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Add initial OpenShift swap enhancement
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--- | ||
title: node-swap | ||
authors: | ||
- "@ehashman" | ||
reviewers: | ||
- "@rphilips" | ||
- "@sjenning" | ||
- "???" | ||
approvers: | ||
- "@mrunalp" | ||
creation-date: "2021-06-23" | ||
status: implementable | ||
--- | ||
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# OpenShift Node Swap Support | ||
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## Release Signoff Checklist | ||
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- [X] Enhancement is `implementable` | ||
- [X] Design details are appropriately documented from clear requirements | ||
- [X] Test plan is defined | ||
- [X] Operational readiness criteria is defined | ||
- [X] Graduation criteria for dev preview, tech preview, GA | ||
- [ ] User-facing documentation is created in [openshift-docs](https://github.com/openshift/openshift-docs/) | ||
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## Summary | ||
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The upstream Kubernetes 1.22 release introduced alpha support for configuring | ||
swap memory usage for Kubernetes workloads on a per-node basis. | ||
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Now that swap use on nodes is supported in upstream, there are a number of use | ||
cases that would benefit from OpenShift nodes supporting swap, including | ||
improved node stability, better support for applications with high memory | ||
overhead but smaller working sets, the use of memory-constrained devices, and | ||
memory flexibility. | ||
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## Motivation | ||
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See [KEP-2400: Motivation]. | ||
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[KEP-2400: Motivation]: https://github.com/kubernetes/enhancements/blob/master/keps/sig-node/2400-node-swap/README.md#motivation | ||
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### Goals | ||
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- Swap can be provisioned and configured for nodes to use in an OpenShift | ||
cluster. | ||
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### Non-Goals | ||
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- Workload-specific swap accounting. | ||
- Any of the non-goals in [KEP-2400: Non-goals]. | ||
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[KEP-2400: Non-goals]: https://github.com/kubernetes/enhancements/blob/master/keps/sig-node/2400-node-swap/README.md#non-goals | ||
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## Proposal | ||
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### User Stories | ||
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See [KEP-2400: User Stories](https://github.com/kubernetes/enhancements/blob/master/keps/sig-node/2400-node-swap/README.md#user-stories). | ||
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### API Extensions | ||
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No new API extensions are required. For tech preview, the only update required | ||
is the addition of a `NodeSwapNoUpgrade` feature set to the OpenShift API. See | ||
[Design Details](#design-details) for the specifics. | ||
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### Implementation Details/Notes/Constraints [optional] | ||
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See [KEP-2400: Notes/Constraints/Caveats](https://github.com/kubernetes/enhancements/blob/master/keps/sig-node/2400-node-swap/README.md#notesconstraintscaveats-optional). | ||
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### Risks and Mitigations | ||
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See [KEP-2400: Risks and Mitigations]. | ||
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[KEP-2400: Risks and Mitigations]: https://github.com/kubernetes/enhancements/blob/master/keps/sig-node/2400-node-swap/README.md#risks-and-mitigations | ||
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## Design Details | ||
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### Enable `NodeSwap` feature gate | ||
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We will add a new FeatureSet `NodeSwapNoUpgrade` with feature gate `NodeSwap` | ||
enabled for [Tech Preview]. | ||
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Note that we will not add this to the existing `TechPreviewNoUpgrade` feature | ||
set list, as we do not wish to enable this feature simultaneously with other | ||
tech preview features. Note that swap cannot be enabled or used without | ||
additional configuration, so there isn't necessarily a risk of adding it to the | ||
existing Tech Preview feature set; rather, for testing swap, we do not want to | ||
require users to also turn on other features. | ||
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Without this change, it is possible to enable the feature gate on any 4.9+ | ||
cluster by defining `NodeSwap` in a `customNoUpgrade` configuration: | ||
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```yaml | ||
apiVersion: config.openshift.io/v1 | ||
kind: FeatureGate | ||
metadata: | ||
name: cluster | ||
spec: | ||
featureSet: CustomNoUpgrade | ||
customNoUpgrade: | ||
enabled: | ||
- NodeSwap | ||
``` | ||
Once the `NodeSwap` feature flag is enabled, a cluster admin can enable swap | ||
usage on the cluster as follows: | ||
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### Ensure component versions support swap | ||
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While swap support has been available in Kubernetes since the 1.22.0 release, | ||
the container runtimes also need support in order for Kubernetes workloads to | ||
be able to use the feature correctly. | ||
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Therefore, a user of OpenShift version >4.9.0 can enable the feature flag, but | ||
they also will require a version of CRI-O that supports the | ||
`MemorySwapLimitInBytes` for best results. This is only supported in the 1.23 | ||
and onwards releases of CRI-O, and should be supported in the first release of | ||
OpenShift 4.10. | ||
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### Configure worker Kubelets | ||
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Swap behaviour on a node can be configured with | ||
[`memorySwap.SwapBehavior`](https://kubernetes.io/docs/concepts/architecture/nodes/#swap-memory). | ||
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The most straightforward way to configure the kubelets with this feature is | ||
