6.824 - Fall 2007

6.824 Lab 3: Reading, Writing and Sharing Files

Last change: $Date: 2007/10/02 02:13:08 $

Due: Friday, Sep 28th, 5:00pm.


In this lab, you'll continue implementing the Frangipani-like file server you started in Lab 2.

You'll start with the code you wrote for Lab 2, and add support for reading and writing file contents. You'll have to choose a representation with which to store file contents in the block server and implement the SETATTR, WRITE, and READ FUSE operations.

Continuing with the YFS Server

There are no new source files needed for this lab: you will continue to implement functionality in your existing server. There is a new tester which tests for reading and writing data correctly. Download the lab tester files from http://pdos.csail.mit.edu/6.824/labs/yfs-lab3.tgz to the yfs directory where you coded in Labs 1 and 2:
% cd ~/yfs
% wget -nc http://pdos.csail.mit.edu/6.824/labs/yfs-lab3.tgz
% rsync -av l2/* l3
% tar xzvf yfs-lab3.tgz

Note that this will overwrite start.sh and stop.sh with some new, minor bug fixes.

Lab 3 builds upon Lab 2 so make sure you pass the Lab 2 tester before proceeding. When you're done with Lab 3, you should be able to read and write files in your file system. For example, you can use the start.sh script as in Lab 2, and it will start up two separate yfs_servers, mounted at "./yfs1" and "./yfs2", which both use the same extent server for organizing its data:

% cd ~/yfs/l3
% ./start.sh
Then, you can write files to one directory and read them from the other:
% ls -l yfs1 yfs2
total 0

total 0
% echo "Hello world" > yfs1/hello.txt
% ls -l yfs1 yfs2
total 0
-rw-rw-rw- 1 root root 12 Sep  6 20:14 hello.txt

total 0
-rw-rw-rw- 1 root root 12 Sep  6 20:14 hello.txt
% cat yfs2/hello.txt 
Hello world

Afterwards, be sure to stop your all the processes:

% ./stop.sh

If you try the above commands on your Lab 2 file server, you will get an error.

Your Job

Your job is to implement SETATTR, WRITE, and READ FUSE operations in fuse.cc. As always see the FUSE lowlevel header file for the necessary function specifications. You must store the file system's contents in the extent server, so that you can share one file system among multiple servers.

You are free to store the file contents however you like within the file's extent; however, it should be very straightforward to make a file's extent simply equal to a std::string containing the contents.

If your server passes the tester (see below), then you are done with this part. If you have questions about whether you have to implement specific pieces of FUSE functionality, then you should be guided by the tester: if you can pass the tests without implementing something, then don't bother implementing it.

Please don't modify the test program or the rpc library in any way; we will be using our own versions of these files during testing.


You should test your file server using the test-lab-3.pl script. The script tests reading, writing, and appending to files, as well as testing whether files written on one server can be read through a second server. To run the tester, first start two YFS servers using the start.sh script, as above:
% ./start.sh

Now you can use test-lab-3.pl to test your file system by passing the two directories that were just created:

% ./test-lab-3.pl ./yfs1 ./yfs2
Write and read one file: OK
Write and read a second file: OK
Overwrite an existing file: OK
Append to an existing file: OK
Write into the middle of an existing file: OK
Check that one cannot open non-existant file: OK
Check directory listing: OK
Read files via second server: OK
Check directory listing on second server: OK
Passed all tests
% ./stop.sh

If test-lab-3.pl exits without printing "Passed all tests!" or hangs indefinitely, then something is wrong with your file server.



The operating system can tell your file system to set one or more attributes with each SETATTR operation. The to_set argument to your SETATTR handler is a mask that informs the method which attributes should be set. There are really only three attributes you need to worry about (but feel free to implement the others if the mood strikes you), and each has a corresponding bitmask: FUSE_SET_ATTR_SIZE, FUSE_SET_ATTR_ATIME, and FUSE_SET_ATTR_MTIME, corresponding to the file size, last access time, and last modification time of a file. To see if a particular field needs to be set, just AND (i.e., &) the to_set mask with that field's bitmask, and see if the result is non-zero. Then you can find the new value for that attribute in the attr parameter that was passed to your SETATTR handler.

You are free to implement setting these different attributes each in their own RPC, or all together in the same RPC to match the FUSE call. Either way, you'll need to somehow get the new values through the YFS client to the YFS server, and then from the server to the extent server, where it will be visible to other YFS servers using the same extent server.

Note that setting the size attribute of a file can correspond to truncating it completely to zero bytes, truncating it to a subset of its current length, or even padding bytes on to the file to make it bigger. Your system should handle all these cases correctly.


These should be pretty straightforward. A non-obvious situation may arise if the client tries to write at a file offset that's past the current end of the file. Linux expects the file system to return '\0's for any reads of unwritten bytes in these "holes". See the manpage for lseek(2) for details.

Collaboration policy

You must write all the code you hand in for the programming assignments, except for code that we give you as part of the assigment. You are not allowed to look at anyone else's solution (and you're not allowed to look at solutions from previous years). You may discuss the assignments with other students, but you may not look at or copy each others' code.

Handin procedure

You will need to email your completed code as a gzipped tar file to 6.824-submit@pdos.csail.mit.edu by the deadline at the top of the page. To do this, execute these commands:
% cd ~/yfs/l3
% make clean
% cd ..
% tar czvf `whoami`-lab3.tgz l3/
That should produce a file called [your_user_name]-lab3.tgz in your yfs/ directory. Attach that file to an email and send it to the 6.824 staff address.
For questions or comments, email 6.824-staff@pdos.csail.mit.edu.
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