Cinder Newton Design Summit Summary

8 minute read

At the Design Summit in Austin, the Cinder team met over three days to go over a variety of topics. This is a general summary of the notes captured from each session.

We were also able to record most sessions. Please see the openstack-cinder YouTube channel for all its minute and tedious glory:

https://www.youtube.com/channel/UCJ8Koy4gsISMy0qW3CWZmaQ

Replication Next Steps

Replication v2.1 was added in Mitaka. This was a first step in supporting a simplified use case. A few drivers were able to implement support for this in Mitaka, with a few already in the queue for support in Newton.

There is a desire to add the ability to replicate smaller groups of volumes and control them individually for failover, failback, etc. Eventually we would also want to expose this functionality to non-admin users. This will allow tenants to group their volumes by application workload or other user specific constraint and give them control over managing that workload.

It was agreed that it is too soon to expose this at this point. We would first like to get broader vendor support for the current replication capabilities before we add anything more. We also want to improve the admin experience with handling full site failover. As it is today, there is a lot of manual work that the admin would need to do to be able to fully recover from a failover. There are ways we can make this experience better. So before we add additional things on top of replication, we want to make sure what we have is solid and at least slightly polished.

Personally, I would like to see some work done with Nova or some third party entity like Smaug or other projects to be able to coordinate activities on the compute and storage sides in order to fail over an environment completely from a primary to secondary location.

Related to the group replication (tiramisu) work was the idea of generic volume groups. Some sort of grouping mechanism would be required to tie in to that. We have a grouping today with consistency groups, but that has its own set of semantics and expectations that doesn’t always fully mesh with what users would want for group replication.

There have also been others looking at using consistency groups to enable vendor specific functionality not quite inline with the intent of what CGs are meant for.

We plan on creating a new concept of a group that has a set of possible types. One of these types will be consistency, with the goal that internally we can shift things around to convert our current CG concept to be a group of type consistency while still keeping the API interface that users are used to for working with them.

But beyond that we will be able to add things like a “replication” type that will allow users to group volumes, that may or not be able to be snapped in a IO order consistent manner, but that can be acted on as a group to be replicated. We can also expand this group type to other concepts moving forward to meet other use cases without needing to introduce a wholly new concept. The mechanisms for managing groups will already be in place and a new type will be able to be added using existing plumbing.

Active/Active High Availability

Work continues on HA. Gorka gave an overview of the work completed so far and the work left to do. We are still on the plan proposed at the Tokyo Summit, just a lot of work to get it all implemented. The biggest variations are around the host name used for the “clustered” service nodes and the idea that we will not attempt to do any sort of automatic cleanup for in-progress work that gets orphaned due to a node failure.

Mitaka Recap

Two sessions were devoted to going over what had changed in Mitaka. There were a lot of things introduced that developers and code reviewers now need to be aware of, so we wanted to spend some time educating everyone on these things.

Conditional DB Updates To try to eliminate races (partly related to the HA work) we will now use conditional updates. This will eliminate the gap between checking a value in setting it, making it one atomic DB update. Better performance than locking around operations.

Microversions API microversions was implemented in Mitaka. The new /v3 endpoint should be used. Any change in the API should now be implemented as a micrversion bump. Devref in Cinder with details of how to use this and more detail as to when a microversion is needed and when it is not.

Rolling Upgrades Devref added for rolling upgrades and versioned objects. Discussed need to make incremental DB changes rather than all in one release. First release add new colume – write to both, read from original. Second release – write to both, read from new column. Third release – original column can now be deleted.

Recommended service upgrade order: cinder-api, cinder-scheduler, cinder-volume, cinder-backup. After all services upgraded, send SIGHUP to each to release version pins.

Multinode grenade tests need to be set up to get regular testing on rolling upgrades to ensure we don’t let anything through that will cause problems.

Some additional patches are in progress to fix a few internal things like unversioned dicts that are passed around. This will help us make sure we don’t change one of those structures in an incompatible way.

