Docs for CONF.block_device_allocate_retries should be updated to note cinder image caching

[Matt Riedemann]

CONF.block_device_allocate_retries in nova.compute.manager is used to control how long nova waits for a volume it created in cinder to be available when booting from volume and source != 'volume'.

In liberty, cinder landed some features for image cache:

https://review.openstack.org/#/c/195795/

And efficient transfer of images from glance:

https://review.openstack.org/#/c/201754/

The nova option help text should be updated to note that enabling the image cache feature in cinder can help with tuning.

Given the complexity involved in https://review.openstack.org/#/c/201754/ it might be best to just note at least enabling the image cache feature in cinder (https://review.openstack.org/#/c/195795/).

We'd also have to note that the feature is only available in cinder starting in liberty, so liberty/mitaka nova talking to cinder < liberty wouldn't work for the image cache feature.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/229056

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/229056
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=5508e11cf873384a28dc7416168d34e85f2c06cf
Submitter: Jenkins
Branch: master

commit 5508e11cf873384a28dc7416168d34e85f2c06cf
Author: Mark Doffman <email address hidden>
Date: Tue Sep 29 12:00:19 2015 -0500

    Point to cinder options in nova block alloc docs.

    Cinder, since Liberty, has had the option to enable volume cache of
    images. This can help with tuning booting instances from volume.
    This documents a place to start with those Cinder options from
    the block device tuning configuration in Nova.

    Change-Id: I067f171130fe383e00585dc574812504efebdd57
    Closes-bug: #1499518