How to implement a very simple filesystem?

Question

I am wondering how the OS is reading/writing to the hard drive.
I would like as an exercise to implement a simple filesystem with no directories that can read and write files.
Where do I start?
Will C/C++ do the trick or do I have to go with a more low level approach?
Is it too much for one person to handle?

Answer 1

Take a look at FUSE: http://fuse.sourceforge.net/

This will allow you to write a filesystem without having to actually write a device driver. From there, I'd start with a single file. Basically create a file that's (for example) 100MB in length, then write your routines to read and write from that file.

Once you're happy with the results, then you can look into writing a device driver, and making your driver run against a physical disk.

The nice thing is you can use almost any language with FUSE, not just C/C++.

Answer 2

I found it quite easy to understand a simple filesystem while using the fat filesystem on the avr microcontroller.

http://elm-chan.org/fsw/ff/00index_e.html

Take look at the code you will figure out how fat works.

Answer 3

For learning the ideas of a file system it's not really necessary to use a disk i think. Just create an array of 512 byte byte-arrays. Just imagine this a your Harddisk an start to experiment a bit. Also you may want to hava a look at some of the standard OS textbooks like http://codex.cs.yale.edu/avi/os-book/OS8/os8c/index.html

Answer 4

The answer to your first question, is that besides Fuse as someone else told you, you can also use Dokan that does the same for Windows, and from there is just a question of doing Reads and Writes to a physical partition (http://msdn.microsoft.com/en-us/library/aa363858%28v=vs.85%29.aspx (read particularly the section on Physical Disks and Volumes)).

Of course that in Linux or Unix besides using something like Fuse you only have to issue, a read or write call to the wanted device in /dev/xxx (if you are root), and in these terms the Unices are more friendly or more insecure depending on your point of view.

From there try to implement a simple filesystem like Fat, or something more exoteric like an tar filesystem, or even some simple filesystem based on Unix concepts like UFS or Minux, or just something that only logs the calls that are made and their arguments to a log file (and this will help you understand, the calls that are made to the filesystem driver during the regular use of your computer).

Now your second question (that is much more simple to answer), yes C/C++ will do the trick, since they are the lingua franca of system development, also a lot of your example code will be in C/C++ so you will at least read C/C++ in your development.

Now for your third question, yes, this is doable by one person, for example the ext filesystem (widely known in Linux world by it's successors as ext2 or ext3) was made by a single developer Theodore Ts'o, so don't think that these things aren't doable by a single person.

Now the final notes, remember that a real filesystem interacts with a lot of other subsystems in a regular kernel, for example, if you have a laptop and hibernate it the filesystem has to flush all changes made to the open files, if you have a pagefile on the partition or even if the pagefile has it's own filesystem, that will affect your filesystem, particularly the block sizes, since they will tend to be equal or powers of the page block size, because it's easy to just place a block from the filesystem on memory that by coincidence is equal to the page size (because that's just one transfer).

And also, security, since you will want to control the users and what files they read/write and that usually means that before opening a file, you will have to know what user is logged on, and what permissions he has for that file. And obviously without filesystem, users can't run any program or interact with the machine. Modern filesystem layers, also interact with the network subsystem due to the fact that there are network and distributed filesystems.

So if you want to go and learn about doing kernel filesystems, those are some of the things you will have to worry about (besides knowing a VFS interface)

P.S.: If you want to make Unix permissions work on Windows, you can use something like what MS uses for NFS on the server versions of windows (http://support.microsoft.com/kb/262965)