急！！ 关于“MTBF”问题

sar6000

这是一个关于“MTBF”（平均故障时间）计算的问题：有那位大侠知道，多谢请教：

>; >; The reliability of a hard-disk drive typically describle in terms of a quantity called mean time between failures(MTBF). Although this quantity is called a "time", the MTBF actually is measured in drive-hours per failure.
>; >;
>; >; a. If a system contain 1000 disk drives, each of which has a 750,000 hour MTBF, which of the following best describes how often a drive failure will occur in that disk farm: once per thousand year, once per century, once per decade, once per year, pnce per month, once per week, once per day, once oerhour, once per minute, or once per second?
>; >;
>; >; b. Mortality statistics indicate that, on the average, a U.S. resident has about 1:1,000 chance of dying between ages 20 and 21 years. Deduce the MTBF hours for 20 years old. Convert this figure from hours to years. What does this MTBF tell you about the expected lifetime of a 20 year old?
>; >;
>; >; c. The manufacturer guarantees a 1-million hour MTBF for a certain model of disk drive. What can you conclude about the number of years for which one fo these drives is under warranty?

toot

是不是这个。
http://216.239.39.100/search?q=cache:Xs3j17o0zr4J:bit.csc.lsu.edu/~prkl/cs4103/hw4103_f02_05.doc+has+a+750,000+hour+MTBF&hl=zh-CN&ie=UTF-8


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  CS4103: Operating Systems

  Fall 2002
  Homework 5(partly) & 6

  The following questions are similar ones that will be given on the tests. Try to answer them all. This homework will not be graded, but it will help you to prepare for the tests. All references are to Silberschatz & Galvin "Operating System Concepts", 6th  Edition, Addison Wesley.

  I/O Systems.

  A typical PC bus structure.

  I/O hardware. Bus, port, controller.

  SCSI, IDE, ATAPI, PCI, ISA, USB.

  Polling. Basic handshaking between the host and a controller (try to name 6 steps).

  Interrupt-driven I/O cycle. System calls implementation.

  DMA transfer basic steps.

  Kernel I/O subsystem: scheduling, buffering, caching, spooling. Kernel I/O data structures.

  Typical life-cycle of a blocking read request.

  13.1 State three advantages of placing functionality in a device controller, rather than in the kernel. State three disadvantages.

  13.2 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 available)

  c. A disk drive containing user files

  d. A graphics card with direct bus connection, accessible through memory-mapped I/O

  For each of these I/O scenarios, would you design the operating system to use buffering, spooling, caching, or a combination? Would you use polled I/O or interrupt-driven I/O? Give reasons for your choices.

  13.4 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 processes busy-wait until their device is ready?

  13.5 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?

  13.7 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. Discuss three pros and three cons of each approach.

  13.8 How does DMA increase system concurrency? How does it complicate the hardware design?

  13.10 Why is it important to scale up system bus and device speeds as the CPU speed increases?

  13.11 Distinguish between a STREAMS driver and a STREAMS module.



  Mass-Storage Structure.


  Disk structure.

  Disk scheduling (FCFS, SSTF, SCAN, C-SCAN, LOOK, C-LOOK).

  Disk management: formatting, boot block, bad blocks.

  Swap-space management.

  RAID structure and levels.

  Tertiary-storage structure.


  14.2 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?

   
FCFS
SSTF
SCAN
LOOK
C-SCAN
C-LOOK




  14.8 Is disk scheduling, other than FCFS scheduling, useful in a single-user environment? Explain your answer.

  14.9 Explain why SSTF scheduling tends to favor middle cylinders over the innermost and outermost cylinders.

  14.11 Why is rotational latency usually not considered in disk scheduling?

  14.13 Why is it important to balance file system I/O among the disks and controllers on a system in a multitasking environment?

  14.14 What are the tradeoffs involved in rereading code pages from the file system, versus using swap space to store them?

  14.15 Is there any way to implement truly stable storage? Explain your answer.

  14.16 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 measured in drive-hours per failure.

  a. If a system contains 1,000 disk drives, each of which has a 750,000 hour MTBF, which of the following best describes how often a drive failure will occur in that disk farm: once per thousand years, once per century, once per decade, once per year, once per month, once per week, once per day, once per hour, once per minute, or once per second?

  b. The manufacturer guarantees a 1-million hour MTBF for a certain model of disk drive. What can you conclude about the number of years for which one of these drives is under warranty?

  14.20 Discuss the relative advantages and disadvantages of sector sparing and sector slipping.

  14.21 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 treatment of disks and tapes.


  14.23 What would be the effects on cost and performance if tape storage had the same areal density as disk storage?

  14.29 Suppose that we agree that 1 KB is 1,024 bytes, 1 MB is 1,0242 bytes, and 1 GB is 1,0243 bytes. This progression continues through terabytes, petabytes, and exabytes (1,0246). Several newly proposed scientific projects plan to be able to record and store a few exabytes of data during the next decade. To answer the following questions, you will need to make a few reasonable assumptions; state the assumptions that you make.

  a. How many disk drives would be required to hold 4 exabytes of data?

  b. How many magnetic tapes would be required to hold 4 exabytes of data?



  Protection & Security.


  Security  threats & mechanisms.

  Scope of system security (focus of control).

  Symmetric and asymmetric cryptosystems. Authentication based on a shared secret key and authentication using public-key cryptography.

  Digital signature using public-key cryptography. Digitally signing a message using a message digest.

  Access control matrix. Access control list. Capabilities.

  Firewalls. Encrypted passwords.

  System threats, worms, viruses. Stack overflow attack. Protection of a Java sandbox.

  Computer security classification.


  Represent the ownerships and permissions shown in this UNIX directory listing as an access control matrix. Treat each of the users and groups as  a domain.

  18.1 What are the main differences between capability lists and access lists?

  18.4 Consider a computer system in which "computer games" can be played by students only between 10 P.M. and 6 A.M., by faculty members between 5 P.M. and 8 A.M., and by the computer center staff at all times. Suggest a scheme for implementing this policy efficiently.

  18.6 What hardware features are needed in a computer system for efficient capability manipulation? Can these be used for memory protection?

  18.7 Consider a computing environment where a unique number is associated with each process and each object in the system. Suppose that we allow a process with number n to access an object with number m only if n >; m. What type of protection structure do we have?

  18.8 What protection problems may arise if a shared stack is used for parameter passing?

  18.12 Why is it difficult to protect a system in which users are allowed to do their own I/O?

  18.13 Capability lists are usually kept within the address space of the user. How does the system ensure that the user cannot modify the contents of the list?

  19.2 The list of all passwords is kept within the operating system. Thus, if a user manages to read this list, password protection is no longer provided. Suggest a scheme that will avoid this problem. (Hint: Use different internal and external representations.)

  19.5 Discuss a means by which managers of systems connected to the Internet could have designed their systems to limit or eliminate the damage done by a worm. What are the drawbacks of making the change that you suggest?

  19.8 What are two advantages of encrypting data stored in the computer system?

radiumguo

faint, what are you talking about?