CHAPTER 7 Storage 5. Tape课件.ppt

1、CHAPTER 7 Storage,Tape,Magnetic Disks,Magnetic Disks,Good: cheap, fairly reliable Primary storage, memory swapping Bad: Can only read/write an entire sector Can not be directly addressed as main memory Disk access time Queuing delay Wait until disk gets to do this operation Seek time Head moves to c

2、orrect track Rotational latency Correct sector must get under the head Data transfer time and controller time,Trends for Magnetic Disks,Capacity: doubles in approx. one year Average seek time 5-12ms, very slow improvement Average rotational latency (1/2 full rotation) 5,000 RPM to 10,000 RPM to 15,0

3、00 RPM Improves slowly, not easy (reliability, noise) Data transfer rate Improves at an OK rate New interfaces, more data per track,Magnetic Disks,What is a head crash?,Spinning creates cushion of air that floats read/write head above platter,Occurs when read/write head touches platter surface,A smo

4、ke particle, dust particle, or human hair could render drive unusable,Clearance between head and platter is approximately two-millionths of an inch,Magnetic Disks,What is a disk cache?,Portion of memory that processor uses to store frequently accessed items,Magnetic Tapes,Very long access latency Mu

5、st rewind tape to correct place for read/write Used to be very cheap Its just miles of tape! But disks have caught up anyway Used for backup (secondary storage) Large capacity & Replaceable,Magnetic Tape,External storage Provides sequential access Information stored in sequence Slower than disks whi

6、ch provide direct access Magnetic tape streamers or tape cartridges used by both mainframes and microcomputers,Magnetic Tape,What is magnetic tape? Thin layer of material capable of storing a magnetic signal Usually contains Iron Oxide,Protected by backing layer called the “Substrate”,Examples of Ma

7、gnetic Media,Some you are probably familiar with: Cassette tapes VHS video tape Computer Tape 8-track DAT,Pros and Cons of Magnetic Tape,Inexpensive to store large amounts of information. A reel of tape could store as much as 225 MB in the early 1980s. No other storage technology could compare to th

8、e price of tape Sequential access Slow access Tapes are highly susceptible to magnetic fields and changes in temperature and humidity Tapes are also susceptible to physical damage Tape miss-feeds Magnetic particle instabilities Substrate deformation,Magnetic Tape Uses Today,Today, tape based systems are usually used for backup purposes only. Tape still provides the most cost effective method of storing larger amounts of backup information Reliable as long as temperature is kept low and humidity conditions are kept right. Altho