Chapter 6

1. Chapter 6 Storage and Multimedia

2. Secondary Storage • Secondary storage is storage is separate from the computer itself • Benefits • Space-generally can store much more than primary storage • Reliability-data is safe because storage is relatively safe • Convenience-can locate data quickly • Economy-less expensive than the non-computerized options

3. Magnetic Disk Storage • Magnetic media based on a technology where data is represented as a magnetized spot on the spinning disk (magnetized spot = 1, non-magnetized=0) • Magnetic spots are transmitted to electrical impulses when the data I being read • Electrical impulses from the processor are converted to magnetic spots when writing takes place

4. Floppy Disk • Made from flexible mylar coated with iron oxide • Most computers today have floppy disks that hold 1.44mb and are 3.5 inches • Has protection of the hard disk • Small size makes it very portable • Sometimes used to back up files • New disk formats include • HIFD (200 MB) • SuperDisk (120 and 240MB) • Zip Drive-largest market share can store up to 750MB

5. Hard Disks • A rigid platter coated with magnetic oxide that can be magnetized to represent data • Several platters can be assembled in to a disk pack • Each disk can be written to on the top and bottom, although many times to top of the top disk and the bottom of the bottom disk are left blank

6. Hard Drives • The hard drive actually refers to the device that enables reading and writing to the disk • A computer can have one to hundreds of disk drives • In the disk pack, all disks rotate at the same time even though only one is being read/written to

7. How the Hard Drive Works • The access arm, which contains the read/write head moves in to position over the magnetic media • The read/write access sits 1 millionth of an inch above the media but does not touch the media • If it does, data is destroyed in a head crash • Read/write heads usually travel in pairs, one pointing up and one pointing down

8. Hard drive storage capacity • Storage is growing and getting cheaper • Current hard drives for PCs are available now in the 250GB range (approx $300) but most common still in the 120GB range ($150ish) • External drives (more expensive) sit outside the computer case whereas internal drives are generally installed inside the case • Removable hard drives such as the Jaz drive are not as common due to expense

9. RAID • Redundant Array of Independent disks • Used to protect data • Two types: • RAID level 0-uses data striping which spreads data from a single file across several drives; allows simultaneous read/write which improves performance • RAID level 1- also called disk mirroring because it makes a mirror image of the data on a second drive

10. How Data is Organized • Track • Refers to the circular portion of the disk • Number of tracks depends on the size of the disk (160 for floppies, 1000s on hard drives) • Sector • Tracks are divided into sectors of a fixed number of bytes, usually 512 • Generally, data is located according to its surface (top or bottom), its track, then its sector on that track

11. Zone Recording • To increase storage, disks are divided into zones and outer zones have more sectors assigned per track • However, note that data stored in outer tracks is less efficient because of the circular nature of the disk

12. Clusters • Groupings of adjacent sectors that are treated as a unit of storage by the OS, generally 2 to 8 sectors • Each file is stored in an integral number of clusters • Example: Our OS clusters 2 sectors as a cluster and our file is 1025 bytes. How many clusters will our file require for storage?

13. Answer…. • Our file will required 2 clusters • One cluster holds 2 sectors which generally are 512 bytes a piece (1024 total) • Our file is 1025 and the data is stored in integral numbers of clusters, so the OS will allocate the second cluster just to accommodate that last byte

14. Cylinders • Defined as the track on each surface beneath the read/write head at a given position of the read/write arms • What this really means is what is under the read/write head on the top of the disk and what is under the read write arm on the bottom of the disk • Data is stored in cylinders to allow for vertical access to data which requires less physical movement (and time!) of the read write arm

15. Disk Access Speed • Access time is the time needed to access data on the disk, generally measured in milliseconds • Three factors: • Seek time-time to get the arm into place over the track • Head Switching-Activation of either the top or bottom head since only one can read or write at a time • Rotational Delay-data must spin under the read/write head

16. Data transfer • After data is accessed, data transfer is either from memory to disk (write) or from disk to memory(read). • Data transfer rate, another measure of performance, is measured in MB/sec and measures how fast data is transferred once found

17. Disk Cache • To improve performance, hard drive reads data around what it was told to read and stores it in disk cache. • Next time the Control Unit tells it to read data, it checks this cache first in hopes of not having to perfrom another physical read • Speeds up performance if it’s in cache

