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Input/Output Systems: Disk Systems, Dependability, and RAID Technologies - Prof. Jiang Li, Study notes of Computer Architecture and Organization

This document, authored by dr. Jiang li from the dept. Of systems & computer science at howard university, provides an in-depth exploration of various aspects of input/output (i/o) systems, focusing on disk systems, dependability, and redundant array of independent disks (raid) technologies. Topics such as bytes/sec and transfers/sec, i/o bus connections, response time, diversity of devices, faults and service interruption, mean time to repair (mttr), mean time between failures (mtbf), availability, disk storage, disk latency, flash storage, flash types, raid 1, 2, 3, 4, 5, and 6, bus types, bus signals and synchronization, interrupts, i/o data transfer, and file system & web benchmarks.

Typology: Study notes

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Uploaded on 08/18/2009

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Jiang Li
Dept. of Systems & Computer Science, Howard Univ. 1
Input/Output, Disk Systems
(8.1, 8.2, 8.4 ~ 8.7, 8.9)
Dr. Jiang Li
Slides adapted from various sources (e.g. VT, RPI, UCSB etc)
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Input/Output, Disk Systems

Dr. Jiang Li

Slides adapted from various sources (e.g. VT, RPI, UCSB etc)

Introduction

 I/O devices can be characterized by

Behaviour: input, output, storage

Partner: human or machine

Data rate: bytes/sec, transfers/sec

 I/O bus connections

Dependability

 Fault: failure of acomponent

May or may not leadto system failure

Service accomplishment

Service delivered

as specified

Service interruption

Deviation from

specified service

Failure

Restoration

Dependability Measures

Reliability: mean time to failure (MTTF)

A measure of the continuous service accomplishment

Service interruption: mean time to repair (MTTR)

Mean time between failures

MTBF = MTTF + MTTR

Availability = MTTF / (MTTF + MTTR)

A measure of the service accomplishment with respect to thealternation between accomplishment and interruption.

Improving Availability

Increase MTTF: fault avoidance, fault tolerance, fault forecasting

Reduce MTTR: improved tools and processes for diagnosis andrepair

Magnetic Disks

 A magnetic disk consists of 1- platters (metal or glass disk covered with magnetic recordingmaterial on both sides), with diameters between 1-3.5 inches  Each platter is comprised of concentric tracks (5- 30K) and each track is divided into sectors (100 – 500 per track, each about 512 bytes)

Each sector records

 Sector ID, data (512 bytes, 4096 bytes proposed), error correcting code(ECC, Used to hide defects and recording errors), synchronizationfields and gaps  A movable arm holds the read/write heads foreach disk surface and moves them all in tandem –a cylinder of data is accessible at a time

Disk Latency

To read/write data, the arm has to be placed on the correcttrack – this

seek time

usually takes 5 to 12 ms on average

  • can take less if there is spatial locality 

Rotational latency

is the time taken to rotate the correct

sector under the head – average is typically more than 2ms (15,000 RPM)

Transfer time

is the time taken to transfer a block of bits out

of the disk and is typically 3 – 65 MB/second

A disk controller maintains a disk cache (spatial localitycan be exploited) and sets up the transfer on the bus(

controller overhead

Queuing delay if other accesses are pending

Disk Access Time Example

 Average seek time: 6ms  Transfer rate: 50 MB/sec  Controller overhead is 0.2ms  What is the average time to read or write a 512-byte sector for a disk of 10000RPM? Average disk access time = Average seek time + Average rotational delay

  • Transfer time + Controller overhead = 6.0ms + 0.5/10000RPM/(60000ms/min)
  • 0.5KB/50MB/sec/1000 + 0.2ms = 9.2ms

Disk Performance Issues

 Manufacturers quote average seek time

Based on all possible seeks

Locality and OS scheduling lead to smaller actualaverage seek times

 Smart disk controller allocate physical sectors ondisk

Present logical sector interface to host

 Disk/motherboard interface

SCSI, ATA, SATA

 Disk drives include caches

Prefetch sectors in anticipation of access

Avoid seek and rotational delay

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

RAID

 Redundant Array of Inexpensive (Independent)Disks

Use multiple smaller disks (c.f. one large disk)

Parallelism improves performance

Plus extra disk(s) for redundant data storage

 Provides fault tolerant storage system

Especially if failed disks can be “hot swapped”

 RAID 0

No redundancy (“AID”?)

Just stripe data over multiple disks

But it does improve performance

RAID 3: Bit-Interleaved Parity

 N + 1 disks

Data striped across N disks at byte level

Redundant disk stores parity

For example: with 9 disks, bit 0 is in disk-0, bit 1 is indisk-1, …, bit 7 is in disk-7; disk-8 maintains parity forall 8 bits

Read access

Read all disks

Write access

Generate new parity and update all disks

On failure

Use parity to reconstruct missing data

 Not widely used

RAID 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

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

Not widely used

RAID 5: Distributed Parity

 N + 1 disks

Like RAID 4, but parity blocks distributed across disks

Avoids parity disk being a bottleneck

 Widely used

RAID 6: P + Q Redundancy

 N + 2 disks

Like RAID 5, but two lots of parity

Greater fault tolerance through more redundancy

 Multiple RAID

More advanced systems give similar fault tolerance withbetter performance