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

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> 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.

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.

A future revision of blocktap could use kiovec's to avoid having to copy
data into user space, and thus would give good performance.

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.

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.

Ian
 


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