Avishai Wool lecture 9 - 1 Introduction to Systems Programming Lecture 9 Input-Output Devices.

Avishai Woollecture 9 - 1

Introduction to Systems Programming Lecture 9

Input-Output Devices


I/O Software Organization


Layers of the I/O system


Device drivers

• Every I/O controller has its own control registers, timing requirements, and command sets

• Device driver is the device-specific software that talks to the controller (the adapter)

• Usually device driver is written by the device manufacturer


Device Drivers

• Logical position of device drivers is shown here• Communications between drivers and device controllers goes over the bus


Generic Device Driver Structure• Check input parameters for validity.• Translate to device terms (e.g., block number

cylinder/track/sector/head).• In use queue. • Activate device if needed (power on, spin motor)• Loop: Send control command, Wait for response.

– Usually implemented via interrupts, not by polling

• Wait for data transfer to complete.• Check for errors.• Pass data and/or status to device-independent code.


Disks

Arm

Platter

Track

Read/Write Head


Disk Terminology

• Disk Cylinders (concentric circles)

• Cylinder Tracks (one per read-write head)

• Tracks Sectors (sequentially around the track)– Sector == Block


Disk Parameters

msec


Improvement in Disk Technology

• All parameters got better, but not at same rate:

• Seek time: x 7

• Transfer time: x 1300

• Capacity: x 50000

• Surface density improved much more than mechanical performance.


Disk Layout Affects PerformanceAverageSeek

Rotationaldelay

Read time(1 track)

Number ofSectors/track

20 ms 8.3 ms 16.7 ms 32

256 sector file stored contiguously

Number of tracks = 8

Total time = seek + 8*(track time)

Track time = Rot. Delay + Read time = 8.3 + 16.7 = 25 ms

Total time = 20ms + 8*25ms = 220 ms

256 sector file stored randomly

Total time = 256 * sector time

sector time = seek + Rot. Delay + Read time = 20 + 8.3 + 0.5

= 28.8 ms

Total time = 256 * 28.8ms = 7372.8 ms

•Only one seek overhead

•Rotational delay once for every 32 sectors

•Seek and rotational delay for every sector


What goes on inside the disk


Disk Geometry

• Physical geometry of a disk with two zones


Virtual Disk Geometry

• OS / Driver does not deal with zone

• Disk exports virtual geometry, all tracks have same number of sectors

• IBM PC (BIOS) only allowed 65535 Cyl / 16 Heads / 63 Sectors per track– Max disk size: 31GB

• To avoid: disks use logical block addressing: – driver refers to blocks by number, not by

(cyl/head/sector)


A disk sector• Before a disk can be used, it needs a low-level format

16 bytes error-correcting code for 512 byte sector is normal.

“my number is sector N”


Performance through geometry

• Assume 32 sectors per track

• To read 40 sequential sectors:– read 32 sectors from track x– move head to track x+1 (slow!)– read 8 sectors

• by the time head is at track x+1, sector 0 will pass

• need to wait a whole rotation for it to return


An illustration of cylinder skew


More geometry tweaks

• Reading a sector includes:1. read data into controller’s buffer

2. compute ECC, correct errors

3. transfer data to main memory

• By the time steps 2+3 done, next sector passes the head [on slow controllers].


Disk Interleaving

• No interleaving• Single interleaving• Double interleaving


Dealing with bad blocks

• Disks have manufacturing defects

--> each track has some spare sectors

• During low-level format, a spare can replace the bad block


Error Handling

• A disk track with a bad sector• Substituting a spare for the bad sector• Shifting all the sectors to bypass the bad one


Disk Scheduling


Controller task: Disk Scheduling

• The controller receives a request for a sector, and issues an interrupt when done.

• Keep a queue of pending requests

• Serve them in some non-FIFO order to reduce average waiting time

• This is a type of a scheduling problem!


