xrgtn/nullfs

OVERVIEW

nullfs is FUSE filesystem driver which discards
all files' data written to it.

Reading from any nullfs file returns EOF.

Regarding directories, different strategies
exist and several implementations ar provided
(see below). 

DETAILED DESCRIPTION

Proper implementation requires nullfs driver to
store metadata for files, directories and special
files (devices/sockets/fifos), and this may not be
desirable in some cases because of memory
footprint.

Therefore 3 implementations are to be provided:

1. nul1fs

nul1fs implements flat filesystem without support
for directories except the "/" one. Readdir (ls)
show only "." and ".." entries in "/".

Any file can be created, read or written, because
lookup for any filename returns TRUE and reports
root:root owner and 0666 permissions.

Building and mounting:

  xrgtn@ux280p:~/jff/nullfs$ make clean
  rm -f nul1fs nullfs nulnfs *.o
  xrgtn@ux280p:~/jff/nullfs$ make
  cc   "-lfuse"  nul1fs.c   -o nul1fs
  g++   "-lfuse" -o nullfs nullfs.c++
  cc   "-lfuse"  nulnfs.c   -o nulnfs
  xrgtn@ux280p:~/jff/nullfs$ mkdir mnt
  xrgtn@ux280p:~/jff/nullfs$ ./nul1fs ./mnt

Using:

  xrgtn@xrgtn-q40:~/jff/nullfs$ ls -al ./mnt
  total 4
  drwxrwxrwx 2 root  root     0 2010-08-12 12:09 .
  drwxr-xr-x 3 xrgtn xrgtn 4096 2010-08-12 12:09 ..
  xrgtn@xrgtn-q40:~/jff/nullfs$ echo foo > ./mnt/bar
  xrgtn@xrgtn-q40:~/jff/nullfs$ cat ./mnt/bar
  xrgtn@xrgtn-q40:~/jff/nullfs$ cat ./mnt/foo
  xrgtn@xrgtn-q40:~/jff/nullfs$ ls -l ./mnt/baz
  -rw-rw-rw- 1 root root 0 2010-08-12 12:10 ./mnt/baz
  xrgtn@xrgtn-q40:~/jff/nullfs$ 

2. nulnfs

nulnfs implements nullfs with limited number of
available inodes. When upper limit is reached,
old inodes are forgotten if possible (here's
example when forgetting is impossible:
cd /tmp/nullfs/ ; while mkdir d ; do cd d ; done).
If nulnfs cannot free some inodes, it returns
ENOSPC in response to mkdir/mknod/create.

NOTE: nulnfs hasn't been finished yet (it crashes
on use) and I have no plans to continue working on
it at the moment. But two other implementations
(minimalistic nul1fs one and C++ nullfs) work just
fine as advertised.

3. nullfs

nullfs permits to create files/directories until
it gets OOM killed or malloc()/new() stop working
(in the later case ot responds with ENOMEM).