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It6.6005 wk 3-2 - device management
1.
Β© NZSE
1 Β© AUT
2.
Learning Objectives ο Features
of dedicated, shared, and virtual devices ο Differences between sequential and direct access media ο Concepts of blocking and buffering and how they improve I/O performance ο Roles of seek time, search time, and transfer time in calculating access time ο Strengths and weaknesses of common seek strategies, including FCFS, SSTF, SCAN, LOOK and how they compare ο Different levels of RAID and what sets each apart from the others Β© NZSE 2 Β© AUT
3.
Β© NZSE
3 Β© AUT
4.
Types of Devices Dedicated
Devices Shared Devices ο Device assigned to one job at a ο Device assigned to several time processes ο For entire time job is active (or ο Example: direct access storage until released) device (DASD) ο Example: tape drives, printers, ο Processes share DASD and plotters simultaneously ο Requests interleaved ο Disadvantage ο Inefficient if device is not used ο Device manager supervision 100% ο Controls interleaving ο Allocated for duration of jobβs ο Predetermined policies execution determine conflict resolution Β© NZSE 4 Β© AUT
5.
Types of Devices Virtual
Devices ο Dedicated and shared device combination ο Dedicated devices transformed into shared devices ο Example: printer ο Converted by spooling program ο Spooling ο Speeds up slow dedicated I/O devices ο Example: universal serial bus (USB) controller ο Interface between operating system, device drivers, applications, and devices attached via USB host Β© NZSE 5 Β© AUT
6.
Types of Devices Storage
media ο Two groups ο Sequential access media ο Records stored sequentially ο Direct access storage devices (DASD) ο Records stored sequentially ο Records stored using direct access files ο Vast differences ο Speed and share-ability Β© NZSE 6 Β© AUT
7.
Sequential Access Storage
Media ο Magnetic tape ο Early computer systems: routine secondary storage ο Todayβs use: routine archiving and data backup ο Records stored serially ο Record length determined by application program ο Record identified by position on tape ο Record access ο Tape mount ο Fast-forwarded to record ο Time-consuming process Β© NZSE 7 Β© AUT
8.
Sequential Access Storage
Media ο Tape density: characters recorded per inch ο Depends upon storage method (individual or blocked) ο Tape reading/writing mechanics ο Tape moves under read/write head when needed Β© NZSE 8 Β© AUT
9.
Sequential Access Storage
Media ο Interrecord gap (IRG) ο Β½ inch gap inserted between each record ο Same size regardless of records it separates ο Blocking: group records into blocks ο Transfer rate: (tape density) x (transport speed) ο Interblock gap (IBG) ο Β½ inch gap inserted between each block ο More efficient than individual records and IRG Β© NZSE 9 Β© AUT
10.
Sequential Access Storage
Media Β© NZSE 10 Β© AUT
11.
Sequential Access Storage
Media ο Blocking advantages ο Fewer I/O operations needed ο Less wasted tape ο Blocking disadvantages ο Overhead and software routines needed for blocking, deblocking, and record keeping ο Buffer space wasted ο When only one logical record needed Β© NZSE 11 Β© AUT
12.
Sequential Access Storage
Media ο Advantages ο Low cost, compact storage capabilities, good for magnetic disk backup and long-term archival ο Disadvantages ο Access time ο Poor for routine secondary storage ο Poor for interactive applications Β© NZSE 12 Β© AUT
13.
Direct Access Storage
Devices ο Directly read or write to specific disk area ο Random access storage devices ο Four categories ο Magnetic disks ο Optical discs ο Flash memory ο Magneto-optical disks ο Access time variance ο Not as wide as magnetic tape ο Record location directly affects access time Β© NZSE 13 Β© AUT
14.
Fixed-Head Magnetic Disk
Storage ο Looks like a large CD or DVD ο Covered with magnetic film ο Formatted ο Both sides (usually) in concentric circles called tracks ο Data recorded serially on each track ο Fixed read/write head positioned over data ο Advantages ο Fast (more so than movable head) ο Disadvantages ο High cost and reduced storage Β© NZSE 14 Β© AUT
15.
Movable-Head Magnetic Disk Storage ο
One read/write head floats over disk surface (e.g. HDD) ο Disks can be single platter or part of disk pack (stack of platters) ο Disk pack platter ο Two recording surfaces ο Surface formatted with concentric tracks (1000+ tracks with high-capacity disks) ο Track surface number ο Track zero: outermost concentric circle on each surface ο Center: contains highest-numbered track ο Arm moves over all heads in unison Β© NZSE 15 Β© AUT
16.