Disk Arm Scheduling Algorithms

• Time required to read or write a disk block determined by 3 factors

Seek time Rotational delay Actual transfer time

• Seek time dominates


Shortest Seek First (SSF)

Initialposition

Pendingrequests

• Go to requested cylinder that is closest to the head position

• Better average seek time than FCFS

• Unfair: cylinders near edges served more slowly


The elevator algorithm

• Go “up” and service all upward requests

• Then go “down” and serve the downward requests


Pre-fetching

• Seek time is much slower than transfer time• Controller often reads the whole track into an

internal read-ahead cache even if only one sector is requested

• Different from OS data cache:– OS caches sectors that were read, hoping they will be

needed again– Disk controller caches sectors that were convenient to

read, hoping they may be needed in future


On-disk sector cache• modern hard disks typically include

512kB to 2MB of RAM, used as a read-ahead cache– when a read request comes in, the accessed

sector is cached– subsequent sectors cached as well

• this has essentially no cost, since the drive heads need only stay over the drive surface for one revolution (approx. 8 ms at 7200 rpm) to read all sectors in a given track– typically takes heads several milliseconds to

move between tracks anyway


RAID


RAID

• Redundant Array of Inexpensive Disks

• Idea: use a box with several disks to get:– parallel I/O (can read/write to several disks at once)– fault-tolerance (disk crashes can be masked without

losing data and without down-time)

• Patterson-Gibson-Katz, 1988, suggested 6 RAID schemes, called RAID level 0-5.


RAID – Cont.

• Appears as a large disk to OS

• All the “intelligence” is in the controller

• Inside the box: e.g., a SCSI bus with several disks


RAID-0: Stripping

• Virtual disk sectors split into strips of k sectors• Strips placed on disks cyclically

• No overhead• Maximal parallelism• No fault tolerance (worse than single disk).• Not “real” RAID – no redundancy


RAID-1: Mirroring

• Every disk has a copy (a mirror)• Simple• Write does 2 I/O, Read can be from either copy

• 100% overhead• Excellent fault tolerance • 2 disk operations per write, can read either copy.


RAID-2: ECC across disks

• Use Error-Correcting Code (ECC) – example: 4 data bits, 3 parity bits

• Spread each word’s bits across the disks

• Disk drives have to be perfectly synchronized• Lower overhead than RAID-1 (depends on ECC)• Complicated, expensive controller


RAID-3: Parity Disk

• Single parity disk, stores XOR of other disks.

• Provides 1-disk crash tolerance – crashed disk position known.

• Disk drives need to be synchronized.

• Low overhead (1/N for N disks)

• No parallelism


RAID-4: Parity + Stripping

• Like RAID 3 but with strip-for-strip parity

• No drive synchronization.

• Each write causes 2 reads (data, parity) and 2 writes (new data, new parity)

• Parity drive can become a bottleneck


Writing to a Raid-4 Disk

• Initially: S0 S1 S2 S3 P03 = 0

• Want to write value V into strip 0:– Read S0– Compute ΔS = S0 V– Write V into strip 0 (replace S0)– Read P03– Write (P03 ΔS) as new parity

• Verify:– V S1 S2 S3 (P03 ΔS) = … = 0


RAID-5: Distributed Parity drive

• Like RAID-4 but parity strips spread over all disks

• More complicated controller & crash recovery process


Clocks


Clock Hardware

A programmable clock


Square-Wave mode

Repeat:

• Holding register copied into counter

• Each pulse decrements the counter

• When counter == 0, interrupt CPU

• Each interrupt is a clock tick.


Maintaining the time of day

• With battery-powered backup clock for when power is off


Simulating multiple timers with a single clock“Delta-list”


Concepts for Review• Device Driver• Disk Platter• Disk Arm• Cylinder/Track/Sector• Seek time• Cylinder skew• Disk Interleaving• Disk arm scheduling:

– SSF– Elevator

• Pre-fetching• RAID• RAID-0: Stripping• RAID-1: Mirroring• RAID-4: Parity +

Stripping• Programmable clock• Timer Delta-list

Avishai Wool lecture 9 - 1 Introduction to Systems Programming Lecture 9 Input-Output Devices.

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