Questions and Answers of Computer Engineering

Consider a file system where a file can be deleted and its disk space reclaimed while links to that file still exist. What problems may occur if a new file is created in the same storage area or with
Some systems automatically delete all user files when a user logs off or a job terminates, unless the user explicitly requests that they be kept; other systems keep all files unless the user
Why do some systems keep track of the type of a file, while others leave it to the user or simply do not implement multiple file types? Which system is “better?”
What are the advantages and disadvantages of recording the name of the creating program with the file’s attributes (as is done in the Macintosh Operating System)?
Could you simulate a multilevel directory structure with a single-level directory structure in which arbitrarily long names can be used? If your answer is yes, explain how you can do so, and contrast
Explain the purpose of the open and close operations.
Some systems automatically open a file when it is referenced for the first time, and close the file when the job terminates. Discuss the advantages and disadvantages of this scheme as compared to the
Give an example of an application in which data in a file should be accessed in the following order: a. Sequentially b. Randomly
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
In some systems, a subdirectory can be read and written by an authorized user, just as ordinary files can be. a. Describe the protection problems that could arise. b. Suggest a scheme for dealing
Consider a system that supports 5000 users. Suppose that you want to allow 4990 of these users to be able to access one file. a. How would you specify this protection scheme in UNIX? b. Could you
Researchers have suggested that, instead of having an access list associated with each file (specifying which users can access the file, and how), we should have a user control list associated with
What is a file?
List sample file types, based on use, on the VAX under VMS.
List some file types on the VAX under VMS.
What does OPEN do?
What does CLOSE do?
List advantages of operating system “knowing” and supporting many file types.
List the disadvantages of operating system “knowing” and supporting many file types.
What is a sequential file?
What is direct access?
How does user specify block to be fetched in direct access?
Can a direct access file be read sequentially? Explain.
How can an index file be used to speed up the access in direct-access files?
Explain what ISAM is.
List two types of system directories
List operations to be performed on directories?
List disadvantages of using a single directory.
What is the MFD? UFD? How are they related?
What advantages are there to this two-level directory?
What disadvantages are there to this two-level directory?
How do we overcome the disadvantages of the two-level directory?
What is a file path name?
If we use the two-level directory, how do we access common files and programs, like FORTRAN compiler? Show two or more ways.
Why would we want a subdirectory in our account?
List steps you need to follow to delete a subdirectory in your account.
What is an acyclic graph?
List ways to share files between directories in operating systems.
What problems might arise on deletion if a file is shared?
What problems might arise on deletion if a file is shared? Discuss.
How can we solve this problem?
What is a general graph?
Give advantages of each directory structure above.
Consider a file currently consisting of 100 blocks. Assume that the file control block (and the index block, in the case of indexed allocation) is already in memory. Calculate how many disk I/O
Consider a system where free space is kept in a free-space list. a. Suppose that the pointer to the free-space list is lost. Can the system reconstruct the free-space list? Explain your answer. b.
What problems could occur if a system allowed a file system to be mounted simultaneously at more than one location?
Why must the bit map for file allocation be kept on mass storage, rather than in main memory?
Consider a system that supports the strategies of contiguous, linked, and indexed allocation. What criteria should be used in deciding which strategy is best utilized for a particular file?
One problem with contiguous allocation is that the user must preallocate enough space for each file. If the file grows to be larger than the space allocated for it, special actions must be taken. One
Fragmentation on a storage device could be eliminated by recompaction of the information. Typical disk devices do not have relocation or base registers (such as are used when memory is to be
How do caches help improve performance? Why do systems not use more or larger caches if they are so useful?
In what situations would using memory as a RAM disk be more useful than using it as a disk cache?
Why is it advantageous for the user for an operating system to dynamically allocate its internal tables? What are the penalties to the operating system for doing so?
Consider the following backup scheme: Day 1: Copy to a backup medium all files from the disk. Day 2: Copy to another medium all files changed since day 1. Day 3: Copy to another medium all files
List three ways of allocating storage, and give advantages of each.
What is contiguous allocation?
What main difficulty occurs with contiguous allocation?
What is a “hole” in contiguous allocation method?
Explain first-fit, best-fit, and worst-fit methods of allocating space for contiguous files.
