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It sounds like a useful feature, and there are a growing number of
touch points for libvirt assisted cinder features. A summit session to
discuss how that interface should work (hopefully get a few nova folks
there as well, the interface has two ends) might be a good idea

On 27 March 2014 16:15, Trump.Zhang <zhangleiqiang at gmail.com> wrote:
> Online-extend volume feature aims to extend a cinder volume which is in-use,
> and make the corresponding disk in instance extend without stop the
> instance.
>
>
> The background is that, John Griffith has proposed a BP ([1]) aimed to
> provide an cinder extension to enable extend of in-use/attached volumes.
> After discussing with Paul Marshall, the assignee of this BP, he only focus
> on OpenVZ driver currently, so I want to take the work of libvirt/qemu based
> on his current work.
>
> A volume can be extended or not is determined by Cinder. However, if we want
> the capacity of corresponding disk in instance extends, Nova must be
> involved.
>
> Libvirt provides "block_resize" interface for this situation. For QEMU, the
> internal workflow for block_resize as follows:
>
> 1) Drain all IO of this disk from instance
> 2) If the backend of disk is a normal file, such as raw, qcow2, etc, qemu
> will do the *extend* work
> 3) If the backend of disk is block device, qemu will first judge if there is
> enough free space on the device, if only so, it will do the *extend* work.
>
> So I think the "online-extend" volume will need QEMU Assisted, which is
> simlar to BP [2].
>
> Do you think we should introduce this feature?
>
> [1]
> https://blueprints.launchpad.net/cinder/+spec/inuse-extend-volume-extension
> [2] https://blueprints.launchpad.net/nova/+spec/qemu-assisted-snapshots
>
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-- 
Duncan Thomas