nova/doc/source/upgrade.rst

Upgrades

Nova aims to provide upgrades with minimal downtime.

Firstly, the data plane. There should be no VM downtime when you upgrade Nova. Nova has had this since the early days, with the exception of some nova-network related services.

Secondly, we want no downtime during upgrades of the Nova control plane. This document is trying to describe how we can achieve that.

Once we have introduced the key concepts relating to upgrade, we will introduce the process needed for a no downtime upgrade of nova.

Current Database Upgrade Types

Currently Nova has 2 types of database upgrades that are in use.

1. Offline Migrations
2. Online Migrations

Offline Migrations consist of:

1. Database schema migrations from pre-defined migrations in nova/db/sqlalchemy/migrate_repo/versions.
2. Deprecated Database data migrations from pre-defined migrations in nova/db/sqlalchemy/migrate_repo/versions.

Online Migrations consist of:

1. Online data migrations from inside Nova object source code.
2. Future Online schema migrations using auto-generation from models.

An example of online data migrations are the flavor migrations done as part of Nova object version 1.18. This included a transient migration of flavor storage from one database location to another.

Note: Database downgrades are not supported.

Migration policy:

The following guidelines for schema and data migrations are followed in order to ease upgrades:

• Additive schema migrations - In general, almost all schema migrations should be additive. Put simply, they should only create elements like columns, indices, and tables.
• Subtractive schema migrations - To remove an element like a column or table during the N release cycle:
  1. The element must be deprecated and retained for backward compatibility. (This allows for graceful upgrade from N to N+1.)
  2. Data migration, by the objects layer, must completely migrate data from the old version of the schema to the new version.
     • Data migration example
     • Data migration enforcement example (for sqlalchemy migrate/deprecated scripts):
  3. The column can then be removed with a migration at the start of N+2.
• All schema migrations should be idempotent. (For example, a migration should check if an element exists in the schema before attempting to add it.) This logic comes for free in the autogenerated workflow of the online migrations.
• Constraints - When adding a foreign or unique key constraint, the schema migration code needs to handle possible problems with data before applying the constraint. (Example: A unique constraint must clean up duplicate records before applying said constraint.)
• Data migrations - As mentioned above, data migrations will be done in an online fashion by custom code in the object layer that handles moving data between the old and new portions of the schema. In addition, for each type of data migration performed, there should exist a nova-manage option for an operator to manually request that rows be migrated.
  • See flavor migration spec for an example of data migrations in the object layer.

Future work - #. Adding plumbing to enforce that relevant data migrations are completed

before running contract in the expand/migrate/contract schema migration workflow. A potential solution would be for contract to run a gating test for each specific subtract operation to determine if the operation can be completed.

Concepts

Here are the key concepts you need to know before reading the section on the upgrade process:

RPC version pinning

Through careful RPC versioning, newer nodes are able to talk to older nova-compute nodes. When upgrading control plane nodes, we can pin them at an older version of the compute RPC API, until all the compute nodes are able to be upgraded. https://wiki.openstack.org/wiki/RpcMajorVersionUpdates

Note

This does not apply to cells deployments since cells does not currently support rolling upgrades. It is assumed that cells deployments are upgraded in lockstep so n-1 cells compatibility does not work.

Online Configuration Reload

During the upgrade, we pin new serves at the older RPC version. When all services are updated to use newer code, we need to unpin them so we are able to use any new functionality. To avoid having to restart the service, using the current SIGHUP signal handling, or otherwise, ideally we need a way to update the currently running process to use the latest configuration.

Graceful service shutdown

Many nova services are python processes listening for messages on a AMQP queue, including nova-compute. When sending the process the SIGTERM the process stops getting new work from its queue, completes any outstanding work, then terminates. During this process, messages can be left on the queue for when the python process starts back up. This gives us a way to shutdown a service using older code, and start up a service using newer code with minimal impact. If its a service that can have multiple workers, like nova-conductor, you can usually add the new workers before the graceful shutdown of the old workers. In the case of singleton services, like nova-compute, some actions could be delayed during the restart, but ideally no actions should fail due to the restart. NOTE: while this is true for the RabbitMQ RPC backend, we need to confirm what happens for other RPC backends.

API load balancer draining

When upgrading API nodes, you can make your load balancer only send new connections to the newer API nodes, allowing for a seamless update of your API nodes.

Expand/Contract DB Migrations

Modern databases are able to make many schema changes while you are still writing to the database. Taking this a step further, we can make all DB changes by first adding the new structures, expanding. Then you can slowly move all the data into a new location and format. Once that is complete, you can drop bits of the scheme that are no long needed, i.e. contract. We have plans to implement this here: https://review.openstack.org/#/c/102545/5/specs/juno/online-schema-changes.rst,cm

Online Data Migrations using objects

In Kilo we are moving all data migration into the DB objects code. When trying to migrate data in the database from the old format to the new format, this is done in the object code when reading or saving things that are in the old format. For records that are not updated, you need to run a background process to convert those records into the newer format. This process must be completed before you contract the database schema. We have the first example of this happening here: http://specs.openstack.org/openstack/nova-specs/specs/kilo/approved/flavor-from-sysmeta-to-blob.html

DB prune deleted rows

Currently resources are soft deleted in the database, so users are able to track instances in the DB that are created and destroyed in production. However, most people have a data retention policy, of say 30 days or 90 days after which they will want to delete those entries. Not deleting those entries affects DB performance as indices grow very large and data migrations take longer as there is more data to migrate.

nova-conductor object backports

RPC pinning ensures new services can talk to the older service's method signatures. But many of the parameters are objects that may well be too new for the old service to understand, so you are able to send the object back to the nova-conductor to be downgraded to a version the older service can understand.

Process

NOTE:

This still requires much work before it can become reality. This is more an aspirational plan that helps describe how all the pieces of the jigsaw fit together.

This is the planned process for a zero downtime upgrade:

1. Prune deleted DB rows, check previous migrations are complete
2. Expand DB schema (e.g. add new column)
3. Pin RPC versions for all services that are upgraded from this point, using the current version
4. Upgrade all nova-conductor nodes (to do object backports)
5. Upgrade all other services, except nova-compute and nova-api, using graceful shutdown
6. Upgrade nova-compute nodes (this is the bulk of the work).
7. Unpin RPC versions
8. Add new API nodes, and enable new features, while using a load balancer to "drain" the traffic from old API nodes
9. Run the new nova-manage command that ensures all DB records are "upgraded" to new data version
10. "Contract" DB schema (e.g. drop unused columns)

Testing

Once we have all the pieces in place, we hope to move the Grenade testing to follow this new pattern.

The current tests only cover the existing upgrade process where: * old computes can run with new control plane * but control plane is turned off for DB migrations

Unresolved issues

Ideally you could rollback. We would need to add some kind of object data version pinning, so you can be running all new code to some extent, before there is no path back. Or have some way of reversing the data migration before the final contract.

It is unknown how expensive on demand object backports would be. We could instead always send older versions of objects until the RPC pin is removed, but that means we might have new code getting old objects, which is currently not the case.