Automatic PV releaser.
- PV Releaser
- Deletes
claimReffrom PVs that belong to Storage Class annotated for this Releaser ID (-controller-id) to move their status fromReleasedtoAvailablewithout deleting any data.
- Deletes
Releaser Controller is by design automatically makes PVs with reclaimPolicy: Retain available to be reclaimed by other consumers without cleaning up any data. Use it with caution - this behavior might not be desirable in most cases. Any data left on the PV after the previous consumer will be available to all the following consumers. You may want to use StatefulSets instead. This controller might be ideal for something like build cache - insensitive data by design required to be shared among different consumers. There is many use cases for this, one of them is documented in examples/jenkins-kubernetes-plugin-with-build-cache.
Essentially Releaser controller allows you to have a storage pool of reusable PVs that retain data between consumers.
The problem statement for creating it was - I needed a pool of CI/CD build caches that can be re-used by my build pods but not allowed to be used concurrently. Normally, similar functionality is performed by a StatefulSet controller, but build pods (such as Jenkins) are not managed by StatefulSet controller.
When PVC that were using PV with reclaimPolicy: Retain is deleted - Kubernetes marks this PV Released. Fortunately, this will not let any other PVC to use it. I say fortunately because imagine if it did - meaning all the data on the volume could be accessed by a new consumer. This is not what reclaimPolicy: Retain is designed for - it only allows cluster administrators to recover the data after accidental PVC deletion. Even now deprecated reclaimPolicy: Recycle was performing a cleanup before making PV Available again. Unfortunately, this just doesn't work for something like a CI/CD build cache, where you intentionally want to reuse data from the previous consumer.
One way to approach this problem statement would be just to create a static PVC with accessModes: ["ReadWriteMany"] - and it would work in most of the cases. But it has certain limitations - let's start with the fact ReadWriteMany is not supported by many storage types (example: EBS). The ones that do support it (another popular choice - EFS) most likely based on NFS which does not support an FS-level locking. Even if you got a storage type that supports both ReadWriteMany and FS level locking - many tools just doesn't seem to use FS level locking (or have other bugs related to concurrent cache usage). All this can lead to various race conditions failing your builds. That can be solved by making different builds to isolate themselves into a different sub-folders, but that reduces overall cache hit ratio - performance gain will be smaller and you'll have a lot of duplicated cache for commonly used transitive build dependencies.
StatefulSets are idiomatic way to reuse PVs and preserve data in Kubernetes. It works great for most of the stateful workload types - unfortunately it doesn't fit very well for CI/CD. Build pods are most likely dynamically generated in CI/CD, each pod is crafted for a specific project, with containers to bring in tools that are needed for this specific project. A simple example - one project might need a MySQL container for its unit tests while another might need a PostgreSQL container - but both are Maven projects so both need a Maven cache. You can't do this with StatefulSets where all the pods are exactly the same. Not to mention, that most of the times - using StatefulSets is just not an option (hello, Jenkins).
Releaser considers PVs associated when their Storage Class is annotated with metadata.annotations."reclaimable-pv-releaser.kubernetes.io/controller-id" pointing to this -controller-id.
Releaser watches for PVs to be released.
The following conditions must be met for a PV to be made Available:
metadata.annotations."reclaimable-pv-releaser.kubernetes.io/controller-id"on Storage Class must be set to this Controller ID.status.phasemust beReleased.
If these conditions are met, Releaser will set spec.claimRef to null. That will make Kubernetes eventually to mark status.phase of this PV as Available - making other PVCs able to reclaim this PV.
Usage of reclaimable-pv-releaser:
-add_dir_header
If true, adds the file directory to the header of the log messages
-alsologtostderr
log to standard error as well as files
-controller-id string
this controller identity name - use the same string for both provisioner and releaser
-kubeconfig string
optional, absolute path to the kubeconfig file
-lease-lock-id string
optional, the lease lock holder identity name (default <computed>)
-lease-lock-name string
the lease lock resource name
-lease-lock-namespace string
optional, the lease lock resource namespace; default to -namespace
-log_backtrace_at value
when logging hits line file:N, emit a stack trace
-log_dir string
If non-empty, write log files in this directory
-log_file string
If non-empty, use this log file
-log_file_max_size uint
Defines the maximum size a log file can grow to. Unit is megabytes. If the value is 0, the maximum file size is unlimited. (default 1800)
-logtostderr
log to standard error instead of files (default true)
-namespace string
limit to a specific namespace - only for provisioner
-one_output
If true, only write logs to their native severity level (vs also writing to each lower severity level)
-skip_headers
If true, avoid header prefixes in the log messages
-skip_log_headers
If true, avoid headers when opening log files
-stderrthreshold value
logs at or above this threshold go to stderr (default 2)
-v value
number for the log level verbosity
-vmodule value
comma-separated list of pattern=N settings for file-filtered logging
Example:
reclaimable-pv-releaser \
-controller-id reclaimable-pvc-test \
-lease-lock-name reclaimable-pvc-releaser-test \
-lease-lock-namespace default
You can deploy Releaser controller using Helm:
helm repo add plumber-cd https://plumber-cd.github.io/helm/
helm repo update
helm install releaser plumber-cd/reclaimable-pv-releaser
See https://github.com/plumber-cd/helm.
Values: