Understanding I/O Systems and Disk Storage

1. Chapter 8 11/12/08

2. IO System • Storage • Must be dependable • Networks • Must tolerate faults in communications by including mechanisms to detect and recover form faults. • Peripherals • Extremely Diverse

3. I/O Systems • Emphasis is placed on dependability and cost. • Processors and memory emphasize performance and cost. • I/O System performance must keep pace with processor performance. • I/O can become a bottleneck.

5. I/O performance • Complex • Access Latency • Throughput • Depends on many aspects of the system. • Device characteristics • Connection between device and rest of system • Memory hierarchy • Operating system • Etc. • I/O benchmarks are primitive compared to processor benchmarks.

8. Transferring data between a device and memory • Polling • I/O interrupts • DMA • Special DMA controller handles transfers • Processor sets up DMA • DMA controller starts operation, arbitrates the bus, and interrupts processor when DMA is complete.

9. Disk Storage • Nonvolatile – Data is not lost when power turned off. • Consists of platters (1-4) each with two recordable disk surfaces, and R/W heads. • Platters are rotated at 5400-15,000 RPM. • Each dist is divided into tracks. • Tracks are divided into sectors.

11. Answer: Second. Track Seek time.

13. Disk manufactures report minimum, maximum and average seek time. The first two are easy to measure. Average is open to wide interpretation because it depends on seek distance. The standard is Sum of the time for all possible seeks divided by number of possible seeks. Actual average may be considerably less.

15. Transfer time – time to transfer a block of bits. Function of sector size, rotation speed and recording density. 30 – 80 MB/sec typical. However, most disk controllers have a built-in cache that stores sectors as they are passed over. Resulting in higher transfer rates. Today most disk transfers are multiple sectors lengths. Controller time – Overhead imposed by controller in performing disk I/O. Disk I/O time consists of the above times plus any wait time because other processes are using the disk.

19. Replace large disk with many small disks.

21. RAID • RAID 0 – Spread data over multiple drives. • Called striping improves performance but no redundancy. • RAID 1 – Mirrors or shadows data to redundant drive. • RAID 2-6 – Incorporates error correction techniques.

22. Error detection and correction. • Single error correction. HD = 3. • P1 = D3  D5 D7 • 2 3 4 5 6 7 P2 = D3  D6 D7 • P1 P2 D3 P4 D5 D6 D7 P4 = D5  D6 D7 • 0 0 0 1 1 1 1 C4 = P4  D5 D6 D7 • 0 1 1 0 0 1 1 C2 = P2  D3 D6 D7 • 1 0 1 0 1 0 1 C1 = P1  D3 D5 D7

23. RAID 6 • Allows for the simultaneous loss of two drives. • Much better data protection than RAID 5. • Implementation is based on Galois Field (GF) mathematics. • Object is to achieve Maximum Distance Separable (MDS) coding. • There are two implementations • Reed-Solomon P+Q • Double Parity

25. GF - mathematics • A GF is a set of values that contains a finite number of elements. • For example GF(28) contains 28-1 elements. • GF is closed under addition, subtraction, multiplication, and division operations. • These operations are defined differently than the corresponding operations normal arithmetic. • Addition is bitwise XOR. • Subtraction same as addition. • Multiplication and division are more complicated, and are typically accomplished by adding or subtracting gflogs.

26. P and Q Parity blocks

27. Problem 8.1 System A – 1500 I/O operations per second. System B – 1000 I/O operations per second. Both systems use a 500 MIPS processor. Each transaction requires 5 I/O operations and each I/O operation requires 10,000 instructions. Ignoring response time and assuming the transactions may be arbitrarily overlapped, what is the maximum transaction-per-second rate that each machine can sustain? Each transaction requires 5 X 10,000 = 50K inst. CPU limit = 500M/50K = 10,000 trans/sec I/0 limit for A is 1500/5 = 300 trans/sec I/O limit for B is 1000/5 = 200 trans/sec Therefore, I/O limits both systems.

28. Buses • A bus is a shared communication link, which uses one set of wires to connect multiple subsystems. • Advantages • Versatile • Low cost • Disadvantage • Communication bottleneck • A bus generally consists of data, and control lines. • Control lines are used to signal request and acknowledgments, and to indicate what type of information is on the data lines. • Data lines carry information between the source and destination. These lines are often separated into address and data.

29. Bus Transactions • A sequence of bus operations that includes a request and may include a response, either of which may carry data. • May require several bus operations to complete. • Includes two parts sending address and sending or receiving data.

30. Processor-memory bus • Connects processor and memory. • Short • High speed • Matched to memory system to maximize memory-processor bandwidth.

31. I/O Buses • Can connect many types of I/O devices. • Can be long. • Wide range of data bandwidths. • Provides a way of extending the machine and adding new peripherals.

32. Backplane Bus • Allows processor, memory and I/O to exist on a single bus.

33. Synchronous Bus • Contains a clock as part of the control lines, and uses a fixed protocol for communicating that is relative to the clock. • Every device must run at the clock rate. • Because of clock skew synchronous busses can not be long. • Processor-memory buses tend to be synchronous.

34. Asynchronous Buses • Not clocked. • Uses handshaking. • Can accommodate wide variety of devices. • Can be long. • Frequently used in I/O buses. • USB and Firewire are asynchronous buses.

36. data address