Chapter 6Storage and Other I/O Topics
IntroductionIntroduction I/O devices can be characterized by
B h i i t t t t Behavior: input, output, storage Partner: human or machine Data rate: bytes/sec, transfers/sec Data rate: bytes/sec, transfers/sec
I/O bus connections
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Interfacing Processors and PeripheralsPeripherals
I/O Design affected by many factors I/O Design affected by many factors (expandability, resilience)P f Performance:
— access latency th h t— throughput
— connection between devices and th tthe system
— the memory hierarchyth ti t
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— the operating system
Interfacing Processors and PeripheralsPeripherals
A variety of different users (e g banks A variety of different users (e.g., banks, supercomputers, engineers)
Processor
Cache
Interrupts
Main memory
I/O controller
I/O controller
I/O controller
Memory– I/O bus
Disk Graphics output
NetworkDisk
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I/O/O
Important but neglected“The difficulties in assessing and designing I/O
systems haveoften relegated I/O to second class status”often relegated I/O to second class status”“courses in every aspect of computing, from
programming toprogramming tocomputer architecture often ignore I/O or give
it scanty coverage”y g“textbooks leave the subject to near the end,
making it easierf t d t d i t t t ki it!”
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for students and instructors to skip it!”
I/O/O
GUILTY!
— we won’t be looking at I/O in much gdetail
— be sure and read Chapter 6 in its entirety.y
— you should probably take a networking
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y p y gclass!
I/O Devices/O e ces
Very diverse devices Very diverse devices— behavior (i.e., input vs. output)
partner (who is at the other end?)— partner (who is at the other end?)— data rate
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I/O Devices/O e ces
Device Behavior Partner Data rate (KB/sec)Device Behavior Partner Data rate (KB/sec)Keyboard input human 0.01Mouse input human 0.02Voice input input human 0.02Scanner inp t h man 400 00Scanner input human 400.00Voice output output human 0.60Line printer output human 1.00Laser printer output human 200.00Graphics display output human 60,000.00Modem input or output machine 2.00-8.00Network/LAN input or output machine 500.00-6000.00Floppy disk storage machine 100.00ppy gOptical disk storage machine 1000.00Magnetic tape storage machine 2000.00Magnetic disk storage machine 2000.00-10,000.00
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I/O System CharacteristicsI/O System Characteristics
Dependability is important Dependability is important Particularly for storage devices
Performance measures Performance measures Latency (response time) Throughput (bandwidth) Throughput (bandwidth) Desktops & embedded systems
Mainly interested in response time & diversity Mainly interested in response time & diversity of devices
Servers
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Mainly interested in throughput & expandability of devices
DependabilityDependabilityService accomplishmentp
Service deliveredas specified
Fault: failure of a component
M t l dFailureRestoration
May or may not lead to system failure
Service interruptionDeviation from
specified service
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p
Dependability MeasuresDependability Measures
Reliability: mean time to failure (MTTF) Reliability: mean time to failure (MTTF) Service interruption: mean time to repair (MTTR) Mean time between failures
MTBF = MTTF + MTTR
Availability = MTTF / (MTTF + MTTR)l b l Improving Availability
Increase MTTF: fault avoidance, fault tolerance, fault forecasting
Reduce MTTR: improved tools and processes for diagnosis and repair
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Disk StorageDisk Storage
Nonvolatile rotating magnetic storage Nonvolatile, rotating magnetic storage
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Disk Sectors and AccessDisk Sectors and Access Each sector records
S t ID Sector ID Data (512 bytes, 4096 bytes proposed) Error correcting code (ECC)Error correcting code (ECC)
Used to hide defects and recording errors Synchronization fields and gaps
A t t i l Access to a sector involves Queuing delay if other accesses are pending Seek: move the heads Seek: move the heads Rotational latency Data transfer
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Controller overhead
Disk Access ExampleDisk Access Example
Given Given 512B sector, 15,000rpm, 4ms average seek time,
100MB/s transfer rate, 0.2ms controller overhead, idle diskidle disk
Average read time 4ms seek time
+ ½ / (15 000/60) = 2ms rotational latency+ ½ / (15,000/60) = 2ms rotational latency+ 512 / 100MB/s = 0.005ms transfer time+ 0.2ms controller delay= 6.2ms 6.2ms
If actual average seek time is 1ms Average read time = 3.2ms
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Disk Performance IssuesDisk Performance Issues
Manufacturers quote average seek time Manufacturers quote average seek time Based on all possible seeks Locality and OS scheduling lead to smaller actual average