Optical Disc Storage
ο Design difference ο Magnetic disk ο Concentric tracks of sectors ο Spins at constant angular velocity (CAV) ο Wastes storage space but fast data retrieval Β© NZSE 16 Β© AUT
17.
Optical Disc Storage
ο Design features ο Optical disc ο Single spiralling track of same-sized sectors running from center to disc rim ο Spins at constant linear velocity (CLV) ο More sectors and more disc data Β© NZSE 17 Β© AUT
18.
Optical Disc Storage ο
Two important performance measures ο Sustained data-transfer rate ο Speed to read massive data amounts from disc ο Measured in megabytes per second (MBps) ο Crucial for applications requiring sequential access ο Average access time ο Average time to move head to specific disc location ο Expressed in milliseconds (ms) ο Third feature ο Cache size (hardware) ο Buffer to transfer data blocks from disc Β© NZSE 18 Β© AUT
19.
CD, DVD, and
Blu-Ray Technology ο Data recorded as zeros and ones ο Pits: indentations ο Lands: flat areas ο Reads with low-power laser ο Light strikes land and reflects to photodetector ο Pit is scattered and absorbed ο Photodetector converts light intensity into digital signal Β© NZSE 19 Β© AUT
20.
CD, DVD, and
Blu-Ray Technology ο Contains several layers ο Gold reflective layer and dye layer ο Permanent marks on dye layer ο Data read on standard discs using a low-power beam ο Data written, changed, erased using a high-powered laser ο Uses phase change technology ο Amorphous and crystalline phase states ο Record data: beam heats up disc ο State changes from crystalline to amorphous ο Erase data: low-energy beam to heat up pits ο Loosens alloy to return to original crystalline state Β© NZSE 20 Β© AUT
21.
CD, DVD, and
Blu-Ray Technology ο Compact Disc (CD) and Digital Versatile Disc (DVD) ο CD-/DVD-Recordable technology (CD-R/DVD-R) ο Records data using write-once technique ο Data cannot be erased or modified ο CD-/DVD-Rewritable technology (CD-RW/DVD-RW) ο Data can be written, changed, and erased multiple times ο CD-ROM/DVD-ROM ο Similar in design, shape, size ο Differs in data capacity ο Dual-layer, single-sided DVD holds 13 CDs ο Single-layer, single-sided DVD holds 8.6 GB (MPEG video compression) ο Differs in laser wavelength ο Uses red laser (smaller pits, tighter spiral) Β© NZSE 21 Β© AUT
22.
Blu-Ray Disc (BD)
Technology ο Same physical size as CD/DVD ο Smaller pits ο More tightly wound tracks ο Use of blue-violet laser allows multiple layers ο 50GB-500GB ο 432 Mbps ο Formats: ο BD-ROM, ο BD-R (writable), BD-RE (rewritable) ο BD-R DL, BD-RE DL (dual-layer) Β© NZSE 22 Β© AUT
23.
Flash Memory Storage ο
Electronically erasable programmable read-only memory (EEP) ο Nonvolatile and removable ο Emulates random access ο Difference: data stored securely (even if removed) ο Data stored on microchip card or βkeyβ ο Compact flash, smart cards, memory sticks ο Often connected through USB port ο Write data: electric charge sent through floating gate ο Erase data: strong electrical field (flash) applied Β© NZSE 23 Β© AUT
24.
Magnetic Disk Drive
Access Times ο File access time factors ο Seek time (slowest) ο Time to position read/write head on track ο Search time ο Rotational delay (latency) ο Time to rotate DASD ο Rotate until desired record under read/write head ο Transfer time (fastest) ο Time to transfer data ο Secondary storage to main memory transfer Β© NZSE 24 Β© AUT
25.
Communication Among Devices ο
Problems to resolve ο Know which components are busy/free ο Solved by structuring interaction between units ο Accommodate requests during heavy I/O traffic ο Handled by buffering records and queuing requests ο Accommodate speed disparity between CPU and I/O devices ο Handled by buffering records and queuing requests Β© NZSE 25 Β© AUT
26.
Communication Among Devices ο
Direct memory access (DMA) ο Allows control unit main memory access directly ο Transfers data without the intervention of CPU ο Used for high-speed devices (disk) ο Buffers ο Temporary storage areas in main memory, channels, control units ο Improves data movement synchronization ο Between relatively slow I/O devices and very fast CPU ο Double buffering: processing of record by CPU while another is read or written by channel Β© NZSE 26 Β© AUT
27.
