[openstack-dev] [Nova] Does Nova really need an SQL database?

Joe Gordon joe.gordon0 at gmail.com
Mon Nov 18 22:09:31 UTC 2013

On Mon, Nov 18, 2013 at 12:14 PM, Jay Pipes <jaypipes at gmail.com> wrote:

> On 11/18/2013 02:35 PM, Mike Spreitzer wrote:
>> There were some concerns expressed at the summit about scheduler
>> scalability in Nova, and a little recollection of Boris' proposal to
>>  keep the needed state in memory.
> While it could be possible to do all of the scheduler state in memory, I
> think a better (or at least, less cumbersome initially) approach would
> be to add some layers of in-memory caching to any existing parts where
> the scheduler currently makes a database query. The problem with this is

Phil Day discussed this at the summit and I have finally gotten around to
posting a POC of this.


It is very very rough, but gives the general idea. Small scale testing in
devstack showed promising initial results.

> that you won't be able to scale out the design -- since the scheduler's
> cached pieces cannot be shared easily across distributed nodes. This is
> where the concept of using cells and a hierarchical "sieve scheduling"
> pattern is used, where higher-level cell schedulers can quickly send a
> scheduling request to another cell's scheduler based on a small amount
> of information that can generally be compared against in-memory things
> (like region, availability zone, type of hypervisor, etc...)
>  I also heard one guy say that he thinks Nova does not really need a
>> general SQL database, that a NOSQL database with a bit of
>> denormalization and/or client-maintained secondary indices could
>> suffice.  Has that sort of thing been considered before?  What is the
>> community's level of interest in exploring that?
> Good luck. :)  I don't think that whomever suggested that a NoSQL
> database with a "bit of denormalization" would suffice for Nova realized
> the extent to which the sets of data within Nova's database are highly
> relational. You will just end up implementing JOIN algorithms in Python
> code and make some of the more advanced search queries much slower, IMO.
> Oh, and BTW, Nova's "database" was originally Redis [1] :)
> Best,
> -jay
> [1]
> https://github.com/openstack/nova/blob/bf6e6e718cdc7488e2da87b21e258c
> cc065fe499/nova/datastore.py
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