Download Chapter 8

Chapter 8
Storage, Networks and Other
Peripherals
1998 Morgan Kaufmann Publishers
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Introduction
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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.
1998 Morgan Kaufmann Publishers
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I/O Devices
Device
Keyboard
Mouse
Voice input
Scanner
Voice output
Line printer
Laser printer
Graphics display
Modem
Network/LAN
Floppy disk
Optical disk
Magnetic tape
Magnetic disk
Behavior
input
input
input
input
output
output
output
output
input or output
input or output
storage
storage
storage
storage
Partner
human
human
human
human
human
human
human
human
machine
machine
machine
machine
machine
machine
Data rate (KB/sec)
0.01
0.02
0.02
400.00
0.60
1.00
200.00
60,000.00
2.00-8.00
500.00-6000.00
100.00
1000.00
2000.00
2000.00-10,000.00
1998 Morgan Kaufmann Publishers
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I/O Performance
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Performance:
— access latency
— throughput
— connection between devices and the system
— the memory hierarchy
— the operating system
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A variety of different users (e.g., banks, supercomputers, engineers)
each has different requirements.
1998 Morgan Kaufmann Publishers
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Typical Collection of I/O Devices
Processor
Interrupts
Cache
Memory– I/O bus
Main
memory
I/O
controller
Disk
Disk
I/O
controller
I/O
controller
Graphics
output
Network
1998 Morgan Kaufmann Publishers
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Importance of I/O in a Networked Society
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Processors are being built from the same basic technology.
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I/O becomes one of the most distinctive features of the machines.
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As the importance of networking and information infrastructure
grows, I/O plays an increasing important role.
1998 Morgan Kaufmann Publishers
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Impact of I/O on System Performance
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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
doesn’t improve, how much faster will our program run at the end of
five years?
Amdahl’s Law again!
1998 Morgan Kaufmann Publishers
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Assessing I/O Performance
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Depends on the application
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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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1998 Morgan Kaufmann Publishers
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I/O Performance Measures
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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 = 10^6 bytes, not 2^20 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 supercomputer’s 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 --> Makedir, Copy, ScanDir,
ReadAll, Make
1998 Morgan Kaufmann Publishers
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Disk Storage and Dependability
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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
1998 Morgan Kaufmann Publishers
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I/O Example: Disk Drives
Platters
Tracks
Platter
Sectors
Track
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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
1998 Morgan Kaufmann Publishers
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Example
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For a disk rotating at 5400 RPM,
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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
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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.
1998 Morgan Kaufmann Publishers
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Disk Read Time
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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)
1998 Morgan Kaufmann Publishers
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Dependability, Reliability and Availability
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A system alternating between states:
1. Service accomplishment: where the service is delivered as
specified
2. 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-torepair
Availability = MTTF/(MTTF+MTTR)
1998 Morgan Kaufmann Publishers
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How to Increase MTTF
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Fault avoidance
Fault tolerance
Fault forecasting
1998 Morgan Kaufmann Publishers
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RAID
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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)
P+Q redundancy (RAID 6)
1998 Morgan Kaufmann Publishers
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RAID 1-6
1998 Morgan Kaufmann Publishers
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Small Write Update on Raid 3 vs. Raid 4
1998 Morgan Kaufmann Publishers
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Networks
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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
1998 Morgan Kaufmann Publishers
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The OSI Model Layers
1998 Morgan Kaufmann Publishers
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TCP/IP Packet Format
1998 Morgan Kaufmann Publishers
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Performance of Two Networks
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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.
1998 Morgan Kaufmann Publishers
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I/O Example: Buses
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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)
1998 Morgan Kaufmann Publishers
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Bus: Connecting I/O Devices to Processor
and Memory
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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
1998 Morgan Kaufmann Publishers
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Input Operation
1998 Morgan Kaufmann Publishers
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Different Machines using Different Types of
Buses
1998 Morgan Kaufmann Publishers
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Synchronous and Asynchronous Buses
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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
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Asynchronous buses
– don’t use a clock and instead use handshaking
– can accommodate a wide variety of devices
1998 Morgan Kaufmann Publishers
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Asynchronous Protocol
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ReadReq
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Data
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Ack
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DataRdy
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Let’s look at some examples from the text
“Performance Analysis of Synchronous vs. Asynchronous”
“Performance Analysis of Two Bus Schemes”
1998 Morgan Kaufmann Publishers
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