Книга: Distributed operating systems

PROBLEMS

PROBLEMS

1. What is the difference between a file service using the upload/download model and one using the remote access model?

2. A file system allows links from one directory to another. In this way, a directory can "include" a subdirectory. In this context, what is the essential criterion that distinguishes a tree-structured directory system from a general graph-structured system?

3. In the text it was pointed out that shared file pointers cannot be implemented reasonably with session semantics. Can they be implemented when there is a single file server that provides UNIX semantics?

4. Name two useful properties that immutable files have.

5. Why do some distributed systems use two-level naming?

6. Why do stateless servers have to include a file offset in each request? Is this also needed for stateful servers?

7. One of the arguments given in the text in favor of stateful file servers is that i-nodes can be kept in memory for open files, thus reducing the number of disk operations. Propose an implementation for a stateless server that achieves almost the same performance gain. In what ways, if any, is your proposal better or worse than the stateful one?

8. When session semantics are used, it is always true that changes to a file are immediately visible to the process making the change and never visible to processes on other machines. However, it is an open question as to whether or not they should be immediately visible to other processes on the same machine. Give an argument each way.

9. Why can file caches use LRU whereas virtual memory paging algorithms cannot? Back up your arguments with approximate figures.

10. In the section on cache consistency, we discussed the problem of how a client cache manager knows if a file in its cache is still up-to-date. The method suggested was to contact the server and have the server compare the client and server times. Does this method fail if the client and server clocks are very different?

11. Consider a system that does client caching using the write-through algorithm. Individual blocks, rather than entire files, are cached. Suppose that a client is about to read an entire file sequentially, and some of the blocks are in the cache and others are not. What problem may occur, and what can be done about it?

12. Imagine that a distributed file uses client caching with a delayed write back policy. One machine opens, modifies, and closes a file. About half a minute later, another machine reads the file from the server. Which version does it get?

13. Some distributed file systems use client caching with delayed writes back to the server or write-on-close. In addition to the problems with the semantics, these systems introduce another problem. What is it? (Hint: Think about reliability.)

14. Measurements have shown that many files have an extremely short lifetime. What implication does this observation have for client caching policy?

15. Some distributed file systems use two-level names, ASCII and binary, as we have discussed throughout this chapter; others do not, and use ASCII names throughout. Similarly some file servers are stateless and some are stateful, giving four combinations of these two features. One of these combinations is somewhat less desirable than its alternatives. Which one, and why is it less desirable?

16. When file systems replicate files, they do not normally replicate all files. Give an example of a kind of file that is not worth replicating.

17. A file is replicated on 10 servers. List all the combinations of read quorum and write quorum that are permitted by the voting algorithm.

18. With a main memory file server that stores files contiguously, when a file grows beyond its current allocation unit, it will have to be copied. Suppose that the average file is 20K bytes and it takes 200 nsec to copy a 32-bit word. How many files can be copied per second? Can you suggest a way to do this copying without tying up the file server's CPU the entire time?

19. In NFS, when a file is opened, a file handle is returned, analogous to a file descriptor being returned in UNIX. Suppose that an NFS server crashes after a file handle has been given to a user. When the server reboots, will the file handle still be valid? If so, how does it work? If not, does this violate the principle of statelessness?

20. In the bit-map scheme of Fig. 5-16, is it necessary that all machines caching a given file use the same table entry for it? If so, how can this be arranged?

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