Project 5a: Distributed File Server
In this assignment, you will be developing a working distributed file
server. We provide you with only the bare minimal UDP communication code;
you have to build the rest.
A Basic File Server
Your file server is built as a stand-alone UDP-based server. It should wait
for a message and then process the message as need be, replying to the given
Your file server will store all of its data in an on-disk file which will
be referred to as the file system image . This image contains the on-disk
representation of your data structures; you should use these system calls
to access it: open(), read(), write(), lseek(), close(), fsync() .
To access the file server, you will be building a client library. The
interface that the library supports is defined in mfs.h . The
library should be called libmfs.so , and any programs that wish to access
your file server will link with it and call its various routines.
On-Disk File System: A Log-Structured File System
Your on-disk file system structures should roughly follow that of the
log-structured file system discussed in class. On-disk, the first structure
should be a singular checkpoint region. The checkpoint region should contain a
disk pointer to the current end of the log; it should also contain pointers
to pieces of the inode map (assume there are a maximum of 4096 inodes; assume
each piece of the inode map has 16 entries).
Other than the checkpoint region, your on-disk image just consists of an
ever-growing log (i.e., we won't be implementing cleaning). Thus, whenever you
write to the disk, you'll just write all file system updates to the end of the
log, and then update the checkpoint region as need be. For example, if you are
adding a new block to a file, you would write the data block, new version of
the inode, and a new piece of the inode map to the end of the log; when this
write completes, you should update the checkpoint region with the requisite
The inode map is just an array, indexed by inode number. Each entry is a
simple 4-byte integer, which is just the disk address of the location of the
inode in question.
Each inode should be simple: a size field (the number of the last byte in
the file), a type field (regular or directory), and 14 direct pointers; thus,
the maximum file size is 14 times the 4KB block size, or 56 KB.
One other structure you'll have to manage on disk are directories. Each
directory has an inode, and points to one or more data blocks that contain
directory entries. Each directory entry should be simple, and consist of 64
bytes: a name and an inode number pair. The name should be a fixed-length
field of size 60 bytes; the inode number is just an integer (4 bytes). When a
directory is created, it should contain two entries: the name . , which
refers to this new directory's inode number, and .. , which refers to the
parent directory's inode number. For directory entries that are not yet in use
(in an allocated 4-KB directory block), the inode number should be set to
-1. This way, utilities can scan through the entries to check if they are
When your server is started, it is passed the name of the file system image
file. If this file does not exist, the file server should create it, and
initialize it properly, and force it to disk. Such initialization includes
creating the checkpoint region, the initial inode map, and creating a single
root directory with proper . and .. entries. The root inode number
should be 0.
When booting off of an existing image, your server should read in the
checkpoint region (and keep an in-memory version of it), as well as the entire
inode map and keep it in-memory too.
The client library should export the following interfaces:
- int MFS_Init(char *hostname, int port): MFS_Init() takes a host name
and port number and uses those to find the server exporting the file system.
- int MFS_Lookup(int pinum, char *name): MFS_Lookup() takes the parent
inode number (which should be the inode number of a directory) and looks up
the entry name in it. The inode number of name is returned. Success:
return inode number of name; failure: return -1. Failure modes: invalid pinum,
name does not exist in pinum.
- int MFS_Stat(int inum, MFS_Stat_t *m): MFS_Stat() returns some
information about the file specified by inum. Upon success, return 0,
otherwise -1. The exact info returned is defined by MFS_Stat_t. Failure modes:
inum does not exist.
- int MFS_Write(int inum, char *buffer, int block): MFS_Write() writes a
block of size 4096 bytes at the block offset specified by block . Returns 0
on success, -1 on failure. Failure modes: invalid inum, invalid block, not a
regular file (because you can't write to directories).
- int MFS_Read(int inum, char *buffer, int block): MFS_Read() reads
a block specified by block into the buffer from file specified by
inum . The routine should work for either a file or directory; directories
should return data in the format specified by MFS_DirEnt_t. Success: 0,
failure: -1. Failure modes: invalid inum, invalid block.
- int MFS_Creat(int pinum, int type, char *name): MFS_Creat() makes a
file ( type == MFS_REGULAR_FILE) or directory ( type == MFS_DIRECTORY)
in the parent directory specified by pinum of name name . Returns 0 on
success, -1 on failure. Failure modes: pinum does not exist, or name is too
long. If name already exists, return success (think about why).
- int MFS_Unlink(int pinum, char *name): MFS_Unlink() removes the file or
directory name from the directory specified by pinum . 0 on success, -1
on failure. Failure modes: pinum does not exist, directory is NOT empty. Note
that the name not existing is NOT a failure by our definition (think about why
this might be).
- int MFS_Shutdown(): MFS_Shutdown() just tells the server to force all
of its data structures to disk and shutdown by calling exit(0). This interface
will mostly be used for testing purposes.
The key behavior implemented by the server is idempotency.
Specifically, on any change to the file system state (such as a MFS_Write,
MFS_Creat, or MFS_Unlink), all the dirtied buffers in the server are committed
to the disk. The server can achieved this end by calling fsync() on the
file system image. Thus, before returning a success code, the file system
should always fsync() the image.
Now you might be wondering: why do this? Simple: if the server crashes, the
client can simply timeout and retry the operation and know that it is OK to do
so. We'll be talking about this more when we talk about NFS, the Network File
System from the company formerly known as Sun.
Now you might be wondering: how do I implement a timeout? Simple, with the
select() interface. The select() calls allows you to wait for a reply
on a certain socket descriptor (or more than one, though that is not needed
here). You can even specify a timeout so that the client does not block
forever waiting for data to be returned from the server. By doing so, you can
wait for a reply for a certain amount of time, and if nothing is returned, try
the operation again until it is successful.
The TAs will provide some images for you to get started with, but you
should be able to create your own empty ones and populate them
accordingly. The TAs will also provide some fun tools to use your file system,
like mfsput (which will take a filename as input, and create a file in your
MFS file system based on that file) and mfscat (which will dump the contents
of a file in MFS to the screen).
Your server program must be invoked exactly as follows:
prompt> server [portnum] [file-system-image]
The command line arguments to your file server are to be interpreted as follows.
- portnum: the port number that the file server should listen on.
- file-system-image: a file that contains the file system image.
If the file system image does not exist, you should create it and properly
initialize it to include an empty root directory.
Your client library should be called libmfs.so and be built as usual. It
should implement the interface as specified by mfs.h , and in particular
deal with the case where the server does not reply in a timely fashion; the
way it deals with that is simply by retrying the operation, after a timeout of
some kind (default: five second timeout).
Some Helper Code
To get you going, we have written some simple UDP code that can send a
message and then receive a reply from a client to a server. It can be found in