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

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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