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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

> > Q:  Why not just use a loop device on top of AFS, with the 'file:' 
> >     VBD type?
> > A:  Loop devices on top of AFS files hang with large volumes of 
> >     I/O -- looks like a deadlock of some sort (in my tests, a dd of
> >     around 2-300 Mb into an AFS-based loop device appears to
> >     consistently hang the kernel, even with a 500Mb or larger AFS
> >     cache).  In addition, an unmodified loop.c will not fsync() the
> >     underlying file; changes won't get written back to the AFS server
> >     until loop teardown.  I've added an fsync() to the worker thread of
> >     loop.c to take care of this every few seconds; that seems to work
> >     but I can't really stress test it much because of the hang problem.
> 
> That it's already possible to use normal files. :)
> 
> So, no need for explicit support in xen. If your dom0 already knows and
> uses afs, just specify the file in the xen configuration:
> 
> disk = [ 'file:/afs/file,sda1,w' ]

The OP did have an explanation why he didn't want to use a loop
device. However, I would say that the correct approach here is
probably to enhance/fix the existign loop support rather than adding a
whole new backend driver.

 -- Keir


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