cinder-specs/specs/juno/affinity-antiaffinity-filter.rst

Affinity and anti-affinity scheduler filter

https://blueprints.launchpad.net/cinder/+spec/affinity-antiaffinity-filter

To add scheduler filter to Cinder that allows scheduler to make placement decision based on affinity relationship between existing volumes and new volume (the one being scheduled). The affinity relationship here means the location of volumes ('host' of volume).

Problem description

Cinder has done a good job hiding the details of storage backends from end users by using volume types. However there are use cases where users who build their application on top of volumes would like to be able to 'choose' where a volume be created on. How can Cinder provide such capability without hurting the simplicity we have been keeping? Affinity/anti-affinity is one of the flexibility we can provide without exposing details to backends.

The term affinity/anti-affinity here is to describe the relationship between two sets of volumes in terms of location. To limit the scope, we describe one volume is affinity with the other one only when they reside in the same volume back-end (this notion can be extended to volume pools if volume pool support lands in Cinder); on the contrary, 'anti-affinity' relation between two sets of volumes simply implies they are on different Cinder back-ends (pools).

This affinity/anti-affinity filter filters Cinder backend based on hint specified by end user. The hint expresses the affinity or anti-affinity relation between new volumes and existing volume(s). This allows end users to provide hints like 'please put this volume to a place that is different from where Volume-XYZ resides in'. Below are two use cases where this new filter can be useful.

1. DB team builds MySQL master onto one volume, they'd prefer to put new volumes for slave DBs to different backends from where the master DB resides in, for the sake of high availability.
2. Log processing project would like to have fast storage as possible, so they create soft RAID across multiple volumes. They want to put these volumes as close to each other as possible, ideally on the same storage backend, for the sake of performance.

Use Cases

Proposed change

Add two new filters to Cinder - AffinityFilter and AntiAffinityFilter. These two filters will look at the scheduler hint provided by end users (via scheduler hint extension) and filter backends by checking the 'host' of old and new volumes see if a backend meets the requirement (being on the same backend as existing volume or not being on the same backend(s) as existing volume(s)).

Alternatives

There had been one proposal to allow admin user to directly specify the backend for new volumes. It doesn't really provide similar functionality as affinity filter 'cos it was admin only and it itself has a few drawbacks (security concern, for example).

Data model impact

None

REST API impact

None

Security impact

Although this change involves using or parsing user-provided - scheduler hints, which is already part of Cinder. This doesn't put Cinder in any more danger as it is now.

Notifications impact

None

Other end user impact

None

Performance Impact

New filters would query DB once per request, it only adds slightly latency to the system and the latency has nothing to do with the size of the system.

Other deployer impact

None

Developer impact

None

Implementation

Assignee(s)

Primary assignee:

zhiteng-huang (winston-d on IRC)

Work Items

1. Filter implementation
2. Internal DB API modifition

Dependencies

None

Testing

Test against AffinityFilter:

• Create one volume A;
• Create another volume B with uuid of A and 'same_backend' as hint;
• Checks if B is created on same backend as A;

Test against AntiAffinityFilter:

• Create one volume A;
• Create another volume C with uuid of A and 'different_backend' as hint;
• Checks if C is created on different backend as A;

Documentation Impact

Need to document the usage of new filters.

References

Nova has been offering simliar feature called SameHostFilter and DifferentHostFilter since Diablo.

https://github.com/openstack/nova/blob/master/nova/scheduler/filters/affinity_filter.py