Re: Xen 4.3 development update, 15 Oct

[Alex Bligh <alex@alex.org.uk>]

--On 18 December 2012 14:28:57 +0000 George Dunlap 
<George.Dunlap@eu.citrix.com> 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.

In our implementation we have a separate hierarchy database from the parent
links stored within the qcow2 files so we get proper usage counting etc.
but that's an 'implementation detail'.

>From memory (and at risk of crossing threads) using this device model and
external snapshots you can use qemu-img resize to resize drives live at
least under KVM. Obviously the information that the drive has been resized
needs to get to the guest somehow.

-- 
Alex Bligh

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