Ceph PG Auto Tuning¶
In the Nautilus release, Ceph has introduced placement group (PG) autotuning. The charms should support this module allowing a Ceph cluster to grow or shrink naturally.
Problem Description¶
If an existing cluster is incorrectly sized at deploy time, it is very difficult for an operator to resolve the imbalance. PG Autotuning was designed to remove the black-magic of PG tuning.
Proposed Change¶
A new configuration option (pg-autotune) will be introduced to enable the new pg_autoscaler module. This config option will have no effect for Ceph releases before Nautilus and will have no default.
In a new Ceph deployment with a release greater than Nautilus, a null value for ‘pg-autotune’ will cause the autoscaler module to be enabled and configured on all pools, with the global default additionally configured as ON. In an upgrade to Nautilus, a null value will not cause the autoscaler module to be loaded, nor will pool metadata change.
For an existing deployment to enable the autoscaler, an administrator should change the config value to ‘True’, after which the charm will process the new option, as well as updating the existing pools as necessary.
The ceph-mon charms already handle sizing the default placement groups via the expected ratios so the required data is already present.
Alternatives¶
The current solution is a viable alternative, as long as the expectations about pool sizing is correct at deploy time. Other alternatives include manual operator intervention to resolve an incorrectly sized pool.
Implementation¶
Assignee(s)¶
- Primary assignee:
chris.macnaughton
Gerrit Topic¶
Use Gerrit topic “pg-autotune” for all patches related to this spec.
git-review -t pg-autotune
Work Items¶
Enable the pg_autoscaler manager module if the Ceph version is Nautilus or higher
Disable the autoscaler globally, by default
Enable the autoscaler and configure pools when toggled via configuration
Repositories¶
No new repositories will need to be created.
The charms.ceph and charm-ceph-mon repositories will be the primary focus of this development.
Documentation¶
There will be a new configuration option added to toggle on the autoscaler for the Ceph cluster. This configuration option should additionally be called out in the README for the ceph-mon charm.
Security¶
There are no expected security risks associated with this change.
Testing¶
An additional test-case will be added enabling this config option to ensure that the functionality works as expected. Additionally, the functionality will be validated via inspection of Ceph’s suggestions for auto-tuning.
Dependencies¶
There are no new dependencies.
