scheduler: disk_filter permits scheduling on full drives

[Andrew Bogott]

I use qcow images and have disk_allocation_ratio == 2.1 to allow large amounts of overcommitting of disk space. To quote the nova config reference:

> If the value is set to >1, we recommend keeping track of the free disk space, as the value approaching 0 may result in the incorrect
> functioning of instances using it at the moment.

Good advice, but 'keeping track' can be a bit impractical at times. I just now had the scheduler drop a large sized instance onto a server with a 98% full drive since the behavior of disk_allocation_ratio intentionally ignores the actual free space on the drive.

I propose that we add an additional config setting to the disk scheduler so that I can overschedule but can /also/ request that the scheduler stop piling things onto an already groaning server.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/242251

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/261108

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/261108
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=13c1301f41edee6559220f2dfa92211714925f8d
Submitter: Jenkins
Branch: master

commit 13c1301f41edee6559220f2dfa92211714925f8d
Author: andrewbogott <email address hidden>
Date: Mon Dec 21 22:04:40 2015 -0600

    Add a disk space weight-based scheduler.

    By default this is turned on and has equal weight to the
    ram weigher.

    DocImpact: Added the disk_weight_multiplier config option
    Closes-Bug: 1513654
    Change-Id: I29ce73122ad1860081b64b75646a297dfbb8d292

[Doug Hellmann (doug-hellmann)]

This issue was fixed in the openstack/nova 13.0.0.0b3 development milestone.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/341198

[OpenStack Infra (hudson-openstack)]

Change abandoned by Stephen Finucane (<email address hidden>) on branch: master
Review: https://review.openstack.org/341198