Creating Filesystems with Ruby and FUSE

Posted by Steve on Fri 7 Nov 2008 at 19:30

The FUSE project allows you to create filesystems in userspace - which means you can create a filesystem without having to get your hands dirty and modify your kernel source. This is insanely cool, and can be used for many purposes. Here we're going to look at using the Ruby bindings to create a simple filesystem.

The FUSE project allows you to create a filesystem entirely in userspace merely by writing a couple of primitives - such as:

Get contents of directory.

Get file permissions.

Write to file.

With enough of the basic primitives implemented you'll end up with a working filesystem. Unfortunately unless you're a fan of working in C this process is a little more complex than it should be. To ease this there are bindings for Perl, Ruby, and other languages which makes getting started very simple.

My recent FUSE experiments have been Ruby-based, using the libfusefs-ruby1.8 packages.

Assuming you're working with a recent kernel you should be able to get started writing a filesystem very quickly. Here's what it took for me:

trashvm:~# apt-get install libfusefs-ruby1.8 fuse-utils ruby1.8 Reading package lists... Done Building dependency tree... Done The following extra packages will be installed: libfuse2 libruby1.8 The following NEW packages will be installed fuse-utils libfuse2 libfusefs-ruby1.8 libruby1.8 0 upgraded, 4 newly installed, 0 to remove and 0 not upgraded. Need to get 1716kB of archives. After unpacking 6476kB of additional disk space will be used. ..

Once I installed the packages I inserted the fuse kernel module:

trashvm:~# modprobe fuse

Assuming this completes without errors you're ready to begin! Lets create a simple filesystem that just proves we can. Create the file hello.rb :

require 'fusefs' class HelloDir def contents(path) ['hello.txt'] end def file?(path) path == '/hello.txt' end def read_file(path) "Hello, World!

" end end hellodir = HelloDir.new FuseFS.set_root( hellodir ) # Mount under a directory given on the command line. FuseFS.mount_under ARGV.shift FuseFS.run

Once you've done that you can mount your filesystem by running:

trashvm:~# mkdir hello/ trashvm:~# ruby hello.rb hello/ &

Once you've done this you should find you've got a directory called "hello/" which has our filesystem mounted in it. This filesystem just creates a file called "hello.txt" - and when you read it you'll get the expected contents:

trashvm:~/hello# ls hello.txt trashvm:~/hello# cat hello Hello, World!

You can now kill the userspace program and all will be gone:

trashvm:~# kill -9 %1 [1]+ Killed ruby1.8 hello.rb hello trashvm:~# umount hello trashvm:~# ls hello trashvm:~#

All gone. Now is time to confess that I didn't create that hello.rb file - it is one of the examples included with the libfusefs-ruby1.8 package. If you take a look beneath /usr/share/doc/libfusefs-ruby1.8 directory you'll find several examples which each work in a similar way to the one we've demonstrated.

So, what could you do with a FUSE filesystem? Well if you're creative there are almost no limits. As a more useful example I've placed a simple filesystem I created online at the foot of this article. This allows you to mount MySQL as a filesystem:

skx@gold:/mysql/yawns$ ls about_pages.data notifications.data related.data about_pages.sql notifications.sql related.sql adverts.data permissions.data scratchpads.data adverts.sql permissions.sql scratchpads.sql articles.data poll_anon_voters.data sessions.data articles.sql poll_anon_voters.sql sessions.sql bookmarks.data poll_answers.data submission_notes.data bookmarks.sql poll_answers.sql submission_notes.sql comments.data poll_questions.data submissions.data comments.sql poll_questions.sql submissions.sql db_version.data poll_submissions_answers.data tags.data db_version.sql poll_submissions_answers.sql tags.sql ip_blacklist.data poll_submissions.data tips.data ip_blacklist.sql poll_submissions.sql tips.sql messages.data poll_voters.data users.data messages.sql poll_voters.sql users.sql news.data preferences.data weblogs.data news.sql preferences.sql weblogs.sql

Each directory beneath /mysql is a database, and each table of that database is presented as two files:

foo.sql - Just the table structure.

- Just the table structure. foo.data - The contents of the table.

The code is very simple and mostly revolves around invoking mysqldump correctly, but despite that it is surprisingly useful.

I'd love to hear suggestions on interesting uses for FUSE, and what you're doing with it!

This is the code which implements the MySQL filesystem. It is very short as it only involves implementing a couple of methods: