Lab 1 Solutions

This handout was adapted from Jerry Cain’s Spring 2018 offering.

The first and last exercises are problem set-esque questions that could easily appear on a midterm or final exam. In fact, all of the questions asked under Problem 3 were on previous midterms and finals. The middle problem is an experiment that’ll require you fire up your laptop and play with some programs.

I’ve created a Slack channel for Lab 1 discussion (aptly named #lab1), and all students (but particularly remote students) are encouraged to share their ideas there.

Problem 1: Direct, Singly Indirect, and Doubly Indirect Block Numbers

Assume blocks are 512 bytes in size, block numbers are four-byte ints, and that inodes include space for 6 block numbers. The first three contain direct block numbers, the next two contain singly indirect block numbers, and the final one contains a doubly indirect block number. (Note: This is different from the scheme we’ve discussed in class.)

Problem 2: Experimenting with the stat utility

This problem is more about exploration and experimentation, and not so much about generating a correct answer. The file system reachable from each myth machine consists of the local file system (restated, it’s mounted on the physical machine) and networked drives that are grafted onto the fringe of the local file system so that all of AFS – which consists of many, many independent file systems from around the globe – all contribute to one virtual file system reachable from your local / directory.

Log into myth53 and use the stat command line utility (which is a user program that makes calls to the stat system call as part of its execution) and prints out oodles of information about a file. Type in the following commands and analyze the output:

The output for each of the five commands above all produce the same device ID but different inode numbers. Read through this to gain insight on what the Device values are.

For each of the above commands, replace stat with stat -f to get information about the file system on which the file resides (block size, inode table size, number of free blocks, number of free inodes, etc).

Now log into myth54 and run the same commands. Why are the outputs of stat and stat -f the same in some cases and different in others?

Now analyze the output of the stat utility when levied against AFS mounts where the master copies of all /usr/class and /usr/class/cs110 files reside. Do this from both myth53 and myth54.

Why are most of the outputs the same for myth53 compared to myth54? Which ones are symbolic links? Why are the device numbers for remotely hosted file systems so small?

What about these commands?

What files can you see within the dartmouth.edu and asu.edu mounts?

Problem 3: Short Answer Questions

Provide clear answers and/or illustrations for each of the short answer questions below. Each of these questions is either drawn from old exams or based on old exam questions. Questions like this will certainly appear on your own midterm.

  1. The dup system call accepts a valid file descriptor, claims a new, previously unused file descriptor, configures that new descriptor to alias the same file session as the incoming one, and then returns it. Briefly outline what happens to the relevant file entry table and vnode table entries as a result of dup being called. (Read man dup if you’d like, though don’t worry about error scenarios).
    • The vnode table entry is left alone, but a new file descriptor is claimed and set to address the same entry in the file entry table session as the incoming one, and the reference count within that session entry would be incremented by one.
  2. Now consider the prototype for the link system call (peruse man link). A successful call to link updates the file system so the file identified by oldpath is also identified by newpath. Once link returns, it’s impossible to tell which name was created first. (To be clear, newpath isn’t just a symbolic link, since it could eventually be the only name for the file.) In the context of the file system discussed in lecture and/or the file system discussed in Section 2.5 of the secondary textbook, explain how link might be implemented.
    • Resolve oldpath to its inode number, append new directory entry to sequence of existing directory entries where newpath resides. Fill in entry with last component of newpath, and fill in inumber with the inumber of old path. Finally, increment the reference count within the inode itself to be clear the file has one more name.
  3. Explain what happens when you type cd .././../. at the shell prompt. Frame your explanation in terms of our discussion of directories in lecture, and the fact that the inode number of the current working directory is the only relevant global variable maintained by your shell.
    • Search cwd’s payload for .., set inumber of cwd to inumber associated with ..; repeat three more times for ., then .., and then . :)
  4. All modern file systems allow symbolic links to exist as shortcuts for longer absolute and relative paths (e.g. sanitycheck might be a symbolic link for /afs/.ir/users/r/e/rebs/cs110/assign1/tools/sanitycheck, and tests.txt might be a symbolic link for ./mytests/tests.txt. Explain how your the absolute pathname resolution process we discussed in lecture would need to change to resolve absolute pathnames to inode numbers when some of the pathname components might be symbolic links.
    • The absolute pathname resolution process (implemented as pathname_lookup in assign2) should piecemeal tokenize pathnames as usual. When component (e.g. tests.txt) is identified as symlink, prepend expansion to unprocessed set of tokens, continue for relative paths from inumber of symlink parent, restart from inode 1 for absolute paths.