Users routinely copy documents to their laptops, edit the files from the road, and save the changes centrally when they get back to the office.

Unfortunately, it is all too easy to lose files, overwrite changes, or forget which version is the most recent. Windows provides an offline file storage option to address this problem, and several alternative tools are also available (see the "Similar Approaches" box).

Now a new project brings the offline storage option to Linux: OFS, the offline filesystem [1]. (Because it began at the Georg-Simon-Ohm University in Nuremberg, Germany, OFS also stands for Ohm Filesystem.)

OFS

To start, simply select the directories you need, and then the offline filesystem copies the contents of these directories to a cache on your local disk. Even if you don't have a connection to the server, you will still appear to be working on the network filesystem. In reality, you will be working with the copies in the cache. Paths stay the same whether or not you have a connection to the network.

When the connection becomes available again, the offline filesystem automatically launches a reintegration session to write the changes out to the server.

OFS is not a network filesystem in the true sense of the word. In fact, it is a layer between the network filesystems (for example, NFS or Samba) and a user view, which means that you can combine OFS with any filesystem.

How It Works

OFS code runs completely in userspace and relies on FUSE (Filesystem in Userspace [2]). FUSE provides an interface the kernel uses to forward file access to userspace programs.

The FUSE interface mainly consists of a kernel module, fuse.ko, and the Libfuse library, both of which are included with any recent Linux distribution. The kernel's VFS (Virtual Filesystem Switch) accepts the FUSE kernel module as a filesystem. FUSE forwards calls to the OFS daemon in userspace via the /dev/fuse device file.

To receive data, OFS relies on Libfuse and the C++ bindings by the Fusexx project [3] (Figure 1). Each action that affects a file or directory on an OFS filesystem triggers a function call though FUSE to the OFS daemon.

In addition to the OFS daemon shown in Figure 1, the OFS project also provides a mount helper and a file browser plugin (Figure 2). To communicate, the file browser plugin, which acts as a user interface to the OFS daemon, uses D-Bus [4]. This design makes it fairly simple to extend OFS by adding more file browser plugins, no matter which desktop you need to support.

The mount helper, mount.ofs, mounts the remote filesystem, for example, a Samba share, and launches the OFS daemon. The OFS daemon resides in userspace and accesses the remote filesystem. As with any normal application, OFS uses the standard filesystem API for this access.

To allow this to happen, the mount helper does not mount the remote filesystem at the location the admin specified in the call, but under /var/ofs/-remote/URL Hash.

The original mount point is controlled by FUSE. Thus, OFS is completely independent of the remote filesystem and will work with any implementation supported by the Linux kernel.

The OFS daemon's internal structure is service oriented, making it easily extensible. In the simplest case, the daemon passes all requests on to the remote filesystem itself. However, it is also responsible for caching and maintaining all persistent data.

When a user enables caching – for the whole filesystem, or for individual directories – OFS creates a copy of any file that is opened in the cache. When a user works with the file, the OFS daemon writes the changes, both to the cache copy and to the remote file on the server if it is available. See Figure 3 for the individual steps for opening, reading, and writing a file.

Practical Applications

To create an OFS-managed mount point, or – to put this a different way – to mount a share via OFS, use the following command:

mount -t ofs type://server/share /mountpoint

The command

mount -t ofs smb://fileserver.comp/data /network/data

mounts the SMB share data on the server fileserver.comp in the local /network/data directory. The mount.ofs mount helper uses FUSE to let the OFS daemon intercept calls to /network/data. At the same time, the mount helper breaks down the URL and executes an additional mount command to mount the remote directory:

mount -t smb //fileserver.comp/data /var/ofs/remote/URL_hash

Each OFS-managed mount point includes a state indicating whether or not the share is available and directing access to the cache or the share itself. When the network cable is removed, Linux uses D-Bus to trigger an event, which OFS then queries before setting the filesystem status to offline.

In the future, the developers would like to integrate more features, such as a timer to identify server problems or a polling component to detect the current online state.