Open Stack

I am interested in following similar patterns with the XenServer SM work (for NFS and local volumes).
We have the same issues around the hypervisor needing to be involved when taking a snapshot (but with VHDs in our case).
https://blueprints.launchpad.net/cinder/+spec/xenapi-storage-manager

We will try get back with some more concrete details once we sit down and flesh out the blueprint a bit more.
We have done the prototyping to discover the issues now.

John

-----Original Message-----
From: Christoph Hellwig [mailto:hch at infradead.org] 
Sent: 05 November 2012 11:38 AM
To: OpenStack Development Mailing List
Subject: Re: [openstack-dev] Island: Local Storage Volume Support for Cinder

On Fri, Nov 02, 2012 at 03:54:02PM -0700, Vishvananda Ishaya wrote:
> So we dealt with this exact issue when doing snapshots for instances. 
> According to the libvirt folks, creating and/or deleting a snapshot 
> using qemu-image while a disk is in use is not safe.

It's indeed not.  Creating snapshots of any image format while qemu is running requires doing it using the qemu monitor.  Note that this is the exact same snapshot code that qemu-img would invoke, but doing it from qemu means it is serialized vs I/O from the guest.

> Unfortunately libvirt does not expose a reasonable method for 
> snapshotting and extracting data from a live disk, so the only way to 
> do this safely is to use qemu-monitor (or patch libvirt to expose the 
> right interface). This is why the instance snapshot command in kvm is 
> doing a managed save before snapshotting the vm. If we had a good way 
> to do this it would greatly help the instance snapshot code as well.

What do you mean with a managed save?


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