Device Management - Device Types & Storage Media (PDF)

Device Management Learning Objectives After completing this chapter, you should be able to describe: Features of dedicated, shared, and virtual devices Differences between sequential and direct access media Concepts of blocking and buffering and how they improve I/O performance...

Device Management Learning Objectives After completing this chapter, you should be able to describe: Features of dedicated, shared, and virtual devices Differences between sequential and direct access media Concepts of blocking and buffering and how they improve I/O performance Roles of seek time, search time, and transfer time in calculating access time Differences in access times in several types of devices Critical components of the input/output subsystem, and how they interact Strengths and weaknesses of common seek strategies, including FCFS, SSTF, SCAN/LOOK, C-SCAN/C-LOOK, and how they compare Different levels of RAID and what sets each apart from the others Types of Devices Dedicated Devices Shared Devices Device assigned to one job at Device assigned to several a time processes For entire time job is active (or Example: direct access storage until released) device (DASD) Example: tape drives, printers, and Processes share DASD plotters simultaneously Requests interleaved Disadvantage Inefficient if device is not used Device manager supervision 100% Controls interleaving Allocated for duration of job’s Predetermined policies determine execution conflict resolution 3 Types of Devices (cont’d) Virtual Devices Storage media Dedicated and shared device Two groups combination Sequential access media Dedicated devices transformed Records stored sequentially into shared devices Direct access storage devices Example: printer (DASD) Converted by spooling program Records stored sequentially Records stored using direct access Spooling files Speeds up slow dedicated I/O Vast differences devices Speed and share ability Example: universal serial bus (USB) controller Interface between operating system, device drivers, applications, and devices attached via USB host Sequential Access Storage Media Magnetic tape Early computer systems: routine secondary storage Today’s use: routine archiving and data backup Records stored serially Tape density: characters recorded per inch Record length determined Depends upon storage method (individual by application program or blocked) Record identified by Tape reading/writing mechanics position on tape Tape moves under read/write head when Record access needed Tape mount Fast-forwarded to record Time-consuming process 5 Sequential Access Storage Media (cont'd.) Interrecord gap (IRG) ½ inch gap inserted between each record Same size regardless of records it separates Blocking: group records into blocks Transfer rate: (tape density) x (transport speed) Interblock gap (IBG) ½ inch gap inserted between each block More efficient than individual records and IRG Sequential Access Storage Media (cont'd.) Sequential Access Storage Media (cont'd.) Blocking advantages Fewer I/O operations needed Less wasted tape Blocking disadvantages Overhead and software routines needed for blocking, deblocking, and record keeping Buffer space wasted When only one logical record needed Sequential Access Storage Media (cont'd.) Advantages Low cost, compact storage capabilities, good for magnetic disk backup and long-term archival Disadvantages Access time Poor for routine secondary storage Poor for interactive applications Direct Access Storage Devices (DASD) Directly read or write to specific disk area Random access storage devices Four categories Magnetic disks Optical discs Flash memory Magneto-optical disks Access time variance Not as wide as magnetic tape Record location directly affects access time Fixed-Head Magnetic Disk Storage Looks like a large CD or DVD Covered with magnetic film Formatted Both sides (usually) in concentric circles called tracks Data recorded serially on each track Fixed read/write head positioned over data Advantages Fast (more so than movable head) Disadvantages High cost and reduced storage Movable-Head Magnetic Disk Storage One read/write head floats Disk pack platter over disk surface Two recording surfaces Example: computer hard drive Exception: top and bottom platters Disks Surface formatted with concentric Single platter tracks Part of disk pack (stack of platters) Track number varies 1000+ (high-capacity disk) Track surface number Record access system Track zero: outermost concentric requirements circle on each surface Cylinder number, surface number, Center: contains highest-numbered record number track Arm moves over all heads in unison Slower: fill disk pack surface-by- surface Faster: fill disk pack track-by-track Virtual cylinder: fill track zero Movable-Head Magnetic Disk Storage (cont'd.) 13 Optical Disc Storage Design difference Magnetic disk Concentric tracks of sectors Spins at constant angular velocity (CAV) Wastes storage space but fast data retrieval Optical Disc Storage (cont'd.) Design features Optical disc Single spiralling track of same- sized sectors running from center to disc rim Spins at constant linear velocity (CLV) More sectors and more disc data Optical Disc Storage (cont'd.) Two important performance measures 1. Sustained data-transfer rate Speed to read massive data amounts from disc Measured in megabytes per second (Mbps) Crucial for applications requiring sequential access 2. Average access time Average time to move head to specific disc location Expressed