[openstack-dev] [Nova] New DB column or new DB table?

Sean Dague sean at dague.net
Thu Jul 18 11:05:10 UTC 2013

On 07/17/2013 10:54 PM, Lu, Lianhao wrote:
> Hi fellows,
> Currently we're implementing the BP https://blueprints.launchpad.net/nova/+spec/utilization-aware-scheduling. The main idea is to have an extensible plugin framework on nova-compute where every plugin can get different metrics(e.g. CPU utilization, memory cache utilization, network bandwidth, etc.) to store into the DB, and the nova-scheduler will use that data from DB for scheduling decision.
> Currently we adds a new table to store all the metric data and have nova-scheduler join loads the new table with the compute_nodes table to get all the data(https://review.openstack.org/35759). Someone is concerning about the performance penalty of the join load operation when there are many metrics data stored in the DB for every single compute node. Don suggested adding a new column in the current compute_nodes table in DB, and put all metric data into a dictionary key/value format and store the json encoded string of the dictionary into that new column in DB.
> I'm just wondering which way has less performance impact, join load with a new table with quite a lot of rows, or json encode/decode a dictionary with a lot of key/value pairs?
> Thanks,
> -Lianhao

I'm really confused. Why are we talking about collecting host metrics in 
nova when we've got a whole project to do that in ceilometer? I think 
utilization based scheduling would be a great thing, but it really out 
to be interfacing with ceilometer to get that data. Storing it again in 
nova (or even worse collecting it a second time in nova) seems like the 
wrong direction.

I think there was an equiv patch series at the end of Grizzly that was 
pushed out for the same reasons.

If there is a reason ceilometer can't be used in this case, we should 
have that discussion here on the list. Because my initial reading of 
this blueprint and the code patches is that it partially duplicates 
ceilometer function, which we definitely don't want to do. Would be 
happy to be proved wrong on that.


Sean Dague

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