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

Lu, Lianhao lianhao.lu at intel.com
Fri Jul 19 02:12:33 UTC 2013

Sean Dague wrote on 2013-07-18:
> 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
Using ceilometer as the source of those metrics was discussed in the
nova-scheduler subgroup meeting. (see #topic extending data in host
state in the following link).

In that meeting, all agreed that ceilometer would be a great source of
metrics for scheduler, but many of them don't want to make the
ceilometer as a mandatory dependency for nova scheduler. 

Besides, currently ceilometer doesn't have "host metrics", like the 
cpu/network/cache utilization data of the compute node host, which
will affect the scheduling decision. What ceilometer has currently
is the "VM metrics", like cpu/network utilization of each VM instance.

After the nova compute node collects the "host metrics", those metrics
could also be fed into ceilometer framework(e.g. through a ceilometer
listener) for further processing, like alarming, etc.


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