Open Stack

On Tue, Sep 20, 2016 at 04:20:49PM +0100, Daniel P. Berrange wrote:
> On Tue, Sep 20, 2016 at 11:01:23AM -0400, Sean Dague wrote:

[...]

> > Here is my reconstruction of the snapshot issue from what I can remember
> > of the conversation.
> > 
> > Nova defaults to live snapshots. This uses the libvirt facility which
> > dumps both memory and disk. And then we throw away the memory. For large
> > memory guests (especially volume backed ones that might have a fast path
> > for the disk) this leads to a lot of overhead for no gain. The
> > workaround got them past it.
> 
> I think you've got it backwards there.
> 
> Nova defaults to *not* using live snapshots:
> 
>     cfg.BoolOpt(
>         'disable_libvirt_livesnapshot',
>         default=True,
>         help="""
> Disable live snapshots when using the libvirt driver.
> ...""")
>
>
> When live snapshot is disabled like this, the snapshot code is unable
> to guarantee a consistent disk state. So the libvirt nova driver will
> stop the guest by doing a managed save (this saves all memory to
> disk), then does the disk snapshot, then restores the managed saved
> (which loads all memory from disk).
> 
> This is terrible for multiple reasons
> 
>   1. the guest workload stops running while snapshot is taken
>   2. we churn disk I/O saving & loading VM memory
>   3. you can't do it at all if host PCI devices are attached to
>      the VM
> 
> Enabling live snapshots by default fixes all these problems, at the
> risk of hitting the live snapshot bug we saw in the gate CI but never
> anywhere else.

Yes, FWIW, I agree.  In addition to the nice details above, enabling the
live snapshots also allows you to quiesce file systems for consistent
disk state, via the Glance image metadata properties
'hw_qemu_guest_agent' and 'os_require_quiesce'.  (Current cold snapshot
mechanism doesn't allow this.)


Anyhow, Sean seems to have submitted the change to toggle the config for
enabling live_snapshots:

    https://review.openstack.org/#/c/373430/ -- "Change
    disable_live_snapshot workaround"


-- 
/kashyap