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Lecture Notes: 31 Fuse Filesystem

··2 mins

FUSE #

Filesystem in UserSpacE

This is a mechanism that allows filesystem drivers to be written as userspace programs that interface with a generic kernel module. This was initially a Linux specific thing, but it looks like there’s also some support now on Mac and FreeBSD.

For HW09, you will build a FUSE filesystem that implements some of the features of a Linux filesystem, specifically:

  • Create files
  • List files in the FS root directory
  • Write to small files
  • Read from small files
  • Delete files
  • Rename files

When a program tries to do any of these things in your mounted filesystem - by making the appropriate system calls - the kernel will call one or more callback functions in your driver program. Those callback functions are where you implement your filesystem.

Let’s give this a try.

  • Open two terminal windows to the unpacked starter code.
  • In terminal A, we run “make” and “make mount”. This starts the FS driver in the foreground so it can print out debugging info.
  • In terminal B, we can run commands that will interact with the mounted filesystem.
  • Note: Try “make unmount”.
  • The fs is mounted at the “mnt” directory.
  • Let’s see what’s in there with “ls mnt”.
  • There’s a file. Let’s read it with “cat mnt/hello.txt”.
  • Let’s add our own file with “echo boat > mnt/boat.txt”.

Now let’s look at the starter code. We start in “nufs.c”.

  • main is at the bottom
  • It initializes a fuse_operations struct with a bunch of pointers to callback functions.
  • What functions do we get?
  • Go through all of them, noting that some are not needed for hw09.
  • Hint 1: Whenever anything doesn’t work, it’s because getattr is wrong. It needs to return the correct error codes.
  • To return an error code, return its negation from the callback.

The Hints Directory #

  • This includes some of my solution.
  • You can look at or ignore it.
  • Some of it is overkill for this assignment (show inode.h)
  • I recommend using the following stuff from hints/pages.c:
    • pages_init
    • pages_get_page
  • That’ll let you pretend you have a block device (pages_get_page gives you a pointer to one 4k disk “block”).

Let’s mess with this a bit #

  • Add another hard-coded file.
  • Make it so that you can create files, but they always contain their name and throw away any data written to them.