I/O devices can be characterized by
– Behavior: input, output, storage
– Partner: human or machine
– Data rate: bytes/sec, transfers/sec
I/O bus connections
Introduction I/O 1
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I/O Device Summary I/O 2
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Dependability is important Particularly for storage devices
Performance measures
Latency (response time) Throughput (bandwidth) Desktops & embedded systems
Mainly interested in response time & diversity of devices Servers
Mainly interested in throughput & expandability of devices
I/O System Characteristics I/O 3
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Service accomplishment Service delivered
as specified
Restoration
Failure
Fault: failure of a component
– May or may not lead to system failure
Service interruption Deviation from specified service
Dependability I/O 4
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Reliability: mean time to failure (MTTF)
Service interruption: mean time to repair (MTTR)
Mean time between failures MTBF = MTTF + MTTR
Availability = MTTF / (MTTF + MTTR)
Improving Availability
Increase MTTF: fault avoidance, fault tolerance, fault forecasting Reduce MTTR: improved tools and processes for diagnosis and repair
Dependability Measures I/O 5
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Nonvolatile, rotating magnetic storage
Disk Storage I/O 6
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Each sector records Sector ID
Data (512 bytes, 4096 bytes proposed) Error correcting code (ECC)
Used to hide defects and recording errors Synchronization fields and gaps
Access to a sector involves
Queuing delay if other accesses are pending Seek: move the heads
Rotational latency
Data transfer
Controller overhead
Disk Sectors and Access I/O 7
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Given
512B sector, 15,000rpm, 4ms average seek time, 100MB/s transfer rate, 0.2ms controller overhead, idle disk
Average read time
4ms seek time
+ 1⁄2 / (15,000/60) = 2ms rotational latency + 512 / 100MB/s = 0.005ms transfer time + 0.2ms controller delay
= 6.2ms
If actual average seek time is 1ms Average read time = 3.2ms
Disk Access Example I/O 8
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Manufacturers quote average seek time
Based on all possible seeks
Locality and OS scheduling lead to smaller actual average seek times
Smart disk controller allocate physical sectors on disk Present logical sector interface to host
SCSI, ATA, SATA
Disk drives include caches
Prefetch sectors in anticipation of access Avoid seek and rotational delay
Disk Performance Issues I/O 9
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Contemporary Examples I/O 10
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Nonvolatile semiconductor storage
100× – 1000× faster than disk
Smaller, lower power, more robust
But more $/GB (between disk and DRAM)
Flash Storage I/O 11
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NOR flash: bit cell like a NOR gate
Random read/write access
Used for instruction memory in embedded systems
NAND flash: bit cell like a NAND gate
Denser (bits/area), but block-at-a-time access Cheaper per GB
Used for USB keys, media storage, …
Flash bits wears out after 10,000 to 1,000,000+ accesses Not suitable for direct RAM or disk replacement Wear leveling: remap data to less used blocks
Flash Types I/O 12
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Contemporary Examples I/O 13
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