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SubjectRe: Multiplexing filesystem

You may want to look at FS filter drivers in Windows 2000 as a good model
for something like this. Intermezzo also seems to do something a lot
like this, but not to the Windows 2000 degree. W2K has something
called a filter driver interface, which is super handy for
encryption, and going remote.

The filter driver idea is pretty cool, and MS has been able to
implement some pretty cool stuff with it, like snap in virus
detection software across different file systems. They also
did something called a "mini-port" file system, which was a
filter driver on top of their very complex IFS interface which
made writing remote file systems, like network clients, as lot
easier and faster.

The disadvantage to their approach was that several new
and nasty "viruses" showed up as filter drivers and used
this interface to trash file systems.

Food for thought.



On Mon, Nov 26, 2001 at 10:59:48PM -0500, Mark Richards wrote:
> Quick question, which I suspect has a long answer.
> I would like to write a multiplexing filesystem. The idea is as follows:
> The filesystem would ideally wrap another filesystem, such as nfs or smbfs or
> ext2. Most operations would just be passed to the native fs call. However, for
> some files, selectable at run time by some control singal, would actually reside
> on another file system. The other filesystem would have to be mounted.
> The idea is for a version controlling filesystem. The server would be a network
> server (hence the desire to wrap nfs) which presents a 'view' of the source
> code. When the user reserves a file for editing, the file is copied to the
> local disk. From that point on, the local file is referred to until the user
> commits the change or unreserves the file. Ideally, the local copy of the file
> could be on any file system, not one that is necessarily local. And this has to
> be totally transparent to the user, except for the step where the user
> 'reserves' the file.
> I've thought about two ways to do this. One is to wrap the 'versioning' file
> system with a multiplexor that checks fs calls to see if they are referring to a
> file that is on a different fs. The other approach is to intercept calls to the
> VFS to do the same trick.
> I'm new to the whole filesystem-coding thing, so bear with me if what i've just
> said makes no sense. So, my question (I guess it wasn't quick after all) is:
> Can it be done, and are either of my two approaches feasible? Any suggestions
> or tips?
> Thanks,
> Mark Richards
> PS please CC me if possible.
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