Move flavor data from system-metadata to a blob

Include the URL of your launchpad blueprint:

https://blueprints.launchpad.net/nova/+spec/flavor-from-sysmeta-to-blob

Flavor data is currently stored inefficiently in system_metadata and should be moved to being stored as a blob in an appropriate place per-instance. This will facilitate storing additional flavor details that we badly need, such as extra_specs.

Problem description

Currently we store some properties of the flavor used to spawn an instance in that instance’s system_metadata area. We do this by namespacing the keys of the flavor like this:

Key Value
instance_type_flavorid ‘foo’
instance_type_memory_mb ‘1024’

This means that everything is stringified and that each key and value is limited to 255 characters. It also makes it very difficult to store complex flavor properties, such as extra_specs which has become increasingly important recently for things like NUMA and PCI passthrough. Without the ability to store these values along with the rest of the flavor information, rescheduling or migrating an instance requires using a combination of the original flavor data (stored with the instance) along with the current values attached to the flavor.

Finally, during a resize operation, we store two additional copies of all of this data for the old and new flavors respectively, all in system_metadata. Since system_metadata is a single row key-value layout, this means we get a lot of rows per instance during a resize. In the past, this led to a decoupling of the instance query from the system_metadata query, since a multiple-instance query joined with system_metadata would inflate an already large result by a factor of approximately 30.

Use Cases

  • As an operator, I want to be able to migrate my instances and have the rescheduling process honor all the original information I provided, such as CPU and NUMA layouts as well as PCI passthrough details.
  • As an operator, I want to have database queries be efficient, both in wire bandwidth as well as in latency. Joining large tables and making multiple queries hurt this goal.

Project Priority

This refactor has been a project priority for several cycles now, fitting under both the “stability” and “reducing technical debt” umbrellas.

Proposed change

Since shortly after we started storing flavor data in system_metadata, we have been discussing moving it to a JSON blob in a suitable place in the database. Late in Juno, we added a table called “instance_extra” specifically for the purpose of holding additional JSON blobs for each instance that are needed at different times. This spec proposes to add another column to that table for “flavor” where we will store a JSONified copy of the flavor on initial boot. Further, we will provide for storage of an ‘old’ and ‘new’ flavor to facilitate resize operations. The top-level structure will look like this:

{'cur': { ... serialized Flavor object ... }
 'new': None,
 'old': None,
}

When a flavor is stored in one of the three slots above, the form used will be the serialized NovaObject result. This means that the content in the database will be versioned and deserializing it from the database will work just like receiving one over RPC.

The database migration for this change will simply add the new column, but not perform a data migration. Instead, the migration from system_metadata to instance_extra will be managed by the objects layer. The objects code will handle honoring the system_metadata flavor data, if present, otherwise using the new format in instance_extra. During a save operation, the system_metadata area will be cleared of any flavor data at which time the instance will be fully migrated.

Alternatives

  • We could keep doing what we’re doing, adding additional hacks to the flavor stashing code to keep the bits of extra_specs that we need
  • We could move to persisting more of the request_spec structure, which would necessarily include the flavor data, as suggested here: https://review.openstack.org/#/c/125484/1. My feeling is that when we get to things internally modeled more as tasks, we’ll have a better base for that and that the incremental improvement offered by the approach chosen in this spec is a worthwhile intermediate step at least.

Data model impact

This will add a new TEXT column to the instance_extra table, and will result in data being migrated out of system_metadata to that column as instances are used.

Additionally, a tool to perform offline migrations of instances that are not changing will need to be written to allow operators to ensure that dormant instances get their data migrated as well as active ones.

The Instance object will gain three new properties: ‘flavor’, ‘old_flavor’, and ‘new_flavor’. These will be honored in the “expected_attributes” parameters of various queries, and will contain a nova.objects.Flavor object when and where appropriate.

REST API impact

There should be no user-visible impact to this change.

Security impact

None.

Notifications impact

None.

Other end user impact

None.

Performance Impact

After all the flavor data is moved out of system_metadata, we can consider going back to joining system_metadata when we query for instances. This will remove a DB hit and potentially improve latency.

Additionally, the objects layer will support conditional loading of the flavor data, allowing code that needs system_metadata, but not all of the flavor data to request that more granular response.

Other deployer impact

Deployers will need to take action to ensure that dormant instances get their data migrated before we remove the compatibility code that supports flavors in system_metadata. This should be something that can be run in the background against a running deployment, so the operator impact is mostly procedural.

Developer impact

The objects layer should mostly hide the complex migration activities from the average developer. However, it will be important for people to realize that accessing the flavor information for an instance through the Instance.flavor object is necessary going forward.

Implementation

Assignee(s)

Primary assignee:
danms

Work Items

  • Clean up existing instance_extra interfaces for consistency, moving pci_requests to be a proper field on Instance, etc.
  • Add a flavor column to instance_extra
  • Add the three flavor fields to Instance
  • Modify the Instance object and instance_* DB API functions to handle compatibility with and migration away from flavors being stored in system_metadata
  • Write a tool to migrate inactive instances in the background, to live in the nova/tools directory

Dependencies

This is mostly isolated work. Other work may depend on this, however.

Testing

Unit tests to cover the online data migration code will be provided and should be relatively straightforward. Further, the existing grenade testing in the gate should cover the data migration case for existing instances, as well as guaranteeing that this migration on newer conductor nodes does not disrupt compute nodes running Juno code.

Documentation Impact

Some documentation of the fact that this is being done will need to appear in the release notes. Further, a small bit of documentation to cover the procedures for migrating the data of dormant instances will need to be integrated into any documents that describe moving from Juno to Kilo.

References