Chapter 6
Storage and Other I/O
Topics
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I/O Devices Characterization: Dependability, Performance, and Storage Technologies - Prof., Study notes of Computer Architecture and Organization
I/o devices' characteristics, focusing on dependability, performance measures like latency and throughput, and various storage technologies such as disk and flash storage. It covers topics like i/o bus connections, dependability measures, disk sectors and access, disk performance issues, and flash types.
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Chapter 6
Storage and Other I/O
Topics
Introduction
I/O devices can be characterized by
Behaviour: input, output, storage
Partner: human or machine
Data rate: bytes/sec, transfers/sec
Example: keyboard, sound output, network, magnetic tape
I/O bus connections
§6.1 Introduction
Dependability
Fault: failure of a
component
May or may not lead
to system failure
§6.2 Dependability, Reliability, and Availability
Service accomplishment
Service delivered
as specified
Service interruption
Deviation from
specified service
Restoration Failure
Dependability Measures
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
Disk Sectors and Access
Each sector records
Sector ID
Data (typically 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 (position head over proper track)
Rotational latency
Data transfer
Controller overhead
Disk Access Example
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
+ ½ / (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
Flash Storage
Nonvolatile semiconductor storage
100 × – 1000 × faster than disk
Smaller, lower power, more robust
But more $/GB (between disk and DRAM)
§6.4 Flash Storage
Flash Types
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 1000’s of accesses
Not suitable for direct RAM or disk replacement
Wear leveling: remap data to less used blocks
Bus Types
Processor-Memory buses
Short, high speed
Design is matched to memory organization
I/O buses
Longer, allowing multiple connections
Specified by standards for interoperability
Do not typically interface directly to memory
Connect to memory using either a processor-
memory or backplane bus
Bus Signals and Synchronization Data lines Carry address and data Multiplexed or separate Control lines Indicate data type, synchronize transactions Synchronous Uses a bus clock Asynchronous Uses request/acknowledge control lines for handshaking