1. Solutions for the Fourth Problem Set COSC 6360 Fall 2014

2. First question Which of the following properties apply to NFS?  Works with diskless workstations.  Requires all client requests to be idempotent.  Disables client caching when conflicting updates are detected.  Requires servers to trust their clients’ kernels

3. Answers  Works with diskless workstations. TRUE  Requires all client requests to be idempotent. TRUE (required by statelessness)  Disables client caching when conflicting updates are detected. FALSE (would violate statelessness)  Requires servers to trust their clients’ kernels TRUE

4. Second question Consider a distributed file system implementing close-to-open consistency. Assuming that  Alice opens the file at 9:30 AM, modifies it and closes it at 10:30 AM,  Bob opens the file at 10:00 AM, modifies it and closes it at 10:40 AM,  Carol opens the file at 10:45 AM, modifies it and closes it at 11:30 AM, Which of these three users would see his or her changes actually incorporated in the final version of the file?

5. Answer 9:3010:30 Alice 10:0010:40 Bob 10:45 11:30 Carol X

6. Third question Comparing callbacks and leases, which one is better suited to which environment? (5 pts per correct line)  The users’ workstations always remain connected to their servers.  The users’ workstations will sometimes operate in disconnected mode.

7. Answer Comparing callbacks and leases, which one is better suited to which environment? (5 pts per correct line)  The users’ workstations always remain connected to their servers. Leases are better  The users’ workstations will sometimes operate in disconnected mode. Callbacks are better

8. Fourth question How does LBFS detect identical chunks?

9. Answer How does LBFS detect identical chunks?  It uses the collision-resistant property of the SHA-1 hash function.

10. Sixth question How does Wolverine create bursty device access patterns?

11. Answer How does Wolverine create bursty device access patterns?  By flushing its entire write queues at once.

12. Seventh question How do Ceph clients find out where their data are stored? What is the main advantage of the approach?

13. Answer How do Ceph clients find out where their data are stored?  Using a pseudo-random function (CRUSH) What is the main advantage of the approach?  Metadata servers do not have to keep track of object addresses

14. Eighth question Assuming that we want to protect a FARSITE distributed file system against two simultaneous machine failures,  What would be the minimum size for all your directory groups? Answer: _____ machines  On how many machines should the contents of your files be replicated? Answer: _____ machines

15. Answers Assuming that we want to protect a FARSITE distributed file system against two simultaneous machine failures,  What would be the minimum size for all your directory groups? Answer: 2×3 + 1 = 7 machines  On how many machines should the contents of your files be replicated? Answer: 2 + 1 = 3 machines