Lecture 45 Syed Mansoor Sarwar

Lecture 45 Syed Mansoor Sarwar
Operating Systems Lecture 45 Syed Mansoor Sarwar

© Copyright Virtual University of Pakistan
Agenda for Today Review of the previous lecture Space Allocation Techniques Free Space Management Disk Structure and Scheduling Access time and disk bandwidth Disk scheduling algorithms (FCFS, SSTF, Scan, C-Scan, Look, C-Look) Course Recap Future Directions 17 September 2018 © Copyright Virtual University of Pakistan

Review of Lecture 44 File Sharing and Protection Read, write, and execute permissions User, group, and others chmod, ls –l, ls –ld, umask, ls –i Per Process File Descriptor Table Space Allocation Techniques Contiguous, Linked, Index 17 September 2018 © Copyright Virtual University of Pakistan

Index Allocation Need index table Random access Linked scheme – Linked list of index blocks … … Directory Entry … 17 September 2018 © Copyright Virtual University of Pakistan

Index Allocation Two-level index table … File Data Blocks Directory Entry … … First Level Index 17 September 2018 © Copyright Virtual University of Pakistan

UNIX Allocation 17 September 2018 © Copyright Virtual University of Pakistan

UNIX Allocation 4 KB block 4-byte disk pointers 10 direct pointers to file blocks Maximum file size? Amount of space needed to store pointers? 17 September 2018 © Copyright Virtual University of Pakistan

File Allocation Table (FAT)
17 September 2018 © Copyright Virtual University of Pakistan

Free-Space Management
Bit vector 1 2 n-1 … 0  block[i] is free 1  block[i] is occupied bit[i] =  Block number calculation (number of bits per word) * (number of 0-value words) + offset of first 1 bit 17 September 2018 © Copyright Virtual University of Pakistan

Example for overhead of bit map Block size = 4 KB = 212 bytes Disk size = 40 GB = 40 * 230 bytes Overhead = 40 * 230/212 = 40 * 218 bits = 40 * 32 KB = 1280 KB Easy to get contiguous files 17 September 2018 © Copyright Virtual University of Pakistan

Linked list (free list) Cannot get contiguous space easily Overhead (number of pointers used to maintain free space) 17 September 2018 © Copyright Virtual University of Pakistan

17 September 2018 © Copyright Virtual University of Pakistan

Grouping First (n-1) pointers to free blocks and last pointer to another such block Counting An entry contain address of the first free block and number of free contiguous blocks that follow the first block—good for contiguous allocation 17 September 2018 © Copyright Virtual University of Pakistan

I/O Operations Number of I/O operations (inserting, deleting, and reading a file block) needed for the various allocation schemes Assumptions Directory, Bit-map, and index blocks are in the main memory Worst-case and best-case scenarios 17 September 2018 © Copyright Virtual University of Pakistan

I/O Operations File size of 100 blocks Insert a block after the 50th block Read 50th block Insert 101st block Delete 50th block Assumptions Directory, Bit-map, and index blocks are in the main memory Worst-case and best-case scenarios 17 September 2018 © Copyright Virtual University of Pakistan

Secondary Storage Management
Maintains the file system, its directories, and keeps track of free secondary storage space Provides device drivers to control transfer of data between memory and secondary storage devices Optimizes the completion of I/O tasks by employing algorithms to facilitate efficient disk usage File Management System I/O System Secondary Storage Management System 17 September 2018 © Copyright Virtual University of Pakistan

Disk Structure Disk drives are addressed as large 1-dimensional arrays of logical blocks, the smallest units of transfer (usually 512 bytes). The 1-dimensional array of logical blocks is mapped into the sectors of the disk sequentially. Block 0 is the first sector of the first track on the outermost cylinder. Mapping proceeds in order through that track, then through the rest of the tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost. 17 September 2018 © Copyright Virtual University of Pakistan

Disk Scheduling Disk Access Time has two major components Seek time is the time for the disk heads to the desired cylinder (track) Rotational latency is the time waiting for the disk to rotate the desired sector under the disk head Want to minimize seek time 17 September 2018 © Copyright Virtual University of Pakistan

Disk Scheduling 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 We can improve access time and bandwidth by scheduling the servicing of disk I/O request in a good order. 17 September 2018 © Copyright Virtual University of Pakistan

Disk Scheduling Several algorithms exist to schedule the servicing of disk I/O requests. These include: First-Come-First-Serve (FCFS) Shortest Seek Time First (SSTF) Scan Circular Scan (C-Scan) Look C-Look 17 September 2018 © Copyright Virtual University of Pakistan

FCFS Illustration shows total head movement of 640 cylinders. 17 September 2018 © Copyright Virtual University of Pakistan

Shortest Seek Time First (SSTF)
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. 17 September 2018 © Copyright Virtual University of Pakistan

SCAN 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. Also known as the elevator algorithm Total head movement is 208 cylinders 17 September 2018 © Copyright Virtual University of Pakistan

LOOK Version of SCAN Arm only goes as far as the last request in each direction, then reverses direction immediately, serving requests while going in the other direction. That is, it looks for a request before continuing to move in a given direction. 17 September 2018 © Copyright Virtual University of Pakistan

Recap of Lecture Space Allocation Techniques Free Space Management Disk Structure and Scheduling Access time and disk bandwidth Disk scheduling algorithms (FCFS, SSTF, Scan, C-Scan, Look, C-Look) 17 September 2018 © Copyright Virtual University of Pakistan

Recap of Course OS services and structures Processes and threads Process scheduling Concurrent processes Deadlocks in computer systems Memory management Virtual memory File system interface File system implementation Secondary storage management 17 September 2018 © Copyright Virtual University of Pakistan

Lecture 45 Syed Mansoor Sarwar

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