chapter 8 n.
Skip this Video
Download Presentation
Chapter 8

Loading in 2 Seconds...

play fullscreen
1 / 39

Chapter 8 - PowerPoint PPT Presentation

  • Uploaded on

Chapter 8. 11/23/09. IO System. Storage Must be dependable Networks Must tolerate faults in communications by including mechanisms to detect and recover form faults. Peripherals Extremely Diverse. I/O Systems. Emphasis is placed on dependability and cost.

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
Download Presentation

PowerPoint Slideshow about 'Chapter 8' - kirtana-devaj

Download Now An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
chapter 8

Chapter 8


io system
IO System
  • Storage
    • Must be dependable
  • Networks
    • Must tolerate faults in communications by including mechanisms to detect and recover form faults.
  • Peripherals
    • Extremely Diverse
i o systems
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.
i o performance
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.
transferring data between a device and memory
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.
disk storage
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.

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.


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.

  • 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.
raid 5
  • To illustrate concepts and implications consider RAID 5.
  • RAID 5 uses striped array with rotating parity.
  • Optimized for short, multithreaded transfers.
  • Capable of recovering from a single drive failure.

RAID 5 system consisting of three data drives and rotating parity.

Four stripes for sectors A, B, C, and D are shown.

rotating parity
Rotating Parity
  • Why rotating parity?
  • The following steps are necessary to update a single data sector in a stripe.
    • The old data sector and the parity sector for the stripe must be read.
    • Compute the new parity using the new data sector, old data sector, and old parity.
    • Write new data sector and new parity sector.
  • Thus, to write to a data sector both the data sector and parity sector must be read and written.
  • Since there are many data drives a fixed parity drive would accessed much more frequently than a data drive.
  • This excessive access of a single parity drive is avoid by rotating parity across all drives.

Parity encoding is given by

Where Di represent a data byte in a sector on drive i.

If both sides of the above equation are exclusive ored with P, then

D5 for example can be recovered by

raid 6
Raid 6
  • Use two parity drives (P and Q).
  • Data can be recovered if two sectors in a stripe are corrupted.
  • P parity is the same as RAID 5 (simple XOR).
    • Easy to encode and easy to recover data.
  • Q parity is more complicated.
q parity encoding
Q parity encoding

The Q parity is a Reed-Solomon code given by

Where  is Galois Field (GF) multiplication and giis a constant. For i < 8 it turns out that gi = 2i. For larger i, it not as simple. For example g8 = 29.

But Q simplifies to

The problem is how to compute the GF multiplication.

gf multiplication
GF multiplication
  • In ordinary arithmetic multiplication can be accomplished summing the logs and taking the inverse log.
  • GF multiplication is typically accomplished using lookup tables to find the GF log and inverse log. The addition in modulo 255.

See Xilinx application note XAPP731 “Hardware Accelerator for RADD 6 Parity Generation / Data Recovery Controller”.

  • 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.
bus transactions
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.
processor memory bus
Processor-memory bus
  • Connects processor and memory.
  • Short
  • High speed
  • Matched to memory system to maximize memory-processor bandwidth.
i o buses
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.
backplane bus
Backplane Bus
  • Allows processor, memory and I/O to exist on a single bus.
synchronous bus
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.
asynchronous buses
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.