Distributed Systems Lab
3: Reading, Writing and Sharing Files
Due: Friday, Nov 25th,
11:59pm
Introduction
In this lab, you will continue to add
functionality to YFS supporting SETATTR, WRITE and READ operations.
Getting started
First, merge your solution to Lab 2 with the new
code for Lab 3. There are no substantive differences in our code between the
labs, except for a new tester. We have also provided stubs for the FUSE
functions you need to implement in order to make your job easier. You can use
the following commands to get the Lab 3 sources; see the directions for Lab 2 for more background on git.
% cd lab
% git commit -am 'my solution to lab2'
Created commit ...
% git pull
remote: Generating pack...
...
% git checkout -b lab3 origin/lab3
Branch lab3 set up to track remote branch lab3 from origin.
Switched to a new branch 'lab3'
% git merge lab2
As before, if git reports any conflicts,
edit the files to merge them manually, then run git commit -a. 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, use the start.sh script to start up two separate yfs_clients with mountpoints at "./yfs1"
and "./yfs2", which both use the same extent server for organizing
its data:
Then, you can write files to one directory and read them from the
other:
% ls -l yfs1 yfs2
yfs1:
total 0
yfs2:
total 0
% echo "Hello world" > yfs1/hello.txt
% ls -l yfs1 yfs2
yfs1:
total 0
-rw-rw-rw- 1 root root 12 Sep 6 20:14 hello.txt
yfs2:
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 all your processes:
If you try the above commands without implementing the
required SETATTR, WRITE and READ operations, you will get an error.
Your Job
Your job is to implement SETATTR, WRITE, and READ FUSE operations
in fuse.cc. You must store the file system's contents in the extent server,
so that you can share one file system among multiple yfs_clients.
If your server passes the tester test-lab-3.pl, then you are done. 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
grading.
The test-lab-3.pl script tests reading, writing, and appending to files, as
well as testing whether files written on one yfs_client instance can be read through a second one. To run the
tester, first start two yfs_clients using the start.sh script. It runs two
yfs_client processes each mounting the same file system under
a different name (yfs1 or yfs2).
Now run test-lab-3.pl by passing the
yfs1 and yfs2 mountpoints. Since the script
tests each yfs_client
sequentially, we do not need to worry about
locking for this lab.
% ./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.
Detailed Guidance
ˇ
Implementing SETATTR
The operating system can tell your file system to set one
or more attributes via the FUSE SETATTR operation. As always, see the FUSE lowlevel header file for the necessary function
specifications. The to_set argument to
your SETATTR handler is a mask that informs the method which attributes should
be set. There is really only one attribute (the file size attribute) you need
to pay attention to (but feel free to implement the others if you like), which
has the corresponding bitmask, FUSE_SET_ATTR_SIZE. Just AND
(i.e., &) the to_set mask
value with an attribute's bitmask to see if the attribute is to be set. The new
value for the attribute to be set is in the attr parameter passed to your SETATTR handler.
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.
ˇ
Implementing READ/WRITE:
You are free to store the file contents however you like
with the extent_server. You may, for example, treat a
file's contents as a std::string. (If so, how would
you support very large files? We would not test your FS for large files, but
it's a good design exercise.)
READ/WRITE operations are 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.
Handin
procedure
You will need to email your completed code (without binaries) as a
gzipped tar file to ds-assignment@mpi-sws.org by the deadline stated at the top of the page. To do this, switch
to the source directory and execute these commands:
% tar czvf MATR1-MATR2-lab3.tgz lab/
That should produce a file called [MATR1-MATR2]-lab3.tgz in that directory, where MATR1
and MATR2 are the matriculation
numbers of the team members. Attach that file to an email and send it to the
address above with the subject "Assignment 3 - LastName1 LastName2".
You will receive full credit
if your software passes the same tests we gave you when we run your software on
our machines.
Questions
or comments regarding this course? Please use the general course mailing list or
the teaching staff mailing list.
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