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

Joshua Harlow harlowja at outlook.com
Fri Jul 31 18:37:59 UTC 2015

Mike Perez wrote:
> On Fri, Jul 31, 2015 at 8:56 AM, Joshua Harlow<harlowja at outlook.com>  wrote:
>> ...random thought here, skip as needed... in all honesty orchestration
>> solutions like mesos
>> (http://mesos.apache.org/assets/img/documentation/architecture3.jpg),
>> map-reduce solutions like hadoop, stream processing systems like apache
>> storm (...), are already using zookeeper and I'm not saying we should just
>> use it cause they are, but the likelihood that they just picked it for no
>> reason are imho slim.
> I'd really like to see focus cross project. I don't want Ceilometer to
> depend on Zoo Keeper, Cinder to depend on etcd, etc. This is not ideal
> for an operator to have to deploy, learn and maintain each of these
> solutions.
> I think this is difficult when you consider everyone wants options of
> their preferred DLM. If we went this route, we should pick one.


> 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?

All very good questions, although IMHO a hash-ring is just a piece of 
the puzzle, and is more equivalent to sharding resources, which yes is 
one way to scale as long as each shard never touches anything from the 
other shards. If those shards ever start to need to touch anything 
shared then u get back into this same situation again for a DLM (and at 
that point u really do need the 'distributed' part of DLM, because each 
shard is distributed).

And an few (maybe obvious) questions:

- How would re-sharding work?
- If sharding (the hash-ring partitioning) is based on entities 
(conductors/other) owning a 'bucket' of resources (ie entity 1 manages 
resources A-F, entity 2 manages resources G-M...), what happens if a 
entity dies, does some other entity take over that bucket, what happens 
if that entity really hasn't 'died' but is just disconnected from the 
network (partition tolerance...)? (If the answer is there is a lock on 
the resource/s being used by each entity, then u get back into the LM 

I'm unsure about how ironic handles these problems (although I believe 
they have a hash-ring and still have a locking scheme as well, so maybe 
thats there answer for the dual-entities manipulating the same bucket 

> --
> Mike Perez
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