Disc Allocation and Disc Arm Scheduling?
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1 CSE 2431: Introduction to Operating Systems Disc Allocation and Disc Arm Scheduling? Study: 9 th (12.4, 12.5, 10.4) 10 th (14.4, 14.5, 11.2) Presentation K Gojko Babić Moving-Head Disk Mechanism Figure 10.1 A sector (usually 512 bytes) is a basic unit of transfer (read/write) g. babic Presentation K 2 1
2 File Allocation Methods For the operating system, disk drives are addressed as large 1-dimensional arrays of logical blocks, where the logical block corresponds to a sector. The 1-dimensional array of logical blocks is usually mapped into the sectors of the disk as follows: Block 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. An allocation method refers to how disk blocks are allocated for files: Contiguous allocation, Linked allocation, Indexed allocation. Presentation K 3 Contiguous Allocation Figure 12.5 Each file occupies a set of contiguous blocks on the disk. Only starting location (block #) and length (in blocks) required. Presentation K 4 2
3 Pros Fast sequential access Easy random access Easy to recover in case of crash Cons Contiguous Allocation (cont.) External fragmentation Hard to grow files Presentation K 5 Linked Allocation Figure 12.6 Needs only starting block address and the last for efficient appending. Each file is a linked list of disk blocks: blocks may be scattered. Presentation K 6 3
4 Linked Allocation (cont.) Pros Can grow files dynamically Space efficient, little fragmentation Cons Random/direct access: horrible Unreliable: losing a block means losing the rest Need some bytes to store pointers Presentation K 7 Indexed Allocation Figure 12.8 Presentation K 8 4
5 Indexed Allocation (cont.) Solves external fragmentation Supports sequential and direct access Disc access always requires an access to index block first. it can be cached in main memory File can be easily extended Requires extra space for index block, possible wasted space Extension to big files issue Presentation K 9 Indexed Allocation (cont.) Link full index blocks together using last entry. Unix i-nodes are similar. Presentation K 10 5
6 Disc Free-Space Management Operating system has to also keep track of unallocated blocks on disk. Two approaches Bit map Linked list Bit map approach: bit[i] = 0 => block [i] free block bit[i] = 1 => block [i] occupied Bit map requires extra space. Easy to get contiguous files Presentation K 11 Free-Space Management: Linked list Figure no waste of space, but cannot get contiguous space easily and needs to protect pointers of a free list g. babic Presentation K 12 6
7 The operating system is responsible for using hardware efficiently for the disk drives, that also means having a fast disc access time. Disc access time has two major components: Seek time: the time for the disk to move the heads to the cylinder containing the desired sector. Rotational latency; the time waiting for the disk to rotate for the desired sector to get under the disk head. Plus negligible transfer time. Goal to minimize seek time Seek time seek distance Disc Arm Scheduling O.S. has a disc queue with track numbers for each sector requiring access Presentation K 13 First Come First Serve (FCFS) Order Selects the request from the queue of requests that arrived first Figure 10.4 Wild head swing may happen. Illustration shows total head movement of 640 cylinders. 14 7
8 Shortest Seek Time First - SSTF Selects the next request with the minimum seek time Figure 10.5 A form of SJF scheduling, and it may cause starvation SSTF tries to minimize seek time. But, is it optimal? Illustration shows total head movement of 236 cylinders. Presentation K 15 SCAN or Elevator Algorithm 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. Figure 10.6 Illustration shows total head movement of 236 cylinders. Presentation K 16 8
9 Circular SCAN or C-SCAN The head moves from one end of the disk to the other, servicing requests as it goes. When it reaches the end, it immediately returns to the beginning of the disk, without servicing any requests on the return trip. Treats the cylinders as a circular list. Figure 10.7 Provides a more uniform wait time than SCAN. Presentation K 17 LOOK and C-LOOK LOOK and C-LOOK are versions of SCAN and C-SCAN, respectively. Arm only goes as far as the last request in each direction, then reverses direction immediately, without going all the way to the end of the disk. C-LOOK Figure 10.8 Presentation K 18 9
10 Is O.S. Disc Arm Scheduling Possible? Modern discs have complex geometry, with multiple surfaces and different recording zones, plus random sectors on the given disc may be corrupted. To hide this complexity, as we have stated, disks are addressed by O.S. as large 1-dimensional arrays of logical blocks. Thus, it is not clear how O.S. could map a given a logical block number into its corresponding track number. But, since in the disc queue for each of previous algorithms it is assumed that track numbers are derived from logical bloc number, it appears that O.S. can t do disc arm scheduling. It is a disc controller who does mapping from a logical block number to its sector physical address, i.e. to its surface, its track number and its sector number. Presentation K 19 Disc Arm Scheduling Algorithm Problem A personal computer salesperson remarked during his/her sales pitch that the company included very much improved version of a disc scheduling algorithm in its newest UNIX version. After buying it, one customer wanted to test a salesman claim. He/ she wrote a program to randomly read 10,000 blocks spread across the disc. To his/her amazement, the performance that he/she measured was identical to what would be expected from FCFS. Was the salesperson lying? Explain your answer. Presentation K 20 10
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