18. Optical Disk Storage • A fancy way to say CDs and DVDs • Optical disks work like this: • When writing, laser hits metal and makes little dots • Then to read, laser hits the disk and the light reflects back from the pits • Disks are categorized by their read/write nature • Read only says it all, you can only read • Write once, read many also called WORM means it can be written once, and then read til the cows come home • Magneto-optical-can be rewritten and read many times

19. Compact Disks • Compact Disks generally store 700MB • CD-ROM versus a regular old CD means Compact Disk Read Only Memory…it’s for a computer and it can only be read • CD-R-write once and can be read by any CD drive • CD-RW-read write many times; may not work on all CD drives

20. DVD ROM • Digital Versatile Disk • A CD on steroids, has same functionality as its CD counterpart • Holds up tp 4.7 GB • Uses shorter laser to read more densely packed dots • Disks have two layers per side, a clear and opaque

21. Writing DVDs • Things are a little cloudier when writing DVDs • Standards are not solid as technology emerges • DVD-RAM-writable • DVD-R -writable • DVD+R –writable • DVD-RW-rewritable • DVD+RW-rewritable • New standards such as Blu-Ray use different laser for even greater storage • More info at http://www.dvdplusrw.org/faq/faq_general.html#1

22. Multinedia • Software that presents data in many different media such as images, sound, and video • Made possible by larger storage media such as the CDROM • What you need for the multimedia experience • CD or DVD Drive • Sound card and speakers • Other options include high speed hard drives, high resolution graphics cards, and software such as an MPEG player (video format stands for Motion Picture Experts Group)

23. Some examples of multimedia • Many educational software titles today are multimedia applications • Electronic versions of journals and books • Business applications including training materials, database applications, marketing software • Educational uses unlimited to increase the education experience for students through different media

24. Magnetic Tape Storage • Tapes that look similar to standard cassette tapes with a magnetic coating • Come in different sizes with different storage capacities • Capacity is expressed in terms of density CPI (characters per inch) and BPI (bytes per inch) • Optical disks have reduced the need for tape and they are mainly used for inexpensive, qucik backup options

25. Backup Systems • Backup system is defined as storing important data in another location in case of failure • Can range from copying your term paper to a floppy disk to a business backing up an entire database on a tape system • Tape is used often because of its storage capacity and its low cost

26. Organizing and Accessing Stored Data • Levels of Data: • Character is a single letter, digit, or special character • Field contains a set of related characters such as a single name, phone number, or grade • Record is set of related fields. Your student record contains your name, personal info, grades, etc. • Files are a collection of related records. All of the student’s records would be a file. • Database is a collection of all of the files. Class information, faculty information, students records and all make up the database.

27. Key Fields-Identifying Records • One field in the record is used to uniquely identify the records, called the key field • Names, product descriptions, phone numbers and addresses all make lousy key fields because they are rarely unique. • Most of the time a product ID or social security number or an arbitrarily assigned number will act as the key

28. Overview of File Plan • Determine how data will be accessed • Organize data according to access plan • Determine storage mechanism according to organization plan • Access determined by the immediate need of the data; direct if needed immediately • If immediate, requires indexing and transaction processing

29. Methods of File Organization • Sequential-records are stored in order and processed in order by key field • Direct File Organization- aka random file organization allows for direct or random access to data anywhere in the file • Requires a DASD, direct access storage device • Allows updating in place, does not require merging of two datasets to make an update • Hashing or randomizing algorithm applies math to the key and sometimes accidentally results in synonyms, two files with the same assigned address and results in a collision

30. One More Method of File Organization • Indexed file organization • Combines the best of sequential and direct file • Has a table of storage addresses • Serves as a directory to that disk

31. Processing the Data • Transactions are instances where a record needs be created, altered, or deleted • Transaction file keeps list of changes until the master file is updated • Batch processing means all of the transactions are held in the transaction file and updated all at once

32. Transaction Processing • Real-time processing • Data is updated or changed very soon after each individual transaction • Common in banking industry, when you use your Texan card, automatic inventory systems, etc.

33. How to choose a method • If data is needed immediately, transaction is the way to. Files are always up to date with new information. • Batch processing is more efficient. It can be scheduled as a background job and run when no one else is on the system