seek timesseek times
Smart disk controller allocate physical sectors on disk Present logical sector interface to hostg SCSI, ATA, SATA
Disk drives include cachesP f t h t i ti i ti f Prefetch sectors in anticipation of access
Avoid seek and rotational delay
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Flash StorageFlash Storage
Nonvolatile semiconductor storage Nonvolatile semiconductor storage 100× – 1000× faster than disk
S ll l b t Smaller, lower power, more robust But more $/GB (between disk and DRAM)
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Flash TypesFlash Types
NOR flash: bit cell like a NOR gate NOR flash: bit cell like a NOR gate Random read/write access Used for instruction memory in embedded systems Used for instruction memory in embedded systems
NAND flash: bit cell like a NAND gate Denser (bits/area), but block-at-a-time accesse se (b ts/a ea), but b oc at a t e access Cheaper per GB Used for USB keys, media storage, …
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Interconnecting ComponentsComponents
Need interconnections between CPU, memory, I/O controllers
Bus: shared communication channelP ll l t f i f d t d Parallel set of wires for data and synchronization of data transfer
Can become a bottleneckCan become a bottleneck Performance limited by physical factors
Wire length, number of connectionsg , More recent alternative: high-speed
serial connections with switchesk k
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Like networks
Bus TypesBus Types
Processor Memory buses Processor-Memory buses Short, high speed
D i i t h d t i ti Design is matched to memory organization
I/O buses Longer, allowing multiple connections Specified by standards for interoperability Connect to processor-memory bus through
a bridge
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Bus Signals and SynchronizationBus Signals and Synchronization
Data lines Data lines Carry address and data Multiplexed or separate
Control lines Indicate data type, synchronize transactions
S h Synchronous Uses a bus clock
Asynchronous Asynchronous Uses request/acknowledge control lines for
handshaking
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I/O Example: Buses/O a p e: uses
Shared communication link (one or more Shared communication link (one or more wires)
Difficult design: Difficult design:— may be bottleneck— length of the busg— number of devices— tradeoffs (buffers for higher bandwidth
i l )increases latency)— support for many different devices— cost
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— cost
I/O Example: Buses/O a p e: uses
Types of buses: Types of buses:— processor-memory (short high
speed custom design)speed, custom design)— backplane (high speed, often
standardized e g PCI)standardized, e.g., PCI)— I/O (lengthy, different devices,
standardized e g SCSI USB Firewire)standardized, e.g., SCSI, USB, Firewire)
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I/O Example: Buses/O a p e: uses
Synchronous vs Asynchronous Synchronous vs. Asynchronous— use a clock and a synchronous
protocol fast and smallprotocol, fast and smallbut every device must operate
at same rate andat same rate andclock skew requires the bus to
be shortbe short— don’t use a clock and instead use
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handshaking
Examplea p e
R dR
Ack
Data
ReadReq 13
4
5
642 2
DataRdy
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I/O Bus ExamplesI/O Bus ExamplesFirewire USB 2.0 PCI Express Serial ATA Serial
Attached SCSI
Intended use External External Internal Internal External
D i 63 127 1 1 4Devices per channel
63 127 1 1 4
Data width 4 2 2/lane 4 4
P k 50MB/ 0 2MB/ 250MB/ /l 300MB/ 300MB/Peak bandwidth
50MB/s or 100MB/s
0.2MB/s, 1.5MB/s, or 60MB/s
250MB/s/lane1×, 2×, 4×, 8×, 16×, 32×
300MB/s 300MB/s
Hot Yes Yes Depends Yes Yespluggable
p
Max length 4.5m 5m 0.5m 1m 8m
Standard IEEE 1394 USB PCI-SIG SATA-IO INCITS TC
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Implementers Forum
T10
Other important issuesO e po a ssues
Bus Arbitration: Bus Arbitration:— daisy chain arbitration (not very
f i )fair)— centralized arbitration (requires
an arbiter), e.g., PCI— self selection, e.g., NuBus used inself selection, e.g., NuBus used in
Macintoshlli i d t ti Eth t
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— collision detection, e.g., Ethernet
Other important issuesO e po a ssues
Operating system: Operating system:— polling— interrupts— DMA
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Other important issuesO e po a ssues
Performance Analysis techniques: Performance Analysis techniques:— queuing theory— simulation— analysis, i.e., find the weakest link a a ys s, e , d t e ea est
(see “I/O System Design”)
M d l Many new developments
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Typical x86 PC I/O SystemTypical x86 PC I/O System
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I/O ManagementI/O Management
I/O is mediated by the OS I/O is mediated by the OS Multiple programs share I/O resources
