CSci 555 / Neuman-Obraczka Fall 1999
Directions for completing exam
 

1. (15 points) Leases, call backs, TS invalidation.
   • (5 points) Are call backs equivalent to leases with infinite term? Justify your answer.
   • (10 points) Do leases avoid file inconsistencies? Use an example to illustrate your answer describing it using the protocol exchange form we covered in class.

2. (10 points) LFS improves file system performance by collecting multiple writes into one big write to the log. Suppose you want to keep a more traditional file layout.
   • (5 points) What technique could you use and why does it improve performance?
   • (5 points) What are its advantages and disadvantages when compared to LFS?

3. (15 points) RAID
   • (5 points) Why can RAID disk systems use simple parity to correct single disk failures?
   • (10 points) RAID levels 1 and 5 are the ones that are actually used in real systems. Why is RAID 1 preferred over RAID 5? And why is RAID 5 preferred over RAID 1?

4. (20 points) Kernels: We have looked at different ways of extending operating systems services.
   • (10 points) What are the two substantially different ways of extending the OS we have covered and what system illustrates each?
   • (10 points) List the advantages and disadvantages of one when compared to the other.

5. (40 points) Home computing in past years predominantly consisted of home PC's, and subscriptions to online services like America Online or Earthlink. The online service providers supported the sharing of data by managing huge systems for electronic mail and the hosting of their customer's web pages.. The customers private data was stored on their personal computer, presumably safe from the world (we know otherwise). With the advent of always-on connections like DSL and cable modems, more and more home users are running servers (e.g., web, mail, and even DNS) directly on their home machines. In this question, you are asked to consider the implications of this change in usage and discuss the steps needed to keep such a system running optimally. Answer the questions below using your own paper.
   • (10 points) How does this new system {\em architecture} affect scalability? Consider all three components of scale, say how this new model causes growth in that component, and describe how that growth affects the operation of the system.
   • (5 points) What affect will this new architecture have on reliability and availability of system services? Consider the resources and expertise available to system administrators, and who these administrators are.
   • (5 points) Do you consider this new architecture better or worse than the current architecture? Why? (both answers are acceptable, it is the justification of your response that is important).
   • (10 points) How would you handle security for such a system and how would it be administered? Consider authentication, authorization, and protection of the users home machine. (10 points)
   • (10 points) How might you improve the availability and reliability of the services provided by the user's home systems? In this improved system, how does the role of the online service providers compare with their role today?