Re: Question about Backing Up RBD Volumes in Openstack

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Dave,

OpenStack does "qemu-img snapshot" command to create a snapshot, here's the method:

https://github.com/openstack/nova/blob/stable/folsom/nova/virt/libvirt/utils.py#L335-L347

So the memory is _not_ saved, only the disk is. Note that it's always hard to make consistent snapshot. I assume that freezing the filesystem itself is the only solution to have a consistent snapshot, and still this doesn't mean that your application's data are consistent , in term of commits for instance with a database. I don't know which applications do you run but at the end, you might consider to stop some daemon as well, freeze the fs (to be sure), take your snapshot, unfreeze, start your daemon. You could reach 100% consistency with this method… I guess :)

Cheers.

––––
Sébastien Han
Cloud Engineer

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On Apr 7, 2013, at 10:13 PM, Wido den Hollander <wido@xxxxxxxx> wrote:

Hi Dave,

Disclaimer: I'm not an OpenStack user.

On 04/05/2013 03:43 PM, Dave Spano wrote:
If I pause my instances in Openstack, then snapshot and clone my volumes, I should have a consistent backup correct? Is freezing on snapshot creation like LVM a potential future feature?

No, probably not. Unless you are also saving the memory state of the instance you can't be 100% sure you snapshot is consistent.

Writes can still be buffered inside the VM and those do not end up in your snapshot.

So to get 100% consistent snapshot you will have to flush the buffers, freeze the filesystem, snapshot and unfreeze.


I've considered Sebastien's method here( http://www.sebastien-han.fr/blog/2012/12/10/openstack-perform-consistent-snapshots/ ), but I would prefer to backup via ceph and outside of the vm.

Dave Spano
Optogenics
Systems Administrator


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Wido den Hollander
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