Need protection and scheduling Need protection and scheduling
I/O causes asynchronous interruptsSame mechanism as exceptions Same mechanism as exceptions
I/O programming is fiddlyOS provides abstractions to programs OS provides abstractions to programs
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I/O CommandsI/O Commands
I/O devices are managed by I/O controller hardware I/O devices are managed by I/O controller hardware Transfers data to/from device Synchronizes operations with software
Command registers Command registers Cause device to do something
Status registers Indicate what the device is doing and occurrence of errors
Data registers Write: transfer data to a device Read: transfer data from a device
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I/O Register MappingI/O Register Mapping
Memory mapped I/O Memory mapped I/O Registers are addressed in same space as memory Address decoder distinguishes between them Address decoder distinguishes between them OS uses address translation mechanism to make
them only accessible to kernel
I/O instructions Separate instructions to access I/O registers Can only be executed in kernel mode Example: x86
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PollingPolling
Periodically check I/O status register Periodically check I/O status register If device ready, do operation
If t k ti If error, take action
Common in small or low-performance l b dd dreal-time embedded systems
Predictable timing Low hardware cost
In other systems, wastes CPU time
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y ,
InterruptsInterrupts
When a device is ready or error occurs When a device is ready or error occurs Controller interrupts CPU
Interrupt is like an exceptionh i d i i i But not synchronized to instruction execution
Can invoke handler between instructions Cause information often identifies the interrupting p g
device Priority interrupts
Devices needing more urgent attention get higher Devices needing more urgent attention get higher priority
Can interrupt handler for a lower priority interrupt
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I/O Data TransferI/O Data Transfer
Polling and interrupt driven I/O Polling and interrupt-driven I/O CPU transfers data between memory and
I/O data registersI/O data registers Time consuming for high-speed devices
Di t (DMA) Direct memory access (DMA) OS provides starting address in memory I/O controller transfers to/from memory
autonomouslyC ll l
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Controller interrupts on completion or error
DMA/Cache InteractionDMA/Cache Interaction
If DMA writes to a memory block that is If DMA writes to a memory block that is cached Cached copy becomes stale Cached copy becomes stale
If write-back cache has dirty block, and DMA reads memory blocky Reads stale data
Need to ensure cache coherence Flush blocks from cache if they will be used for
DMAO h bl l ti f I/O
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Or use non-cacheable memory locations for I/O
DMA/VM InteractionDMA/VM Interaction OS uses virtual addresses for memory
DMA blocks may not be contiguous in physical memory
h ld l dd Should DMA use virtual addresses? Would require controller to do translation
If DMA uses physical addresses May need to break transfers into page-
i d h ksized chunks Or chain multiple transfers
Or allocate contiguous physical pages for
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Or allocate contiguous physical pages for DMA
Measuring I/O PerformanceMeasuring I/O Performance
I/O performance depends on I/O performance depends on Hardware: CPU, memory, controllers, buses Software: operating system, database Software: operating system, database
management system, application Workload: request rates and patterns
I/O system design can trade-off between response time and throughput Measurements of throughput often done with
constrained response-time
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Transaction Processing BenchmarksTransaction Processing Benchmarks
Transactions Small data accesses to a DBMS Interested in I/O rate, not data rate
Measure throughputg p Subject to response time limits and failure handling ACID (Atomicity, Consistency, Isolation, Durability) Overall cost per transaction
Transaction Processing Council (TPC) benchmarks (www.tcp.org) TPC-APP: B2B application server and web services TCP-C: on-line order entry environment TCP-E: on-line transaction processing for brokerage firm TPC-H: decision support — business oriented ad-hoc queries
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File System & Web BenchmarksFile System & Web Benchmarks SPEC System File System (SFS)
Synthetic workload for NFS server, based on monitoring real systemsR lt Results Throughput (operations/sec) Response time (average ms/operation) Response time (average ms/operation)
SPEC Web Server benchmark Measures simultaneous user sessions Measures simultaneous user sessions,
subject to required throughput/session Three workloads: Banking, Ecommerce,
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g, ,and Support
