

9 High-Capacity Portable Disks Can hold larger files Zip disk MB or 250 MB Super Disk MB Jazz Disk - 1 or 2 GB Data compression can be used to store more data in the same area

10 Hard Disk Rigid platter with metallic coating Typical PC has 20 GB to 100 GB hard drive Non-portable, permanently mounted inside the system unit

11 Redundant Array of Independent Disks (RAID) Disk mirroring Simplest form of RAID reduces possibility of data loss maintains multiple copies of the same data on different drives Data striping Higher level of RAID Spreads data across several disks Uses check disk to rebuild lost data

12 How Disk Drives Work Disks rotate, then access arms move read/write heads in or out All access arms move together Only one read/write head can operate at a time Read = Data transfer from the drive to memory Write = Data transfer from memory to the drive

13 Logical Layout of a Disk Tracks Concentric circles Standard floppy has 80 tracks on each side Hard disks may have 1000 or more tracks per surface

14 Logical Layout of a Disk Sector Fixed size: usually 512 bytes Zone recording assigns more sectors to outer tracks ↓↓↓↓↓↓

15 Logical Layout of a Disk Cluster 2 to 8 adjacent sectors Data is read into memory one cluster at a time Data is stored one cluster at a time # of sectors in a cluster depends on the OS

16 Logical Layout of a Disk Cylinder Same track on each platter Large files stored in cylinder Access arms all move together Faster access if data is on the same cylinder

17 Disk Access Speed Access time (3 factors) Seek time Head switching Rotational delay Data transfer rate Disk cache

18 Data Organization Character – letter, digit, or symbol ($, ?, *, etc) Field – set of related characters Record – collection of related fields Key field – a field that is different for each record File – collection of records Database – collection of files

19 Data Access Sequential Records stored in order by a key field Slow retrieval Used on tape storage

20 Data Access Direct Also called random access Hashing algorithm determines the address of a record Collision – if an address is already used, another record must find a different address Cannot be used on tape

21 Data Access Indexed Compromise between sequential and direct Records stored sequentially An index tells where to find each key

22 Data Processing Batch processing Collects transactions and processes them later Master file Transaction file Used when changes don’t have to be processed immediately Bank statements, Payroll, etc

23 Data Processing Transaction processing Real-time, happens now Online – computers / terminals connected directly

24 Multimedia Hardware Requirements CD-ROM, DVD-ROM Drive speed Sound card Speakers MPEG – Motion Picture Experts Group Widely accepted format for storing video