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

["Stephen C. Tweedie" <sct@xxxxxxxxxx>]

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?

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

--Stephen



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