Scalable Backup

The backup service was decoupled from the c-vol service in Mitaka. This allows us to move backup to other nodes to offload that work. We can also scale out to allow more backup operations to work in parallel.

Also some discussion of whether KVMs change block tracking could be used to further optimize this process.

Some other issues were identified with backup. It is currently a sequential process, so backups can take a long time. Some work is being done to break out the backup chunks into separate processes to allow concurrent processing.

The idea of having different backup types was brought up. This will require more discussion. The idea is to have a way to configure different volumes to be able to be backed up to different backup target types (i.e., one to Swift backend, one to Ceph backend, one to GCS, etc.).

There are some implications for the HA work being done and how to do clean up. Those issues will need to be fully identified and worked through.

Testing Process

We discussed our various available testing and how we can get better test coverage. We also want to optimize the testing process so everyone isn’t stuck running long running tests just to validate small changes.

For unit tests, tests that take more than 3-5 seconds to run will be moved out to our integration tests. Most of these are doing more than just “unit” testing so makes more sense for them to be separated out.

There’s some desire to gate on coverage, but it was discussed how that can be a difficult thing to enforce. There was a lot of concern that a hard policy around coverage would lead to a lot of useless unit tests that don’t really add valuable testing, just adds coverage over a given path to get the numbers up.

It may be nice to have something like our non-voting pylint jobs to help detect when our test coverage decreases, but not sure if the additional infra load to run this would be worth it.

We have a functional test job that is currently non-voting. It has been broken recently and due to it being non-voting it was missed for some time. This needs to be fixed and the job changed to be voting. Since the majority of these tests were moved from the unit tests, they should be fine to make voting once passing again.

Having in-tree tempest tests were also discussed. We’ve used tempest for testing things not in line with the mission of tempest. We also have tests in there that may not really be relevant or needed for other projects getting them by running all tempest tests. For the ones that are really Cinder specific we should migrate them from being in tempest to being in tree with Cinder. That also gives us full control over what is included.

We also discussed plans for how to test API microversions and adding pylint testing to os-brick.

CinderClient and OpenStackClient

A group of folks are working through a list of commands to make sure osc has functional parity with cinderclient. We need to have the same level of support in osc before we can even consider deprecating the cinder client CLI.

The idea was raised to create a wiki page giving a “lookup” of cinder client commands to openstackclient commands to help users migrate over to the new client. Will need to decided how to support this given long term plans for the wiki, but having the lookup posted somewhere should help.

We do have an extra challenge in that we added a cinderclient extension for working with os-brick as a stand alone storage management tool. We will work with the osc team to see what the options are for supporting something like this and decide what the best end user experience would be for this. It may be that we don’t deprecate python-cinderclient CLI just for this use case.

Unconference

Made midcycle plans based on survey results. (Has since changed since we ran in to complications with hotal availability)

Talked briefly about non-Cinder volume replication and how to recover during DR. Discussed whether all backends support managing existing volumes. How to discover available volumes. What to do with snapshots on manage.

Nova Cross Project

All about multiattach. Matt Riedman wrote up a nice recap of this discussion already:

http://lists.openstack.org/pipermail/openstack-dev/2016-May/094018.html

##Contributors Meetup API capability and feature discovery. What to do for backends that only support allocation in units greater than 1G. Next steps for user message reporting. Release priorities:

  • User messaging
  • Active/active high availability
  • Improved functional test coverage
  • Better support for cheesecake (repl v2.1)

Extending attached volume. Discussed qemu-img convert error a couple drivers were seeing. FICON protocol support. Generic group discussion. Force detach. Replication questions. QoS support for IOPs per GB. Per tenant QoS. Cinder callbacks to Nova for task completion. Config options inheritance (defined in Default, apply to subsections) Deadlines (http://releases.openstack.org/newton/schedule.html) Status of image caching. Driver interface compliance checks and interface documentation.