OVERVIEW OF MASS STORAGE STRUCTURE

Magnetic disks provide bulk of secondary storage of modern computers Drives rotate at 60 to 250 times per second Transfer rate is rate at which data flow between drive

and computer Positioning time (random-access time) is time to

move disk arm to desired cylinder (seek time) and time for desired sector to rotate under the disk head (rotational latency)

Head crash results from disk head making contact with the disk surface -- That’s bad

Disks can be removable

MOVING-HEAD DISK MECHANISM

DISK STRUCTURE

DISK STRUCTURE Disk drives are addressed as large 1-dimensional

arrays of logical blocks, where the logical block is the smallest unit of transfer Low-level formatting creates logical blocks on

physical media The 1-dimensional array of logical blocks is

mapped into the sectors of the disk sequentially Sector 0 is the first sector of the first track on the

outermost cylinder Mapping proceeds in order through that track, then the

rest of the tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost

Logical to physical address should be easy Except for bad sectors Non-constant no. of sectors per track via constant angular

velocity

DISK SCHEDULING The operating system is responsible for using

hardware efficiently — for the disk drives, this means having a fast access time and large disk bandwidth.

Access time has two major components Seek time Rotational latency

Disk bandwidth is the total number of bytes ()transferred, divided by the total time between the first request for service and the completion of the last transfer.

DISK SCHEDULING When a read/write job is requested, the disk may currently

be busy All pending jobs are placed in a disk queue

could be scheduled to improve the utilization Disk scheduling increases the disk’s bandwidth

(the amount of information that can be transferred in a set amount of time)

DISK SCHEDULING

• Several algorithms exist to schedule the servicing of disk I/O requests. •We illustrate them with a request queue (0-199).

98, 183, 37, 122, 14, 124, 65, 67

Head pointer 53

FCFS SCHEDULING

Illustration shows total head movement of 640 cylinders.

45+85+146+85+108+110+59+2=640

SSTF SCHEDULING Shortest Seek Time First selects the request with

the minimum seek time from the current head position

SSTF scheduling is a form of SJF scheduling; may cause starvation of some requests

Illustration shows total head movement of 236 cylinders

SSTF SCHEDULING

SCAN SCHEDULING The disk arm starts at one end of the disk, and

moves toward the other end, servicing requests until it gets to the other end of the disk, where the head movement is reversed and servicing continues.

SCAN algorithm Sometimes called the elevator algorithm

Illustration shows total head movement of 208 cylinders

SCAN SCHEDULING

C-SCAN SCHEDULING Circular scan provides a more uniform wait time

than SCAN. The head moves from one end of the disk to the

other. servicing requests as it goes. When it reaches the other end, however, it immediately returns to the beginning of the disk, without servicing any requests on the return trip.

Treats the cylinders as a circular list that wraps around from the last cylinder to the first one.

C-SCAN SCHEDULING

C-LOOK SCHEDULING Version of C-SCAN Arm only goes as far as the last request in each

direction, then reverses direction immediately, without first going all the way to the end of the disk.

EXAMPLE A disk queue with requests for I/O to blocks on

cylinders 23, 89, 132, 42, 187 With disk head initially at 100

EXAMPLE A disk system has 100 cylinders, numbered 0 to 99.

Assume that the read / write head is at cylinder 50 and determine the order of head movement for each algorithm to satisfy the following stream of requests. They are listed in the order received.

40, 12, 22, 66, 67, 33, 80, 75, 85, 65, 8

DISK MANAGEMENT Disk formatting Boot block Bad blocks

System’s peripheral devices falls into three categories:

Dedicated Devices Shared Devices Virtual Devices Every device is different. The most important

differences among them are speed and degree of sharability.

Storage media are divided into 2 groups: Sequential Access Media Direct Access Storage devices (DASD)

DEVICE MANAGEMENT

Dedicated devices are assigned to do only one job at a time.

They serve that job for the entire time it’s active. Some devices, such as tape drives, printers, and

plotters demand this kind of allocation scheme. A Shared plotter might produce half of one user’s

graph and half of another. The disadvantage of dedicated devices is that they

must be allocated to a single user for the duration of job’s execution, which can be quite inefficient, especially when the device isn’t used 100 percent of the time.

DEDICATED DEVICES

Shared devices can be assigned to several processes.

For instance, a disk pack, or any other direct access storage device, can be shared by several processes at the same time by interleaving their requests, but this interleaving must be carefully controlled by the Device Manager.

All conflicts- - such as when process A and Process B each need to read

from the same disk pack.- Must be resolved based on predetermined policies to

decide which request will be handled first.

SHARED DEVICES

They are combinations of Dedicated and shared devices.

For example, Printers (dedicated device) are converted into sharable devices through spooling program that reroutes all print requests to a disk.

Only when all of a job’s output is complete, and the printer is ready to print out the entire document, is the output sent to the printer for printing.

Because disk are sharable devices, this technique can convert one printer into several virtual printers, thus improving both its performance and use.