What is external fragmentation in a system with contiguous files?
How can we overcome fragmentation?
What is preallocation? Why do it?
What is linked allocation, as detailed in text?
Can linked allocation have external fragmentation? Internal fragmentation?
Can linked allocation be used for direct-access files?
What is indexed allocation?
Rank the allocation methods on speed.
List four ways a system could use to determine which sectors are free. Give advantages of each way.
List kinds of information we’d likely want to keep in a directory, and estimate number of bytes needed to store each compactly.
What data structures can be used for directory information?
What problems might arise with above data structures?
State three advantages of placing functionality in a device controller, rather than in the kernel state three disadvantages
Consider the following I/O scenarios on a single-user PC. a. A mouse used with a graphical user interface b. A tape drive on a multitasking operating system (assume no device preallocation is
Describe three circumstances under which blocking I/O should be used. Describe three circumstances under which nonblocking I/O should be used. Why not just implement nonblocking I/O and have
Why might a system use interrupt-driven I/O to manage a single serial port, but polling I/O to manage a front-end processor, such as a terminal concentrator?
UNIX coordinates the activities of the kernel I/O components by manipulating shared in-kernel data structures, whereas Windows NT uses object-oriented message passing between kernel I/O components.
How does DMA increase system concurrency? How does it complicate hardware design?
Why is it important to scale up system bus and device speeds as the CPU speed increases?
None of the disk-scheduling disciplines, except FCFS, is truly fair (starvation may occur). a. Explain why this assertion is true. b. Describe a way to modify algorithms such as SCAN to ensure
Suppose that a disk drive has 5000 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
From elementary physics, we know that when an object is subjected to a constant acceleration a, the relationship between distance d and time t is given by d = ½ at2. Suppose that, during a seek,
Suppose that the disk in Exercise 14.3 rotates at 7200 RPM. a. What is the average rotational latency of this disk drive? b. What seek distance can be covered in the time that you found for part a?
The accelerating seek described in Exercise 14.3 is typical of hard-disk drives. By contrast, floppy disks (and many hard disks manufactured before the mid-1980s) typically seek at a fixed rate.
Compare the performance of C-SCAN and SCAN scheduling, assuming a uniform distribution of requests. Consider the average response time (the time between the arrival of a request and the completion of
Is disk scheduling, other than FCFS scheduling, useful in a single-user environment? Explain your answer.
Explain why SSTF scheduling tends to favor middle cylinders over the innermost and outermost cylinders.
Requests are not usually uniformly distributed. For example, a cylinder containing the file system FAT or in odes can be expected to be accessed more frequently than a cylinder that only contains
Why is rotational latency usually not considered in disk scheduling? How would you modify SSTF, SCAN, and C-SCAN to include latency optimization?
How would use of a RAM disk affect your selection of a disk-scheduling algorithm? What factors would you need to consider? Do the same considerations apply to hard-disk scheduling, given that the
Why is it important to balance file system I/O among the disks and controllers on a system in a multitasking environment?
What are the trade offs involved in rereading code pages from the file system versus using swap space to store them?
Is there any way to implement truly stable storage? Explain your answer.
The reliability of a hard-disk drive is typically described in terms of a quantity called mean time between failures (MTBF). Although this quantity is called a “time,” the MTBF actually is
The term “fast wide SCSI-II” denotes a SCSI bus that operates at a data rate of 20 megabytes per second when it moves a packet of bytes between the host and a device. Suppose that a fast wide
More than one disk drive can be attached to a SCSI bus. In particular, a fast wide SCSI-II bus (see Exercise 14.17) can be connected to atmost 15 disk drives. Recall that this bus has a bandwidth of
Remapping of bad blocks by sector sparing or sector slipping could influence performance. Suppose that the drive in Exercise 14.17 has a total of 100 bad sectors at random locations and that each
Discuss the relative advantages and disadvantages of sector sparing and sector slipping.
The operating system generally treats removable disks as shared file systems but assigns a tape drive to only one application at a time. Give three reasons that could explain this difference in
In a disk jukebox, what would be the effect of having more open files than the number of drives in the jukebox?
If magnetic hard disks eventually have the same cost per gigabyte as do tapes, will tapes become obsolete, or will they still be needed? Explain your answer.

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