[openstack-dev] [Cinder] A possible solution for HA Active-Active

Duncan Thomas duncan.thomas at gmail.com
Fri Jul 31 18:31:15 UTC 2015

On 31 July 2015 at 20:40, Mike Perez <thingee at gmail.com> wrote:

> Regardless, I want to know if we really need a DLM. Does Ceilometer
> really need a DLM? Does Cinder really need a DLM? Can we just use a
> hash ring solution where operators don't even have to know or care
> about deploying a DLM and running multiple instances of Cinder manager
> just works?

There's a lot of circling around here about what we're trying to achieve
with 'H/A'.

Some people are interested in performance. For them, a hash ring solution
(deterministic load balancing) is fine. If the aim is availability (as mine
is) then I can't see how it helps. I might be missing something, of course
- if so, I'm happy to be corrected.

To be clear, my aim with H/A is to remove the situation where a single node
failure removes the control path for my storage. Currently, the only way to
avoid this is to use something like pacemaker to monitor the c-vol
services. Extensive experience suggests that pacemaker is a complex,
fragile piece of software. Every component of cinder except c-vol can be
deployer active/active[/active/...] - I'm aiming for consistency of
approach if nothing else.

If it ends up that trying to fix this adds too much complexity and/or
fragility to cinder itself, then I can accept that - once whatever we do
ends up being worse than pacemaker, we've taken a significant step

Regardless of how H/A discussions go, the first part of Gorka's patch can
certainly be used to fix a few of the API races we have, and can do so with
rather nice, elegant, easy to understand code, so I think the whole process
has been productive whatever the H/A outcome.

Duncan Thomas
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