Slides from a talk given at PythonPune meetup on 21st Feb, 2015, and later at PyConIndia on 04th Oct, 2015. FUSE-Python is probably the quickest way to prototype new filesystem ideas and rapidly build a new filesystem from scratch. In these slides I cover the basic FUSE methods and data structures necessary for implementing a working filesystem.

1. Writing flexible filesystems with
FUSE-Python
—Anurag Patel Red Hat, Pune

2. Also available at
http://xinh.org/fuse-python

3. Talk overview
UNIX based filesystems
What's FUSE?
FUSE API overview
ToyFS
Q&A

4. On filesystems
“One of the real contributions of UNIX has been
the view that everything is a file.”

5. Filesystems
FS Platform
vfat Windows
hfs+ OS X
ext4 Linux
xfs IRIX, Linux
nfs Linux*
iso9660 CD-ROM
fuse Linux*
# cat /proc/filesystems

6. Files
Symbol Meaning
d Directory
l Symbolic link
c Character device
b Block device
s Socket
p Named pipe
- Regular file

7. FUSE
Filesystem in USErspace

8. Virtual memory segregation

9. Why FUSE?
Usable by unprivileged users
Easier development cycle
Easy to install (apt-get install sshfs)
Multiple language support and bindings
C, C++, Python, Java, Ruby, Perl, Golang, Lua
Ported to FreeBSD, Mac OSX, OpenSolaris

10. FUSE overview

11. How do I install FUSE?
# yum install fuse fuse-python
# apt-get install fuse python-fuse

12. Loading fuse
# modprobe fuse
# lsmod | grep fuse

13. Filesystem class
Subclass fuse.Fuse class and implement a
number of methods.

14. Mount filesystem
Instantiate the Filesystem class and call main()

15. mount
$ python toyfs.py /tmp/toy

16. getattr(path)
Defining this method is mandatory for a working filesystem.

17. The stat structure
Member Description
st_mode Inode protection mode
st_ino File serial number
st_dev Device ID
st_nlink Number of hard links
st_uid User ID of file
st_gid Group ID of file
st_size File size in bytes
st_atime Time of last access
st_mtime Time of last data modification
st_ctime Time of last status change
ref: <sys/stat.h>

18. $ stat /etc/fstab
[anurag@zomg toyfs]$ stat /etc/fstab
File: ‘/etc/fstab’
Size: 481 Blocks: 8 IO Block: 4096 regular file
Device: fd01h/64769d Inode: 259076 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Context: system_u:object_r:etc_t:s0
Access: 2015-02-20 14:46:16.248920273 +0530
Modify: 2015-02-11 20:42:51.750210844 +0530
Change: 2015-02-11 21:56:52.520767293 +0530
Birth: -
[anurag@zomg toyfs]$

19. st_mode
st.st_mode = stat.S_IFDIR | 0755
Each bit of the output is 0 if the corresponding bit
of x AND of y is 0, otherwise it's 1

20. ST_MODE flags
Type Flag
d stat.S_IFDIR
l stat.S_IFLNK
c stat.S_IFCHR
b stat.S_IFBLK
s stat.S_IFSOCK
p stat.S_IFIFO
- stat.S_IFREG
ref: Python stat module

21. readdir(path)
Read directory contents

22. Reading files
open(path, flags)
read(path, length, offset)

23. Filesystem methods
General File operation
getattr(path) open(path, flags)
mkdir(path, mode) create(path, flags, mode)
rename(old, new) read(path, length, offset)
mknod(path, mode, rdev) write(path, buf, offset)
link(target, name) fgetattr(path)
symlink(target, name) ftruncate(path, len)
readlink(path) flush(path)
unlink(path) release(path)
fsinit(self) fsync(path, fdatasync)

24. FUSE filesystems
fuse-zip, rarfs, mysqlfs, cryptfs, httpfs, sshfs,
imapfs, gmailfs, flickrfs, ntfs-3g, gitfs, and many
many more...

25. THE END
ToyFS on Github →
FUSE Project →
FUSE Python reference →
—@gnurag anurag@redhat.com