[Linux-cluster] Re: GFS, what's remaining

[Mark Fasheh]

On Sun, Sep 04, 2005 at 12:23:43AM -0700, Andrew Morton wrote:
> > What would be an acceptable replacement? I admit that O_NONBLOCK -> trylock
> > is a bit unfortunate, but really it just needs a bit to express that -
> > nobody over here cares what it's called.
> 
> The whole idea of reinterpreting file operations to mean something utterly
> different just seems inappropriate to me.
Putting aside trylock for a minute, I'm not sure how utterly different the
operations are. You create a lock resource by creating a file named after
it. You get a lock (fd) at read or write level on the resource by calling 
open(2) with the appropriate mode (O_RDONLY, O_WRONLY/O_RDWR).
Now that we've got an fd, lock value blocks are naturally represented as
file data which can be read(2) or written(2).
Close(2) drops the lock.

A really trivial usage example from shell:

node1$ echo "hello world" > mylock
node2$ cat mylock
hello world

I could always give a more useful one after I get some sleep :)

> You get a lot of goodies when using a filesystem - the ability for
> unrelated processes to look things up, resource release on exit(), etc.  If
> those features are valuable in the ocfs2 context then fine.
Right, they certainly are and I think Joel, in another e-mail on this
thread, explained well the advantages of using a filesystem.

> But I'd have thought that it would be saner and more extensible to add new
> syscalls (perhaps taking fd's) rather than overloading the open() mode in
> this manner.
The idea behind dlmfs was to very simply export a small set of cluster dlm
operations to userspace. Given that goal, I felt that a whole set of system
calls would have been overkill. That said, I think perhaps I should clarify
that I don't intend dlmfs to become _the_ userspace dlm api, just a simple
and (imho) intuitive one which could be trivially accessed from any software
which just knows how to read and write files.
	--Mark

--
Mark Fasheh
Senior Software Developer, Oracle
mark.fasheh at oracle.com