[openstack-dev] [nova] Distributed locking

John Garbutt john at johngarbutt.com
Wed Jun 25 08:08:00 UTC 2014

So just to keep the ML up with some of the discussion we had in IRC
the other day...

Most resources in Nova are owned by a particular nova-compute. So the
locks on the resources are effectively held by the nova-compute that
owns the resource.

We already effectively have a cross nova-compute "lock" holding in the
capacity reservations during migrate/resize.

But to cut a long story short, if the image cache is actually just a
copy from one of the nova-compute nodes that already have that image
into the local (shared) folder for another nova-compute, then we can
get away without a global lock, and just have two local locks on
either end and some "conducting" to co-ordinate things.

Its not perfect, but its an option.


On 17 June 2014 18:18, Clint Byrum <clint at fewbar.com> wrote:
> Excerpts from Matthew Booth's message of 2014-06-17 01:36:11 -0700:
>> On 17/06/14 00:28, Joshua Harlow wrote:
>> > So this is a reader/write lock then?
>> >
>> > I have seen https://github.com/python-zk/kazoo/pull/141 come up in the
>> > kazoo (zookeeper python library) but there was a lack of a maintainer for
>> > that 'recipe', perhaps if we really find this needed we can help get that
>> > pull request 'sponsored' so that it can be used for this purpose?
>> >
>> >
>> > As far as resiliency, the thing I was thinking about was how correct do u
>> > want this lock to be?
>> >
>> > If u say go with memcached and a locking mechanism using it this will not
>> > be correct but it might work good enough under normal usage. So that¹s why
>> > I was wondering about what level of correctness do you want and what do
>> > you want to happen if a server that is maintaining the lock record dies.
>> > In memcaches case this will literally be 1 server, even if sharding is
>> > being used, since a key hashes to one server. So if that one server goes
>> > down (or a network split happens) then it is possible for two entities to
>> > believe they own the same lock (and if the network split recovers this
>> > gets even weirder); so that¹s what I was wondering about when mentioning
>> > resiliency and how much incorrectness you are willing to tolerate.
>> From my POV, the most important things are:
>> * 2 nodes must never believe they hold the same lock
>> * A node must eventually get the lock
> If these are musts, then memcache is a no-go for locking. memcached is
> likely to delete anything it is storing in its RAM, at any time. Also
> if you have several memcache servers, a momentary network blip could
> lead to acquiring the lock erroneously.
> The only thing it is useful for is coalescing, where a broken lock just
> means wasted resources, erroneous errors, etc. If consistency is needed,
> then you need a consistent backend.
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