I/O vs CPU PerformanceI/O vs. CPU Performance Amdahl’s Law
Don’t neglect I/O performance as parallelism increases compute performance
Example Example Benchmark takes 90s CPU time, 10s I/O time Double the number of CPUs/2 years
I/O nchanged I/O unchanged
Year CPU time I/O time Elapsed time % I/O timenow 90s 10s 100s 10%+2 45s 10s 55s 18%+4 23s 10s 33s 31%
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+6 11s 10s 21s 47%
RAIDRAID
Redundant Array of Inexpensive Redundant Array of Inexpensive (Independent) Disks Use multiple smaller disks (c.f. one large disk)
Parallelism improves performance Parallelism improves performance Plus extra disk(s) for redundant data storage
Provides fault tolerant storage systemg y Especially if failed disks can be “hot swapped”
RAID 0N d d (“AID”?) No redundancy (“AID”?) Just stripe data over multiple disks
But it does improve performance
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RAID 1 & 2RAID 1 & 2
RAID 1: Mirroring RAID 1: Mirroring N + N disks, replicate data
Write data to both data disk and mirror disk Write data to both data disk and mirror disk On disk failure, read from mirror
RAID 2: Error correcting code (ECC) RAID 2: Error correcting code (ECC) N + E disks (e.g., 10 + 4)
S lit d t t bit l l N di k Split data at bit level across N disks Generate E-bit ECC
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Too complex, not used in practice
RAID 3: Bit Interleaved ParityRAID 3: Bit-Interleaved Parity
N + 1 disks N + 1 disks Data striped across N disks at byte level Redundant disk stores parity Redundant disk stores parity Read access
Read all disks
Write access Generate new parity and update all disks
On failure On failure Use parity to reconstruct missing data
Not widely used
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Not widely used
RAID 4: Block Interleaved ParityRAID 4: Block-Interleaved Parity
N + 1 disks Data striped across N disks at block level Redundant disk stores parity for a group of blocks Read access
Read only the disk holding the required block
Write access Write access Just read disk containing modified block, and parity disk Calculate new parity, update data disk and parity disk
On failure Use parity to reconstruct missing data
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Not widely used
RAID 3 vs RAID 4
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RAID 5: Distributed ParityRAID 5: Distributed Parity N + 1 disks
Lik RAID 4 b t it bl k di t ib t d Like RAID 4, but parity blocks distributed across disks Avoids parity disk being a bottleneck
Wid l d Widely used
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RAID 6: P + Q RedundancyRAID 6: P + Q Redundancy
N + 2 disks N + 2 disks Like RAID 5, but two lots of parity
G t f lt t l th h Greater fault tolerance through more redundancy
M lti l RAID Multiple RAID More advanced systems give similar fault
t l ith b tt ftolerance with better performance
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RAID SummaryRAID Summary
RAID can improve performance and RAID can improve performance and availability
High il bilit eq i e hot pping High availability requires hot swapping
Assumes independent disk failures Too bad if the building burns down!
See “Hard Disk Performance, Quality and Reliability” http://www.pcguide.com/ref/hdd/perf/index.htm
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p p g p
I/O System DesignI/O System Design
Satisfying latency requirements Satisfying latency requirements For time-critical operations If system is unloaded
Add l f Add up latency of components
Maximizing throughput Find “weakest link” (lowest-bandwidth component)( p ) Configure to operate at its maximum bandwidth Balance remaining components in the system
If system is loaded simple analysis is insufficient If system is loaded, simple analysis is insufficient Need to use queuing models or simulation
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Server ComputersServer Computers Applications are increasingly run on pp g y
servers Web search, office apps, virtual worlds, …, pp , ,
Requires large data center servers Multiple processors networks connections Multiple processors, networks connections,
massive storage Space and power constraints Space and power constraints
Server equipment built for 19” racksMultiples of 1 75” (1U) high
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Multiples of 1.75 (1U) high
Rack Mounted ServersRack-Mounted ServersSun Fire x4150 1U server
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Sun Fire x4150 1U serverSun Fire x4150 1U server
4 cores4 cores each
16 x 4GB =
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16 x 4GB = 64GB DRAM
I/O System Design ExampleI/O System Design Example Given a Sun Fire x4150 system with
Workload: 64KB disk reads Each I/O op requires 200,000 user-code instructions and
100,000 OS instructions Each CPU: 109 instructions/sec FSB: 10.6 GB/sec peak
DRAM DDR2 667MHz: 5 336 GB/sec DRAM DDR2 667MHz: 5.336 GB/sec PCI-E 8× bus: 8 × 250MB/sec = 2GB/sec Disks: 15,000 rpm, 2.9ms avg. seek time, p g
112MB/sec transfer rate What I/O rate can be sustained?
