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

On Fri, 2006-06-30 at 20:37 +0100, Stephen C. Tweedie wrote:
> Hi,
> On Fri, 2006-06-30 at 07:17 -0700, Dan Smith wrote:
> > SCT> It depends on the environment.  To support cold/live migration,
> > SCT> having network-attached storage will be required; and file images
> > SCT> on NFS would be an extremely simple-to-setup way to achieve that.
> > 
> > Ah, but block devices can play too.  With dm-userspace, we could
> > migrate a domain from one machine to another, faulting the needed
> > blocks from its block devices on-demand, and copying the rest in the
> > background.  This would give us a peer-to-peer setup where block
> > devices could slowly move from machine to machine, following its
> > owner.  Once your block was accessed (or copied in the background),
> > it's local and fast.  A peer-to-peer NAS setup.
> Could be useful in places, but it introduces a number of new
> dependencies.  The destination host now relies on the source host for
> data, so if the source crashes, you crash the destination too; and if
> you power-cycle, how do you track where in your cluster the latest copy
> of the block device is?

It's easy.  You run code to coordinate the mapping inside a
fault-tolerant virtual machine which persists across node failures and
cluster power cycles.

> A true NAS solution isolates the Xen hosts from these problems.
> --Stephen
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