in milliseconds (ms) Third feature Cache size (hardware) Buffer to transfer data blocks from disc CD and DVD Technology CD Data recorded as zeros and ones Pits: indentations Lands: flat areas Reads with low-power laser Light strikes land and reflects to photodetector Pit is scattered and absorbed Photodetector converts light intensity into digital signal CD and DVD Technology (cont'd.) CD-Recordable technology (CD-R) Requires expensive disk controller Records data using write-once technique Data cannot be erased or modified Disk Contains several layers Gold reflective layer and dye layer Records with high-power laser Permanent marks on dye layer CD cannot be erased after data recorded Data read on standard CD drive (low-power beam) CD and DVD Technology (cont'd.) CD-Rewritable technology (CD-RW) Data written, changed, erased Uses phase change technology Amorphous and crystalline phase states Record data: beam heats up disc State changes from crystalline to amorphous Erase data: low-energy beam to heat up pits Loosens alloy to return to original crystalline state Drives read standard CD-ROM, CD-R, CD-RW discs Drives store large quantities of data, sound, graphics, multimedia CD and DVD Technology (cont'd.) DVD technology (Digital Versatile Disc) CD-ROM comparison Similar in design, shape, size Differs in data capacity Dual-layer, single-sided DVD holds 13 CDs Single-layer, single-sided DVD holds 8.6 GB (MPEG video compression) Differs in laser wavelength Uses red laser (smaller pits, tighter spiral) DVDs cannot be read by CD or CD-ROM drives DVD-R and DVD-RW provide rewritable flexibility Blu-Ray Disc Technology Same physical size as DVD/CD Smaller pits More tightly wound tracks Use of blue-violet laser allows multiple layers 50GB-500GB 432 Mbps Formats: BD-ROM, BD-R, BD-RE Flash Memory Storage Electronically erasable programmable read-only memory (EEP) Nonvolatile and removable Emulates random access Difference: data stored securely (even if removed) Data stored on microchip card or “key” Compact flash, smart cards, memory sticks Often connected through USB port Write data: electric charge sent through floating gate Erase data: strong electrical field (flash) applied Magnetic Disk Drive Access Times File access time factors Seek time (slowest) Time to position read/write head on track Does not apply to fixed read/write head devices Search time Rotational delay Time to rotate DASD Rotate until desired record under read/write head Transfer time (fastest) Time to transfer data Secondary storage to main memory transfer Fixed-Head Devices Record access requires two items Track number and record number Access time = search time + transfer time Total access time Rotational speed dependent DASDs rotate continuously Three basic positions for requested record In relation to read/write head position DASD has little access variance Good candidates: low activity files, random access Blocking used to minimize access time Fixed-Head Devices (cont'd.) Movable-Head Devices Record access requires three items Seek time + search time + transfer time Search time and transfer time calculation Same as fixed-head DASD Blocking is a good way to minimize access time Components of the I/O Subsystem I/O Channel Programmable units Positioned between CPU and control unit Synchronizes device speeds CPU (fast) with I/O device (slow) Manages concurrent processing CPU and I/O device requests Allows overlap CPU and I/O operations Channels: expensive because so often shared Components of the I/O Subsystem (cont'd.) I/O channel programs Specifies action performed by devices Controls data transmission Between main memory and control units I/O control unit: receives and interprets signal Disk controller (disk drive interface) Links disk drive and system bus Entire path must be available when I/O command initiated I/O subsystem configuration Multiple paths increase flexibility and reliability Components of the I/O Subsystem (cont'd.) Communication Among Devices Problems to resolve Know which components are busy/free Solved by structuring interaction between units Accommodate requests during heavy I/O traffic Handled by buffering records and queuing requests Accommodate speed disparity between CPU and I/O devices Handled by buffering records and queuing requests Communication Among Devices (cont'd.) I/O subsystem units finish independently of others CPU processes data while I/O performed Success requires device completion knowledge Hardware flag tested by CPU Channel status word (CSW) contains flag Three bits in flag represent I/O system component (channel, control unit, device) Changes zero to one (free to busy) Flag tested using polling and interrupts Interrupts are more efficient way to test flag Communication Among Devices (cont'd.) Direct memory access (DMA) Allows control unit main memory access directly Transfers data without the intervention of CPU Used for high-speed devices (disk) Buffers Temporary storage areas in main memory, channels, control units Improves data movement synchronization Between relatively slow I/O devices and very fast CPU Double buffering: processing of record by CPU while another is read or written by channel Communication Among Devices (cont'd.) Management of I/O Requests I/O traffic controller I/O scheduler Watches status of devices, Same job as process scheduler control units, channels Allocates devices, control units, Three main tasks channels Determine if path available If requests greater than If more than one path available, available paths determine