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Re: [Xen-devel] Xen 4.3 development update, 15 Oct



On 18/12/12 16:03, Alex Bligh wrote:


--On 18 December 2012 14:28:57 +0000 George Dunlap
<George.Dunlap@xxxxxxxxxxxxx> wrote:

* Make storage migration possible
   owner: ?
   status: ?
   There needs to be a way, either via command-line or via some hooks,
   that someone can build a "storage migration" feature on top of libxl
   or xl.

We have this working with qemu-xen, qcow2 and snapshot rebase. At a libxl
level (but not an xl level), everything seems to be there.

Can you describe in more detail how you implement this?  Do you have a
script or something?

We have a pile of C code :-)

A script would do something like:

1. Ask qemu to do a live snapshot using snapshot_blkdev putting the
snapshot in a new file on the new storage device. This ensures that all new
writes go to the new storage device.

2. Rebase the snapshot to a null backing file (I think that's qemu-img
rebase with -b '' though we had to submit a couple of lines of patch to
qemu to make it work) which fills the non-written blocks from the new
snapshot from the old base image and breaks the link to the the old base
image.

So it sounds like in this case you're talking about moving the disk to a different storage device connected to the same host, leaving the VM running on the same host.

What I was talking about in this was migrating the VM and storage together to a new host. People who want this typically have all VMs on local storage, so (if I'm understanding you right) the snapshot trick won't work, because host A (where it's running) can't directly access host B's disk (to which we want to migrate it).

Although I suppose one could always hack something together with sshfs or something. :-)

 -George

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