Slave share (or SMB caching proxy) suggestion

[Jeremy Allison]

On Wed, Feb 18, 2009 at 06:46:50PM +0700, Aleksander Budzynowski wrote:
> Hi all,
> 
> Forgive me if something like this has been suggested before or already
> implemented, or was deemed unfeasible for some reason. I briefly ran my
> idea past Tridgell at LCA last month but I don't think I explained it very
> well.
> The idea is to have a share in one location that works essentially as a
> real-time replica of another share, which is located say across the WAN.
> 
> The approach, to me, seems simple: the "slave" server pretends to be just
> another ordinary client of the "master", and requests locks and oplocks as
> appropriate, to ensure that concurrent access is fairly safe. Since the
> server needs no modification (could be a Windows box), and we
> already have SMB client code, I believe this would be fairly simple to
> implement.
> 
> I'm not sure if it would provide acceptable performance in general, but I'm
> sure there are *some* applications where this would work very well, or at
> least be better than the, um, alternatives.
> 
> While the details could vary, this is the picture in my mind:
> 
> The slave server stores a local copy of the entire share, checking if files
> are up-to-date each time they are accessed. So it could double as a simple
> off-site backup, and could be accessed even if the link to the master is
> down. You might have a script or some such which ensures that everything is
> up-to-date weekly.
> 
> My understanding of SMB is not extensive, but here's how I imagine a typical
> request will flow:
> 
> -Local client wants to get a file (and oplock it)
> -Slave gets an oplock for the file from the master
> -Slave checks if local copy has same date as master copy (if not, local copy
> is updated)
> -File is sent to client
> -If client modifies file, slave can cache changes locally, or immediately
> send them on to master
> -If master breaks oplock with slave, slave breaks oplock with client
> 
> So. Does this idea have merit? Has it already been done? Is there some fatal
> flaw?

This sounds very similar to WAN accelerator products that
are already on the market (some of which already use Samba
internally). But an FLOSS version would  be most welcome.

Jeremy.