which one to select Decides which request to satisfy If paths all busy, determine when first: based on different criteria one is available In many systems Maintain database containing I/O requests not preempted unit status and connections For some systems Allow preemption with I/O request subdivided Allow preferential treatment for high-priority requests Management of I/O Requests (cont'd.) I/O device handler Performs actual data transfer Processes device interrupts Handles error conditions Provides detailed scheduling algorithms Device dependent Each I/O device type has device handler algorithm Management of I/O Requests (cont'd.) Device Handler Seek Strategies Predetermined device handler Determines device processing order Goal: minimize seek time Types First-come, first-served (FCFS), shortest seek time first (SSTF), SCAN (including LOOK, N-Step SCAN, C-SCAN, and C-LOOK) Scheduling algorithm goals Minimize arm movement Minimize mean response time Minimize variance in response time Device Handler Seek Strategies (cont'd.) FCFS On average: does not meet three seek strategy goals Disadvantage: extreme arm movement Device Handler Seek Strategies (cont'd.) Shortest Seek Time First (SSTF) Request with track closest to one being served Minimizes overall seek time Postpones traveling to out of way tracks Device Handler Seek Strategies (cont'd.) SCAN Directional bit Indicates if arm moving toward/away from disk center Algorithm moves arm methodically From outer to inner track, services every request in its path If reaches innermost track, reverses direction and moves toward outer tracks Services every request in its path Device Handler Seek Strategies (cont'd.) LOOK Arm does not go to either edge Unless requests exist Eliminates indefinite postponement Device Handler Seek Strategies (cont'd.) N-Step SCAN Holds all requests until arm starts on way back New requests grouped together for next sweep C-SCAN (Circular SCAN) Arm picks up requests on path during inward sweep Provides more uniform wait time C-LOOK Inward sweep stops at last high-numbered track request No last track access unless required Device Handler Seek Strategies (cont'd.) Best strategy FCFS best with light loads Service time unacceptably long under high loads SSTF best with moderate loads Localization problem under heavy loads SCAN best with light to moderate loads Eliminates indefinite postponement Throughput and mean service times SSTF similarities C-SCAN best with moderate to heavy loads Very small service time variances Search Strategies: Rotational Ordering Rotational ordering Optimizes search times Orders requests once read/write heads positioned Read/write head movement time Hardware dependent Reduces time wasted Due to rotational delay Request arrangement First sector requested on second track is next number higher than one just served Search Strategies: Rotational Ordering (cont'd.) Search Strategies: Rotational Ordering (cont'd.) Search Strategies: Rotational Ordering (cont'd.) RAID (Redundant Array of Independent Disks) Physical disk drive set viewed as single logical unit Preferable over few large-capacity disk drives Improved I/O performance Improved data recovery Disk failure event Introduces redundancy Helps with hardware failure recovery Significant factors in RAID level selection Cost, speed, system’s applications Increases hardware costs RAID (cont'd.) RAID (cont'd.) Level Zero Uses data striping (not considered true RAID) No parity and error corrections No error correction/redundancy/recovery Benefits Devices appear as one logical unit Best for large data quantity non-critical data Level One Uses data striping (considered true RAID) Mirrored configuration (backup) Duplicate set of all data (expensive) Provides redundancy and improved reliability Level Two Uses small stripes (considered true RAID) Hamming code: error detection and correction Expensive and complex Size of strip determines number of array disks Level Three Modification of Level 2 Requires one disk for redundancy One parity bit for each strip Level Four Same strip scheme as Levels 0 and 1 Computes parity for each strip Stores parities in corresponding strip Has designated parity disk Level Five Modification of Level 4 Distributes parity strips across disks Avoids Level 4 bottleneck Disadvantage Complicated to regenerate data from failed device Level Six Provides extra degree of error protection/correction Two different parity calculations (double parity) Same as level four/five and independent algorithm Parities stored on separate disk across array Stored in corresponding data strip Advantage: data restoration even if two disks fail Nested RAID Levels Combines multiple RAID levels (complex) Nested RAID Levels (cont'd.) Summary Device Manager Manages every system device effectively as possible Devices Vary in speed and sharability degrees Direct access and sequential access Magnetic media: one or many read/write heads Heads in a fixed position (optimum speed) Move across surface (optimum storage space) Optical media: disk speed adjusted Data recorded/retrieved correctly Summary (cont'd.) Flash memory: device manager tracks USB devices Assures data sent/received correctly I/O subsystem success dependence Communication linking channels, control units, devices SCAN: eliminates indefinite postponement problem Best for light to moderate loads C-SCAN: very small service time variance Best for moderate to heavy loads RAID: redundancy helps hardware failure recover Consider cost, speed, applications Summary (cont'd.)