> Here are a few -- > instance_get_all_by_filters joins manually with > instances_fill_metadata -- > https://github.com/openstack/nova/blob/master/nova/db/sqlalchemy/api.py#L1890 > https://github.com/openstack/nova/blob/master/nova/db/sqlalchemy/api.py#L1782 > > Almost all instance query functions manually join with instance_metadata.
This was done because joining metadata with instances was resulting in very large result sets from the database. Instance objects are very large, and each can have many rows in the metadata tables (especially in older versions of nova). When joining these tables and doing queries for all instances on a host or across a time interval, the result sets are very large (we had a big issue with this at the grizzly release, as reported by real deployments). So, we're trading some overhead (and atomicity) for significantly lower DB traffic. I don't think this is likely to be the source of your measured CPU overhead though. The metadata joining is O(n) where n is the number of instances in the result. I would think that is much smaller than the overhead of processing the result of the query into ORM objects. --Dan
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