Communication Among Devices Β©
NZSE 27 Β© AUT
28.
Device Handler Seek
Strategies ο Predetermined device handler ο Determines device processing order ο Goal: minimize seek time ο Types ο First-come, first-served (FCFS), shortest seek time first (SSTF), SCAN (including LOOK, N-Step SCAN, C-SCAN, and C-LOOK) ο Scheduling algorithm goals ο Minimize arm movement ο Minimize mean response time ο Minimize variance in response time Β© NZSE 28 Β© AUT
29.
Device Handler Seek
Strategies ο First-Come, First-Served (FCFS) ο On average: does not meet three seek strategy goals ο Disadvantage: extreme arm movement Β© NZSE 29 Β© AUT
30.
Device Handler Seek
Strategies ο Shortest Seek Time First (SSTF) ο Request with track closest to one being served ο Minimizes overall seek time ο Postpones traveling to out of way tracks Β© NZSE 30 Β© AUT
31.
Device Handler Seek
Strategies ο SCAN ο Directional bit ο Indicates if arm moving toward/away from disk center ο Algorithm moves arm methodically ο From outer to inner track, services every request in its path ο If reaches innermost track, reverses direction and moves toward outer tracks ο Services every request in its path Β© NZSE 31 Β© AUT
32.
Device Handler Seek
Strategies ο LOOK ο Arm does not go to either edge ο Unless requests exist ο Eliminates indefinite postponement Β© NZSE 32 Β© AUT
33.
Device Handler Seek
Strategies ο N-Step SCAN ο Holds all requests until arm starts on way back ο New requests grouped together for next sweep ο C-SCAN (Circular SCAN) ο Arm picks up requests on path during inward sweep ο Provides more uniform wait time ο C-LOOK ο Inward sweep stops at last high-numbered track request ο No last track access unless required Β© NZSE 33 Β© AUT
34.
Summary (cont'd.) Β© NZSE
34 Β© AUT
35.
Β© NZSE
35 Β© AUT
36.
RAID ο Physical disk
drive set viewed as single logical unit ο Preferable over few large-capacity disk drives ο Improved I/O performance ο Improved data recovery ο Disk failure event ο Introduces redundancy ο Helps with hardware failure recovery ο Significant factors in RAID level selection ο Cost, speed, systemβs applications ο Increases hardware costs Β© NZSE 36 Β© AUT
37.
Β© NZSE
37 Β© AUT
38.
Level Zero ο Uses
data striping (not considered true RAID) ο No parity and error corrections ο No error correction/redundancy/recovery ο Benefits ο Devices appear as one logical unit ο Best for large data quantity non-critical data Β© NZSE 38 Β© AUT
39.
Level One ο Uses
data striping (considered true RAID) ο Mirrored configuration (backup) ο Duplicate set of all data (expensive) ο Provides redundancy and improved reliability Β© NZSE 39 Β© AUT
40.
Level Five ο Distributes
parity strips across disks ο If all the parity bits were in one disk (Level Four), all write/rewrite operations will cause a bottleneck on the parity disk ο Disadvantage ο Complicated to regenerate data from failed device Β© NZSE 40 Β© AUT
41.
Nested RAID Levels ο
Combines multiple RAID levels (complex) Β© NZSE 41 Β© AUT
42.
Β© NZSE
42 Β© AUT
43.
References ο McHoes, A.M.,
and Flynn, I.M. (2011). Chapter 7: Device Management. Understanding Operating Systems, 6th ed. USA: Course Technology. pp. 203-240. Β© NZSE 45 Β© AUT
Editor's Notes
http://www.dtidata.com/resourcecenter/hard-drive-recovery.jpg
http://di1-2.shoppingshadow.com/images/pi/f5/3b/85/20694538-260x260-0-0_Sony+EDM+5200B.jpg
http://www.blu-raydisc.com/Assets/Picture/dvd2-15224.gif
http://www.bookcoverco.com.au/images/cdlayers.jpghttp://volga.eng.yale.edu/uploads/CDsAndDVDs/crystal-vs-amorphous.gif
http://www.gogonomo.com/wp-content/uploads/2009/01/floating_gate_transistor.png
http://2.bp.blogspot.com/_gD6l0n9hYbA/SecIpYYTBGI/AAAAAAAAA6Q/qWf7DSpAmQY/s400/Rotational+latency+and+seek+time.jpg
http://www.itblogs.in/files/media/Disk-scheduling-scan_scheduling.JPG
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