[Openstack] HA(High Availability ) mode - OpenStack

Adam Lawson alawson at aqorn.com
Mon Oct 13 19:53:13 UTC 2014

Odd numbers because with clusters you can't have a majority to determine
how conflicts should be resolved if the number of votes (cluster nodes) is
even (resulting in a potential of 50/50 where nothing breaks a tie).

But VM resiliency touches on a sensitive area for me that still hasn't been

Compute nodes rely on a resilient control plane - 3 nodes achieves that in
theory, making failure domains within the compute tier limited to whatever
is hosted on individual compute nodes. I'm unhappy that VM's with shared
storage, by design apparently, do not automatically fail to another host
when the host node fails but that is likely to change if I can influence
that in some way. Although not ideal, VM resiliency is not our long term
goal I get it but it should at least be optional to accommodate TODAY's
reality. But apparently the technical direction of Openstack has made a
determination for some reason that you either deploy cloud-aware apps
across the board or you agree not to have any resiliency for VM's. That's
unrealistic. And legacy apps are in every single enterprise and not going
away for a long long time so intentionally not supporting them is a massive
question mark for me.

In fact, I'm going to revive the previous thread on this and try to
understand this a little better.


*Adam Lawson*

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On Mon, Oct 13, 2014 at 7:40 AM, Gowri LN <gowri.ln123 at gmail.com> wrote:

> I was refering to the HA mode of deployment with multiple computes and not
> HA of VM s and the need of 3 Computes in particular . But Thanks a lot for
> the information.
> Just curious, Why odd numbers ? :)
> On Tue, Aug 26, 2014 at 12:54 AM, Radek Smigielski <
> radek.smigielski at ymail.com> wrote:
>> On Monday, 25 August 2014, 10:04:20, Gowri LN <gowri.ln123 at gmail.com>
>> wrote:
>> > Can anyone suggest me where I can get information about how HA is
>> implement is compute node ?
>> I am guessing, you are looking for HA of the guest VMs, is that correct?
>> I am afraid OpenStack does not offer HA for guest VM in this moment.
>> > I know that minimum 3 controllers are required to configure HA mode but
>> my question is,
>> > if I add more compute nodes as well will my cluster be more stable?
>> > Are the VM s replicated over compute nodes?
>> The number 3 is a generic, minimum HA cluster requirement. It helps
>> establish quorum of the cluster. It's always good to have odd number of
>> cluster members.
>> - Radoslaw Smigielski
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