Spooling is a technique that is often used to speed up slow dedicated devices.

VIRTUAL DEVICES

I/O HARDWARE Three categories:

Human Readable (display, keyboard) Machine Readable (disk) Communication (modem, network)

Other Differences: Data Rate

Keyboard 80 b/s CD-ROM 6 Mb/s Video display 1Gb/s

Application Disk requires file system support

Complexity of Control Keyboard driver simpler than disk driver

Unit of Transfer (byte, block) Data Representation (char set, parity) Error Conditions

I/O TECHNIQUES Programmed I/O

Processor issues I/O command, waits for the operation to complete

Often handles I/O transfer details Interrupt-Driven I/O

Processors issues I/O command, then proceeds to another process or thread

Device interrupts the CPU when the data is ready to be moved to memory

Direct Memory Access (DMA) Processor issues I/O command Device transfers data to/from memory (CPU will wait for

memory) Device interrupts the CPU when the I/O transfer is

completed

PROGRAMMED I/OIssue Read

Command to I/O Module

Read Status of I/O module

Read Word from I/O Module

Write Word into Memory

Check Status

Done?

Yes

No

CPU Memory

I/O CPU

Error Condition

I/O CPU

CPU I/O

Not Ready

Ready

PROGRAMMED I/O In the programmed I/O, the I/O device does not

have direct access to memory. The data transfer from an I/O device to memory

requires the execution of a program or several instructions by CPU, So that this method is said to be a programmed I/O.

In this method, the CPU stays in a program loop until the I/O unit indicates that it is ready for data transfer.

It is time consuming process for the CPU. The programmed I/O method is particularly useful

for low speed computers.

INTERRUPT I/OCPU issue read command to I/O

device

Read Status of I/O device

Read Word from I/O device

Write Word into Memory

Check Status

Done?

Yes

No

CPU Memory

I/O CPU

Error Condition

I/O CPU

CPU I/O

Ready

Not Ready

Do something else

INTERRUPT I/O In programmed I/O CPU has to wait for the ready

signal from the I/O device. In interrupt driven I/O, CPU issue a read command

to I/O device about the status, and then go to do some important task.

When I/O device is ready the device sends an interrupt signal to the processor.

When the CPU received the interrupt signal from I/O device, it checks the status, if the status is ready then the CPU read the word from I/O device and write the word in to the main memory.

If the operation done successfully, then the processor go on to the next instruction.

DMA

DMA Interrupt initiated I/O is better than programmed I/O

but, the CPU is on the path at the time of data transfer. So, it is not suitable for large amount of data transfers. An alternative approach is DMA. In this method, remove the CPU from path, and letting

the DMA controller to manage the memory buses directly.

It would improve the speed of transfer. The DMA controller takes over the buses to manage the

transfer directly between the I/O device and memory. The DMA is first initialized by CPU and CPU sends some

information which includes: The starting address of memory block where data are

available for read or where the data are to be stored. The word count, which is the number of word in the block. Mode of transfer such as read or write. A control to start the DMA transfer.

I/O BUFFERING A “buffer” is a temporary storage area. Buffer stores the data when data transferred

between two devices or one device and one application.

For example, a user want to take a printout of his program, the user issue a print command, then the program temporarily stored in the printer, the printer consume the data from the Printer Buffer.

This type of mechanism is called buffering.

I/O BUFFERING Buffering is done for three reason:

If the producer device is high speed, the consumer device is low speed at that time, buffering is required. Example: Hard Disk (Producer) – Printer (Consumer)

If two devices having different data transfer sizes (Block Size), then buffering required. In networks a large message is divided in to a number of small

packets. The packets are sent over the network, the receiving side places this packets into buffer, to convert in to the packet sizes of source devices.

A third case of buffering is to support copy semantics for application I/O. Example: Copying the data between kernel buffers and

application data space.

Buffers are of 3 types: Single Buffering Double Buffering Circular Buffering

SINGLE BUFFERING In this buffering only one buffer is used to transfer the

data between two devices. The producer produces the one block of data into the

buffer after that the consumer consumes the buffer, only when the buffer is empty, the processor again produce the data.

The main drawback of this method is the data transfer rate is very low because the producer has to wait while the consumer consuming the data.

ProducerConsum

erSingle Buffer

Input Device Output Device

Operating System

DOUBLE BUFFERING In this scheme, two buffers are used in the place of

one. Producer produces one buffer, at the same time

consumer consumes another buffer. So the producer, need not to wait for filling the buffer.

ProducerConsum

erBuffer-1

Input Device Output Device

Operating System

Buffer-2

CIRCULAR BUFFERING When more than two buffers are used, the collection

of buffers is itself referred to as a circular buffer. Each individual buffer is being one unit in the circular

buffer. The data transfer rate will increase using the circular buffer rather than double buffering.

Operating System

ProducerConsum

er

Buffer-1

Input Device Output Device

Buffer-2

Buffer-3

Buffer-N

1 1