Chapter 111 Chapter 11: Hardware (Slides by Hector Garcia-Molina,
Chapter 112 Outline Hardware: Disks Access Times Optimizations Other Topics: –Storage costs –Using secondary storage –Disk failures
Chapter 113 The Big Picture: DBMS Data Storage
Chapter 114 P M Typical Computer Secondary Storage...
Chapter 115 Processor Fast, slow, reduced instruction set, with cache, pipelined… Speed: 100 500 1000 MIPS Memory Fast, slow, non-volatile, read-only,… Access time: sec. 1 s 1 ns
Chapter 116 Secondary storage Many flavors: - Disk: Floppy (hard, soft) Removable Packs Winchester Ram disks Optical, CD-ROM… Arrays - TapeReel, cartridge Robots
Chapter 117 Focus on: “Typical Disk” Terms: Platter, Head, Cylinder, Track Sector (physical), Block (logical), Gap …
Chapter 118 Top View
Chapter 119 “Typical” Numbers Diameter: 1 inch 15 inches Cylinders:100 2000 Surfaces:1 (CDs) (Tracks/cyl) 2 (floppies) 30 Sector Size:512B 50K Capacity:360 KB (old floppy) 30 GB (I use)
Chapter 1110 Disk Access Time block x in memory ? I want block X
Chapter 1111 Time = Seek Time + Rotational Delay + Transfer Time + Other
Chapter 1112 Rotational Delay Head Here Block I Want
Chapter 1113 Average Rotational Delay R = 1/2 revolution “typical” R = 8.33 ms (3600 RPM)
Chapter 1114 Transfer Rate: t “typical” t: 1 3 MB/second transfer time: block size t
Chapter 1115 Other Delays CPU time to issue I/O Contention for controller Contention for bus, memory “Typical” Value: 0
Chapter 1116 So far: Random Block Access What about: Reading “Next” block?
Chapter 1117 If we do things right (e.g., Double Buffer, Stagger Blocks…) Time to get = Block Size + Negligible block t - skip gap - switch track - once in a while, next cylinder
Chapter 1118 Rule ofRandom I/O: Expensive Thumb Sequential I/O: Much less Ex:1 KB Block »Random I/O: 20 ms. »Sequential I/O: 1 ms.
Chapter 1119 Cost for Writing similar to Reading …. unless we want to verify! need to add (full) rotation + Block size t
Chapter 1120 To Modify a Block? To Modify Block: (a) Read Block (b) Modify in Memory (c) Write Block [(d) Verify?]
Chapter 1121 Block Address: Physical Device Cylinder # Surface # Sector
Chapter 1122 Outline Hardware: Disks Access Times Optimizations Other Topics –Storage Costs –Using Secondary Storage –Disk Failures here
Chapter 1123 Optimizations (in controller or O.S.) Double Buffering Disk Scheduling Algorithms: sec –e.g., elevator algorithm
Chapter 1124 Double Buffering Problem: Have a File » Sequence of Blocks B1, B2 Have a Program » Process B1 » Process B2 » Process B3...
Chapter 1125 Single Buffer Solution (1) Read B1 Buffer (2) Process Data in Buffer (3) Read B2 Buffer (4) Process Data in Buffer...
Chapter 1126 SayP = time to process/block R = time to read in 1 block n = # blocks Single buffer time = n(P+R)
Chapter 1127 Double Buffering Memory: Disk: ABCDGEFA B done process A C B done
Chapter 1128 Say P R What is processing time? P = Processing time/block R = IO time/block n = # blocks Double buffering time = R + nP Single buffering time = n(R+P)
Chapter 1129 Block Size Selection? Big Block Amortize I/O Cost Big Block Read in more useless stuff! and takes longer to read Unfortunately...
Chapter 1130 Trend As memory prices drop, blocks get bigger... Trend
Chapter 1131 Storage Cost access time (sec) cache electronic main electronic secondary magnetic optical disks tape optical disks tape typical capacity (bytes) from Gray & Reuter
Chapter 1132 Storage Cost access time (sec) cache electronic main electronic secondary magnetic optical disks tape optical disks tape dollars/MB from Gray & Reuter
Chapter 1133 Using secondary storage effectively Example: Sorting data on disk Conclusion: –I/O costs dominate –Design algorithms to reduce I/O Also: How big should blocks be?
Chapter 1134 Disk Failures (Sec 11.6) Partial Total Intermittent Permanent
Chapter 1135 Coping with Disk Failures Detection –e.g. Checksum Correction Redundancy
Chapter 1136 At what level do we cope? Single Disk –e.g., Error Correcting Codes Disk Array Logical Physical
Chapter 1137 Operating System e.g., Stable Storage Logical BlockCopy A Copy B
Chapter 1138 Database System e.g., Log Current DBLast week’s DB
Chapter 1139 Summary Secondary storage, mainly disks I/O times I/Os should be avoided, especially random ones….. Summary
Chapter 1140 Outline Hardware: Disks Access Times Optimizations Other Topics –Storage Costs –Using Secondary Storage –Disk Failures here