Re: [Xen-devel] [PATCH] Blktap: Userspace file-based image support.(RFC)

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> I would ask how exactly do you propose to do this ? Today at least
> file-backed domains seems to be the only real world way of doing
> migrations. Migrating block devices seems a little hairy (what if the
> other machine is already using sda for example), and may not be all the
> practical to do.

Well, it doesn't really matter what the destination dom0 is using as block 
devices provided the node name doesn't have to stay the same on the 
destination machine - and if you use the hotplug scripts to set up block 
devices then it doesn;t.

I think being able to demand-fault virtual disks across would be quite cool 
(with the copy eventually completing in the background, eliminating the 
origin as a point of failure.  For smaller, or more ad-hoc setups this could 
be quite useful (especially if you had a daemon trickle updates across the 
network continuously at low bandwidth to minimise the diffs during migration)

Cheers,
Mark

>
> > ST> The destination host now relies on the source host for data, so if
> > ST> the source crashes, you crash the destination too;
> >
> > Sure, which a NAS solves, assuming the NAS is stable.
> >
> > ST> and if you power-cycle, how do you track where in your cluster the
> > ST> latest copy of the block device is?
> >
> > I think that keeping metadata on that and invalidating blocks when you
> > pull them off the source host could be done without too much trouble.
> > Plus, I'm not talking about multiple-writers, so I think you could
> > ignore a lot of the normal locking issues.
> >
> > ST> A true NAS solution isolates the Xen hosts from these problems.
> >
> > Absolutely.  So what's the benefit of having image files on NFS (as
> > you mentioned) if you can use nbd or iSCSI?
> >
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>
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-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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