1. O VERVIEW OF M ASS S TORAGE S TRUCTURE 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

2. M OVING - HEAD D ISK M ECHANISM

3. D ISK S TRUCTURE

4. 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

5. D ISK S CHEDULING 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.

6. D ISK S CHEDULING 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)

7. D ISK S CHEDULING 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

8. FCFS S CHEDULING Illustration shows total head movement of 640 cylinders. 45+85+146+85+108+110+59+2=640

9. SSTF S CHEDULING 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

10. SSTF S CHEDULING

11. SCAN S CHEDULING 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

12. SCAN S CHEDULING

13. C-SCAN S CHEDULING 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.

14. C-SCAN S CHEDULING

15. C-LOOK S CHEDULING 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.

16. E XAMPLE A disk queue with requests for I/O to blocks on cylinders 23, 89, 132, 42, 187 With disk head initially at 100

17. E XAMPLE 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

18. D ISK M ANAGEMENT Disk formatting Boot block Bad blocks

19. 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) D EVICE M ANAGEMENT

20. 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. D EDICATED D EVICES

21. 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. S HARED D EVICES

22. 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. V IRTUAL D EVICES

23. I/O H ARDWARE Three categories: Human Readable (display, keyboard) Machine Readable (disk) Communication (modem, network) Other Differences: Data Rate Keyboard80 b/s CD-ROM6 Mb/s Video display1Gb/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

24. I/O T ECHNIQUES 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

25. P ROGRAMMED I/O Issue 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

26. P ROGRAMMED 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.

27. I NTERRUPT I/O CPU 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 Read y Do something else

28. I NTERRUPT 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.

29. DMA

30. 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.

31. I/O B UFFERING 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.

32. I/O B UFFERING 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

33. S INGLE B UFFERING 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. Producer Consume r Single Buffer Input DeviceOutput Device Operating System

34. D OUBLE B UFFERING 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. Producer Consume r Buffer-1 Input DeviceOutput Device Operating System Buffer-2

35. C IRCULAR B UFFERING 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 Producer Consume r Buffer-1 Input DeviceOutput Device Buffer-2 Buffer-3 Buffer-N 11