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1 Some systems provide file sharing by maintaining a single copy of a file; other systems maintain several copies, one for each of the users sharing...
10.1 Some systems provide file sharing by maintaining a single copy of a
file; other systems maintain several copies, one for each of the users
sharing the file. Discuss the relative merits of each approach?
10.6 What are the advantages and disadvantages of providing, mandatory locks instead of advisory locks whose usage is left to users' discretion?
10.10 Discuss the advantages and disadvantages of associating with remote file systems (stored on file servers) a set of failure semantics different from that associated with local file systems?
11.1 In what situations would using memory as a RAM disk be more useful than using it as a disk cache?
11.2 Consider a file systenc that uses a modifed contiguous-allocation scheme with support for extents. A file is a collection of extents, with each extent corresponding to a contiguous set of blocks. A key issue in such systems is the degree of variability in the size of the extents. What are the advantages and disadvantages of the following schemes? a. All extents are of the same size, and the size is predetermined. b. Extents can be of any size and are allocated dynamically. c. Extents can be of a few fixed sizes, and these sizes are predetermined.
11.3 Some file systems allow disk storage to be allocated at different levels of granularity. For instance, a file system could allocate 4 KB of disk space as a single 4-KB block or as eight 512-byte blocks. How could we take advantage of this flexibility to improve performance? What modifications would have to be made to the free-space management scheme in order to support this feature?
11.4 What are the advantages of the variant of linked allocation that uses a FAT to chain together the blocks of a file?
11.5 Consider a file currently consisting of 100 blocks. Assume that the filecontrol block (and the index block, in the case of indexed allocation) is already in memory. Calculate how many disk I/0 operations are required for contiguous, linked, and indexed (single-level) allocation strategies, if, for one block, the following conditions hold. In the contiguous-allocation case, assume that there is no room to grow at the beginning but there is room to grow at the end. Also assume that the block information to be added is stored in memory. a. The block is added at the beginning. b. The block is added in the middle. c. The block is added at the end. d. The block is removed from the beginning. e. The block is removed from the middle. f. The block is removed from the end.
12.1 What would be the effects on cost and performance if tape storage had the same areal density as disk storage? (Areal density is the number of gigabits per square inch.)
12.2 It is sometimes said that tape is a sequential-access medium, whereas a magnetic disk is a random-access medium. In fact the suitability of a storage device for random access depends on the transfer size. The term streaming transfer rate denotes the rate for a data transfer that is underway, excluding the effect of access latency. By contrast, the effective transfer rate is the ratio of total bytes per total seconds, including overhead time such as access latency. Suppose that, in a computer, the level-2 cache has an access latency of 8 nanoseconds and a streaming transfer rate of 800 megabytes per second, the main memory has an access latency of 60 nanoseconds and a streaming transfer rate of 80 megabytes per second, the magnetic disk has an access latency of 15 milliseconds and a streaming transfer rate of 5 megabytes per second, and a tape drive has an access latency of 60 seconds and a streaming transfer rate of 2 megabytes per seconds. a. Random access causes the effective transfer rate of a device to decrease, because no data are transferred during the access time. For the disk described, what is the effective transfer rate if an average access is followed by a streaming transfer of (1) 512 bytes, (2) 8 kilobytes, (3) 1 megabyte, and (4) 16 megabytes? b. The utilization of a device is the ratio of effective transfer rate to streaming transfer rate. Calculate the utilization of the disk drive for each of the four transfer sizes given in part a. c. Suppose that a utilization of 25 percent (or higher) is considered acceptable. Using the performance figures given, compute the smallest transfer size for disk that gives acceptable utilization. d. Complete the following sentence: A disk is a random-access device for transfers larger than ______ bytes and is a sequentialaccess device for s1naller transfers. e. Compute the minimum transfer sizes that give acceptable utilization for cache, memory, and tape. f. When is a tape a random-access device, and when is it a sequential-access device?
12.7 Compare the performance of write operations achieved by a RAID level 5 organization with that achieved by a RAID level 1 organization?
12.8 Suppose that a disk drive has 5,000 cylinders, numbered 0 to 4999. The drive is currently serving a request at cylinder 143, and the previous request was at cylinder 125. The queue of pending requests, in FIFO order, is: 86,1470,913,1774,948,1509,1022,1750,130 Starting from the current head position, what is the total distance (in cylinders) that the disk arm moves to satisfy all the pending requests for each of the following disk-scheduling algorithms? a. FCFS b. SSTF c. SCAN d. LOOK e. C-SCAN f. C-LOOK
12.9 Elementary physics states that when an object is subjected to a constant acceleration a, the relationship between distance d and time t is given by d = ~at2. Suppose that, during a seek, the disk in Exercise 12.8 accelerates the disk arm at a constant rate for the first half of the seek, then decelerates the disk arm at the same rate for the second half of the seek. Assume that the disk can perform a seek to an adjacent cylinder in 1 n"lillisecond and a full-stroke seek over all 5,000 cylinders in 18 milliseconds. a. The distance of a seek is the number of cylinders that the head moves. Explain why the seek time is proportional to the square root of the seek distance. b. Write an equation for the seek time as a function of the seek distance. This equation should be of the form t = x + y~, where t is the time in milliseconds and L is the seek distance in cylinders. c. Calculate the total seek time for each of the schedules in Exercise 12.8. Determine which schedule is the fastest (has the smallest total seek time). d. The percentage speedup is the time saved divided by the original time. What is the percentage speedup of the fastest schedule over FCFS?
12.11 Suppose that the disk in Exercise 12.9 rotates at 7,200 RPM. a. What is the average rotational latency of this disk drive? b. What seek distance can be covered in the tim.e that you found £or part a?
12.12 Suppose that a one-sided 5.25-inch optical-disk cartridge has an areal density of 1 gigabit per square inch. Further suppose that a magnetic tape has an areal density of 20 megabits per square inch and is 1/2 inch wide and 1,800 feet long. Calculate an estimate of the storage capacities of these two kinds of storage media. Suppose that an optical tape exists that has the same physical size as the magnetic tape but the same storage density as the optical disk. What volume of data could the optical tape hold? What would be a marketable price for the optical tape if the magnetic tape cost $25?