For random reads and for sequential reads
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For random reads, and for sequential reads
Design Example (cont)Design Example (cont)
I/O rate for CPUs I/O rate for CPUs Per core: 109/(100,000 + 200,000) = 3,333 8 cores: 26,667 ops/sec
Random reads, I/O rate for disks Assume actual seek time is average/4 Time/op = seek + latency + transfer Time/op seek + latency + transfer
= 2.9ms/4 + 4ms/2 + 64KB/(112MB/s) = 3.3ms 303 ops/sec per disk, 2424 ops/sec for 8 disks
Sequential reads Sequential reads 112MB/s / 64KB = 1750 ops/sec per disk 14,000 ops/sec for 8 disks
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Design Example (cont)Design Example (cont)
PCI-E I/O rate PCI E I/O rate 2GB/sec / 64KB = 31,250 ops/sec
DRAM I/O rate5 336 GB/sec / 64KB 83 375 ops/sec 5.336 GB/sec / 64KB = 83,375 ops/sec
FSB I/O rate Assume we can sustain half the peak rate 5.3 GB/sec / 64KB = 81,540 ops/sec per FSB 163,080 ops/sec for 2 FSBs
Weakest link: disks 2424 ops/sec random, 14,000 ops/sec sequential Other components have ample headroom to accommodate
these rates
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Fallacy: Disk DependabilityFallacy: Disk Dependability If a disk manufacturer quotes MTTF as
1,200,000hr (140yr) A disk will work that long
Wrong: this is the mean time to failure What is the distribution of failures? What is the distribution of failures? What if you have 1000 disks
How many will fail per year?How many will fail per year?
0.73%ehrs/failur1200000
hrs/disk 8760disks 1000(AFR) Rate Failure Annual
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ehrs/failur1200000
Fallacies Fallacies
Disk failure rates are as specified Disk failure rates are as specified Studies of failure rates in the field
Schroeder and Gibson: 2% to 4% vs. 0.6% to 0.8%Pinhei o et al 1 7% (fi st ea ) to 8 6% (thi d ea ) s 1 5% Pinheiro, et al.: 1.7% (first year) to 8.6% (third year) vs. 1.5%
Why?
A 1GB/s interconnect transfers 1GB in one sec But what’s a GB? For bandwidth, use 1GB = 109 B For storage use 1GB = 230 B = 1 075×109 B For storage, use 1GB = 2 B = 1.075×10 B So 1GB/sec is 0.93GB in one second
About 7% error
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Pitfall: Offloading to I/O ProcessorsPitfall: Offloading to I/O Processors
Overhead of managing I/O processor request may dominate Quicker to do small operation on the CPU But I/O architecture may prevent that
I/O processor may be slowerI/O processor may be slower Since it’s supposed to be simpler
Making it faster makes it into a major Making it faster makes it into a major system component
Might need its own coprocessors!
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Might need its own coprocessors!
Pitfall: Backing Up to TapePitfall: Backing Up to Tape
Magnetic tape used to have advantages Magnetic tape used to have advantages Removable, high capacity
Ad d d b di k h l Advantages eroded by disk technology developments
Makes better sense to replicate data E.g, RAID, remote mirroring
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Fallacy: Disk SchedulingFallacy: Disk Scheduling
Best to let the OS schedule disk Best to let the OS schedule disk accesses
B t mode n d i e de l ith logi l blo k But modern drives deal with logical block addresses Map to physical track cylinder sector locations Map to physical track, cylinder, sector locations Also, blocks are cached by the drive
OS is unaware of physical locations OS is unaware of physical locations Reordering can reduce performance Depending on placement and caching
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p g p g
Pitfall: Peak PerformancePitfall: Peak Performance Peak I/O rates are nearly impossible to
achieve Usually, some other system component
limits performance E.g., transfers to memory over a bus
Collision with DRAM refresh Arbitration contention with other bus masters
b k b d d h E.g., PCI bus: peak bandwidth ~133 MB/sec
In practice max 80MB/sec sustainable
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In practice, max 80MB/sec sustainable
Concluding RemarksConcluding Remarks
I/O performance measures I/O performance measures Throughput, response time Dependability and cost also important
Buses used to connect CPU, memory,I/O controllers
P lli i t t DMA Polling, interrupts, DMA I/O benchmarks
TPC SPECSFS SPECWeb TPC, SPECSFS, SPECWeb RAID
Improves performance and dependability
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p p p y
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