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

On Fri, 2006-06-30 at 13:06 -0700, Dan Smith wrote:
> ST> Could be useful in places, but it introduces a number of new
> ST> dependencies.  
> I was mostly commenting about making migrating block devices as easy
> as (or easier) than file-backed domains, especially from a migration
> point of view.  Being able to use local LVMs but still migrate easily
> without a NAS would be cool, I think, where appropriate.

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.

> 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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