Storage, Networks and Other Peripherals

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Chapter 8

• Storage, Networks and Other Peripherals

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Introduction

• I/O Design affected by many factors
  (expandability, resilience)
• Three characteristics for organizing I/O devices
• Behavior input (read once), output (write only),
  or storage
• Partner Either a human or a machine is at the
  other end of the I/O device
• Data rate the peak rate at which data can be
  transferred between the I/O device and the main
  memory or processor.

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I/O Devices
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I/O Performance

• Performance access latency throughput
  connection between devices and the system the
  memory hierarchy the operating system
• A variety of different users (e.g., banks,
  supercomputers, engineers)each has different
  requirements.

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Typical Collection of I/O Devices
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Importance of I/O in a Networked Society

• Processors are being built from the same basic
  technology.
• I/O becomes one of the most distinctive features
  of the machines.
• As the importance of networking and information
  infrastructure grows, I/O plays an increasing
  important role.

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Impact of I/O on System Performance

• Suppose we have a benchmark that executes in 100
  seconds of elapse time, where 90 seconds is CPU
  time and the rest is I/O time. If the CPU
  improves by 50 per year for the next five years
  but I/O time doesnt improve, how much faster
  will our program run at the end of five
  years?Amdahls Law again!

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Assessing I/O Performance

• Depends on the application
• System throughput
• I/O bandwidth
• how much data can we move through the system in a
  certain time?
• How many I/O operations can we do per unit of
  time?
• Response time

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I/O Performance Measures

• Examples from Disk and File Systems
• affected by disk technology, how disk are
  connected, the memory, the processor, and the
  file system provided by the OS.
• Benchmark relatively primitive compared with
  those for the CPU.
• Note transfer rate 1 MB 106 bytes, not 220
  bytes
• Supercomputer I/O benchmarks dominated by access
  to large files on magnetic disks. Data
  throughput, of bytes per second that can be
  transferred between a supercomputers main memory
  and disks.
• Transaction Processing(TP) I/O benchmarks
• involve both response time requirement and a
  performance based on throughput.
• Concerned with I/O rate, measured as of disk
  accesses per second.
• TPC has developed several benchmarks.
• File System I/O benchmarks five phases --gt
  Makedir, Copy, ScanDir, ReadAll, Make

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Disk Storage and Dependability

• Disk storage is nonvolatile, meaning that the
  data remains even when power is removed.
• Platters in hard disk are metal (or glass),
  offering several advantages over floppy disks
• can be larger because it is rigid
• has higher density because it can be controlled
  more precisely
• Has a higher data rate because it spins faster
• can incorporate more platter

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I/O Example Disk Drives

• To access data seek position head over the
  proper track (3 to 14 ms. avg.) rotational
  latency wait for desired sector (.5 / RPM)
  transfer grab the data (one or more sectors)
  30-80 MB/sec

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Example

• For a disk rotating at 5400 RPM, average
  rotational latency 0.5 rotation / 5400 RPM
  0.5 rotation/(5400 RPM/ 60) 5.6ms
• For a disk rotating at 15,000 RPM, average
  rotational latency 2.0ms
• Note detailed control of the disk and the
  transfer between the disk and the memory is
  usually handled by a disk controller. The
  controller adds the final component of disk
  access time, controller time.
• The average time to perform an I/O operation will
  consist of these four times plus any wait time
  incurred because other processes are using the
  disk.
• Many recent disks have included caches directly
  in the disk to speed up the access time.

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Disk Read Time

• What is the average time to read or write a
  512-byte sector for a typical disk rotating at
  10,000 RPM? The advertised average seek time is 6
  ms, the transfer rate is 50MB/sec, and the
  control overhead is 0.2ms. (Assuming no waiting
  time)

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Dependability, Reliability and Availability

• A system alternating between states
• Service accomplishment where the service is
  delivered as specified
• Service interruption where the service is
  different from the specified service
• Transitions from state 1 to state 2 are caused by
  failures
• Transitions from state 2 to state 1 are called
  restorations.
• Reliability is a measure of the continuous
  service accomplishment.
• Mean-time-between-failures Mean-time-to-failure
  Mean-time-to-repair
• Availability MTTF/(MTTFMTTR)

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How to Increase MTTF

• Fault avoidance
• Fault tolerance
• Fault forecasting

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RAID

• Leveraging redundancy to improve the availability
  of disk storage is captured in the phrase
  Redundant Array of Inexpensive Disks
• No redundancy (RAID 0) allocation of logically
  sequential blocks to separate disks to allow
  higher performance than a single disk can deliver
• Mirroring (RAID 1) writing the identical data to
  multiple disks to increase data availability.
• Error Detecting and Correcting Code (RAID 2)
• Bit-Interleaved Parity (RAID 3) Add enough
  redundant information to restore the lost
  information on a failure.
• Block-interleaved Parity (RAID 4)
• Distributed Block-interleaved Parity (RAID 5)
• PQ redundancy (RAID 6)

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RAID 1-6
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Small Write Update on Raid 3 vs. Raid 4
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Networks

• Key characteristics of typical networks
• distance 0.01 to 10,000 kilometers
• speed 0.001 MB/sec to 1GBit/sec
• topology bus, ring, star, tree
• shared lines none (point-to-point) or shared
• RS232 slow but cheap
• LAN (ethernet) up to 1GBit/sec
• Ethernet is a bus with multiple masters and a
  scheme for determining who gets bus control.
• ATM scalable network technology (155 Mbits/sec
  to 2.5 Gbits/sec)
• Example Performance of two networks

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The OSI Model Layers
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TCP/IP Packet Format
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Performance of Two Networks

• Bandwidth 100 Mbit/s vs. 1000 Mbit/s
• Interconnect latency 10us
• HW latency from/to network 2 us
• SW overhead sending to network 100 us
• SW overhead receiving from network 80 us
• Question Find the host-to-host latency for a 250
  byte message using each network.

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I/O Example Buses

• Shared communication link (one or more wires)
• Difficult design may be bottleneck length
  of the bus number of devices tradeoffs
  (buffers for higher bandwidth increases
  latency) support for many different devices
  cost
• Types of buses processor-memory (short high
  speed, custom design) backplane (high speed,
  often standardized, e.g., PCI) I/O (lengthy,
  different devices, standardized, e.g., SCSI)

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Bus Connecting I/O Devices to Processor and
Memory

• A bus generally contains a set of control lines
  and a set of data lines.
• Control lines are used to signal requests and
  acknowledges, and to indicate what type of
  information is on the data lines
• Data lines carry information between the source
  and the destination. The information may consist
  of data, complex commands or addresses.
• Bus transaction includes two parts sending the
  address and receiving or sending the data.
• Read transaction transfers data from memory
• Write transaction writes data to the memory
• Input operation input to memory so the
  processor can read it
• Output operation output to device from memory

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Input Operation
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Different Machines using Different Types of Buses
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Synchronous and Asynchronous Buses

• Synchronous buses
• use a clock and a synchronous protocol, fast and
  small
• but every device must operate at same rate
• clock skew requires the bus to be short
• processor-memory buses are often synchronous
• Asynchronous buses
• dont use a clock and instead use handshaking
• can accommodate a wide variety of devices

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Asynchronous Protocol

• Lets look at some examples from the
  text Performance Analysis of Synchronous vs.
  Asynchronous Performance Analysis of Two Bus
  Schemes