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Re: [Xen-devel] AFS-based VBD backend

Hi Ian,

On Thu, Dec 23, 2004 at 11:41:08AM -0000, Ian Pratt wrote:
> > I think most of this could be done in python -- the backend 
> > driver itself might be a relatively thin layer to translate 
> > block addressing to and from file byte locations, talk to the 
> > frontend, and do a periodic
> > fsync() on the underlying file to write the changes back to 
> > the AFS server.  
> You don't want to do it in python, but you do want to do it in user
> space to avoid the deadlocks with AFS.

I said that badly -- by "most of this" I mean ticket and token
management etc.  I was still thinking C for the backend itself.  But now
that you mention it...

> The blocktap backend driver is what you want. I'm not sure of it's
> current state, but the plan is to enable you to terminate a blk device
> channel in user-space.

Cool!  Where was the most recent version of that?

> The kernel loop driver works fine with most file systems, but AFS is
> very 'special' and doesn't really follow the design of the other file
> systems.

Yep.  :-}
> Of course, you could use 'unfsd -r' and export your AFS root file
> systems via same-machine NFS. This works pretty well, but I can't say
> I've hammered it.

I'm one of the folks who ran into the NFS root hangs early on -- trying
very hard to get off of it, hence this messing about with alternatives.
Granted, I'm not using same-machine NFS, don't know how much that would
reduce the hangs.

One variation on that theme that I have tested is an enbd server running
on an AFS client, serving block devices to dom0 on another machine.  ;-)
Slow, but seems stable.  Haven't tried same-machine with that either,
because it eats about 30% CPU on the enbd server under load.

Blocktap sounds closer to ideal; at least we'd get rid of the network
stack transit.

Stephen G. Traugott  (KG6HDQ)
UNIX/Linux Infrastructure Architect, TerraLuna LLC
http://www.stevegt.com -- http://Infrastructures.Org

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