Metadata cache time should be configurable

[Chris St. Pierre]

The nova metadata request handler uses an in-memory cache of 15 seconds. Under very heavy usage of the metadata service, this can drastically limit the cache hit rate, since it expires so quickly.

Adding the ability to control the cache timeout has, in our tests, increased the average cache hit rate from around 20% to 80% or better with approximately a thousand metadata calls per minute.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/119421

[Sean Dague (sdague)]

Question, do you think that there is a better default than 15s? Especially in your experience in the environments you've got. I think 15s was probably arbitrary and upping the default is probably sane.

[Chris St. Pierre (stpierre)]

I don't think we have any particular guidance for that; we have a couple of customers who hit the metadata service *really* hard, so making this tunable is important, but the default mostly seems to Just Work.

[OpenStack Infra (hudson-openstack)]

Change abandoned by Chris St. Pierre (<email address hidden>) on branch: master
Review: https://review.openstack.org/119421
Reason: I'm going to roll this into a bigger spec to use oslo.cache to provide a true shared cache for the metadata API in Kilo.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/159264
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=5a93bfd484ec8402f906f13fefd4d03f04426d99
Submitter: Jenkins
Branch: master

commit 5a93bfd484ec8402f906f13fefd4d03f04426d99
Author: Chris St. Pierre <email address hidden>
Date: Wed Feb 25 15:52:34 2015 -0600

    Make metadata cache time configurable

    The metadata cache expiration is now a config option.

    New Option:
    metadata_cache_expiration - Default 15
        Time in seconds to cache metadata objects.

    This also adds basic unit tests for the MetadataRequestHandler, which
    might be nice to have.

    Co-Authored-By: Chet Burgess <email address hidden>
    Co-Authored-By: Nicolas Simonds <email address hidden>
    DocImpact
    Closes-Bug: 1366139
    Change-Id: I4e829ec52cf1445339f325b5895987c82dc7a8f4