Add support to configure weight for each store

https://blueprints.launchpad.net/glance-store/+spec/store-weight

Add support to configure weight to each store. The store with highest weight will be given preference to download the image.

Problem description

Since introduction of importing single image into multiple stores or copying existing image into multiple stores, single image can be stored at multiple locations or different stores configured by glance. Current download image is based on the default location_strategy which is location_order, traverse through the image locations one by one if there are multiple locations (in this case it will return the image stored at the first location). If user prefers to download the image from a specific store then they can use store_type as location strategy to give preference to download the image from that store only. For example if location_strategy is set as store_type and store_type has rbd as preference then the image will be downloaded from the rbd store only. Now the problem with store_type location strategy is that there can be multiple stores of same type (multiple rbd or file stores). So again user will not able to download the image from the specific store even if location_strategy is set to store_type.

Consider the following use cases for providing download from specific store support:

  • I have a large image and want to download it from the SSD store since it’s fast.

  • I do multiple concurrent downloads on a particular image so want to download it from the RELIABLE store since the i/o handling is better.

Proposed change

This proposal requires changes in glance_store as well as in glance.

Glance store side change:

Add new configuration option weight default to zero for each store. Operator can change it for each store if they wish. The store with highest weight will be given preference to download the image from.

[default] enabled_backends = robust:rbd,cheap:file

[robust] rbd_store_pool=images weight=10

[cheap] filesystem_store_datadir=/opt/stack/data/glance/images/ weight=5

In above example, robust store will always be a preferred store to download the image from. If image is not available in robust store then it will be searched in cheap store which is next inline.

If no weight is provided for stores (i.e. all stores have default weight 0) then the image will be searched based on image creation order (similar as location_order).

Glance side change:

Once glance_store side changes are implemented then we need to modify GET API of image to sort the image locations based on the wegith assigned to each store. If weight is default then the location order will not be changed.

Alternatives

Add new location strategy ‘store_identifier’ to existing default two strategies. This will add comma separated list of store identifiers which will be given preference to download the image from. New configuration option store_identifier_preference under group store_identifier_location_strategy will be added where user/deployer can reference their preferences based on store identifiers.

Data model impact

None

REST API impact

None

Security impact

None

Notifications impact

None

Other end user impact

None

Performance Impact

None

Other deployer impact

Deployer/Operator need to configure weight for each glance-store.

Developer impact

None

Implementation

Assignee(s)

Primary assignee:

dansmith

Other contributors:

abhishek-kekane

Work Items

  • Add new configuration option weight for each store

  • GET API change to sort locations based on store weight

  • Unit and functional tests

  • Tempest coverage

Dependencies

None

Testing

Sufficient unit/functional and tempest tests will be added.

Documentation Impact

Need to document how location order will be changed based on weight assigned to each store.

References

None