Hi everyone, for some time I have been working on an implementation of a filter that would allow to force instances to hosts which contain specific volumes. A blueprint can be found here: https://blueprints.launchpad.net/nova/+spec/volume-affinity-filter and an implementation here: https://review.openstack.org/#/c/29343/
The filter works for LVM driver and now it picks either a host containing specified volume or nothing (thus effectively failing instance scheduling). Now it fails primarily when it can't find the volume. It has been pointed to me that sometimes it may be desirable not to fail instance scheduling but to run it anyway. However this softer behaviour fits better for weighter function. Thus I have registered a blueprint for the weighter function: https://blueprints.launchpad.net/nova/+spec/volume-affinity-weighter-function I was thinking about both the filter and the weighter working together. The former could be used in cases when we strongly need storage space associated with an instance and need them placed on the same host. The latter could be used when storage space is nice to have and preferably on the same host with an instance, but not so crucial as to have the instance running. During reviewing a question appeared whether we need the filter and wouldn't things be better if we removed it and had only the weighter function instead. I am not yet convinced that the filter is useless and needs to be replaced with the weighter, so I am asking for your opinion on this matter. Do you see usecases for the filter, or the weighter will answer all needs? With kind regards, Alexey.
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