[ENBD] multiple clients connect to single server device

[Peter T. Breuer]

"Aris Sotiropoulos wrote:"
> > > For clients to avoid caching in a situation where multiple clients are 
> > > ENBDed connected to the same server device, they could use a special 
> > > file system (shared disk file system?) like GFS (openGFS?).
> > 
> > If you could try it and let me know, I would be grateful! I 
> > suspect that GFS would be confused.
> 
> Why do you suspect that GFS would be confused?

Because I guess that it expects the several distributed devices it
manages not to be images of the same thing!

> I would like to try GFS, but I can't. It is proprietary
> (www.sistina.com) and I have to buy the software.
> My knowledge is from web pages and publications.

> GFS is supposed to be a shared disk file system.
> For example, multiple nodes are connected through fibre channel or SCSI
> to the same disk (array).
> So, each node sees the same block device.

In that case, it sounds like it should work!

> From what I read, GFS can make those nodes (clients to the same device)
> see the same files (read/write like in an ordinary file system).
> 
> I was wondering whether there are any other file systems that
> can do the same thing.

Well, there are mirroring file systems.  I forget the name of one ...
ogre?  Could that be it?

> > Depends on the kernel. It can use O_DIRECT in its open calls 
> > (if supported by the kernel, if buffers are aligned properly, 
> > if there are no bugs, etc.), or it can do i/o instead to a 
> > raw device bound to the target.
> 
> Please, clarify the part about the raw device.

I can't. A raw device is a raw device. You bind them to other devices.
That's all you can do with them. Apart from unbinding them. Then when
you write to the raw device it is like writing to the device it is
bound to, except "raw", i.e. uncached.

> What does "i/o to a raw device bound to target" mean?

The parse is "i/o to a (raw device, bound to the target)".

Peter