with a custom `KubeletConfig` that will automatically be applied by the MCO: | ||
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```bash | ||
# Enable the custom kubelet config on the worker pool | ||
oc label machineconfigpool worker custom-kubelet=enabled | ||
``` | ||
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```yaml | ||
apiVersion: machineconfiguration.openshift.io/v1 | ||
kind: KubeletConfig | ||
metadata: | ||
name: custom-config | ||
spec: | ||
machineConfigPoolSelector: | ||
matchLabels: | ||
custom-kubelet: enabled | ||
kubeletConfig: | ||
failSwapOn: false | ||
memorySwap: | ||
swapBehavior: UnlimitedSwap # LimitedSwap is also supported | ||
``` | ||
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Note that enabling swap on control plane nodes is possible but **not** recommended. | ||
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### Add swap to nodes | ||
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There are a few different ways this can be accomplished. The most | ||
straightforward is to add a kubelet configuration that enables a swapfile at | ||
startup with a custom machine config, the same way we enable swap in upstream | ||
CI: | ||
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```yaml | ||
apiVersion: machineconfiguration.openshift.io/v1 | ||
kind: MachineConfig | ||
metadata: | ||
labels: | ||
machineconfiguration.openshift.io/role: worker | ||
name: 90-worker-swap | ||
spec: | ||
config: | ||
ignition: | ||
version: 3.2.0 | ||
systemd: | ||
units: | ||
- contents: | | ||
[Unit] | ||
Description=Enable swap on CoreOS | ||
Before=crio-install.service | ||
ConditionFirstBoot=no | ||
[Service] | ||
Type=oneshot | ||
ExecStart=/bin/sh -c "sudo dd if=/dev/zero of=/var/swapfile count=1024 bs=1MiB && sudo chmod 600 /var/swapfile && sudo mkswap /var/swapfile && sudo swapon /var/swapfile && free -h" | ||
[Install] | ||
WantedBy=multi-user.target | ||
enabled: true | ||
name: swap-enable.service | ||
``` | ||
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It is also possible to use [ignition | ||
configs](https://coreos.github.io/ignition/configuration-v3_3/) to add swap | ||
partitions to worker nodes with `filesystems.format = swap`. | ||
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Beyond tech preview, we may want to look into adding swap support to the | ||
installer, and consider adding a partition on the root node, or perhaps | ||
[provision and mount an NVMe | ||
volume](https://github.com/openshift/machine-config-operator/issues/1619). | ||
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### Open Questions [optional] | ||
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- Will we eventually want to enable swap on all OpenShift nodes by default? | ||
- Should swap just be limited to worker nodes, or should we consider adding it | ||
to control plane nodes too? | ||
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Testing in reliability clusters and in various configurations will help us | ||
answer these questions. Stability metrics determined through the upstream beta | ||
will assist. | ||
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### Test Plan | ||
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In addition to the upstream e2e tests, we will need to add e2e suites to | ||
OpenShift in order to exercise provisioning and use of swap. This may include | ||
unit tests where appropriate, such as the MCO. | ||
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### Graduation Criteria | ||
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#### Dev Preview -> Tech Preview | ||
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Requires alpha support in upstream Kubernetes (1.22+) and support in CRI-O | ||
(1.23+). | ||
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- Support provisioning OpenShift nodes with swap enabled for all available | ||
upstream swap configurations (currently `LimitedSwap`, `UnlimitedSwap`). | ||
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JIRA: https://issues.redhat.com/browse/OCPNODE-470 | ||
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_Graduation criteria below are tentative._ | ||
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#### Tech Preview -> GA | ||
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Requires beta/GA support in upstream Kubernetes. (1.25?+) | ||
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- More testing (upgrade, downgrade, scale) | ||
- Sufficient time for feedback | ||
- Available by default | ||
- Backhaul SLI telemetry | ||
- Document SLOs for the component | ||
- Conduct load testing | ||
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#### Removing a deprecated feature | ||
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- Announce deprecation and support policy of the existing feature | ||
- Deprecate the feature | ||
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Deprecation of this feature would likely follow upstream deprecation, which is | ||
not currently planned. | ||
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### Upgrade / Downgrade Strategy | ||
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The `NodeSwap` feature flag is not supported in Kubernetes versions prior to | ||
1.22/OpenShift 4.9. We will add the upstream `NodeSwap` feature flag as a | ||
"NoUpgrade" feature flag to prevent upgrades. | ||
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Note that swap support does not require coordination between components and the | ||
configuration is limited to individual nodes/machine pools. | ||
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See also [KEP-2400: Upgrade/Downgrade Strategy]. | ||
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[Tech Preview]: https://github.com/openshift/enhancements/blob/ce4d303db807622687159eb9d3248285a003fabb/guidelines/techpreview.md#official-processmechanism-for-delivering-a-tp-feature | ||
[KEP-2400: Upgrade/Downgrade Strategy]: https://github.com/kubernetes/enhancements/blob/master/keps/sig-node/2400-node-swap/README.md#upgrade--downgrade-strategy | ||
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### Version Skew Strategy | ||
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N/A, this is a compatible API change limited to the Kubelet that does not | ||
require coordination with the API Server. | ||
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### Operational Aspects of API Extensions | ||
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None. The only API extension is a feature gate change. There are no plans to | ||
enable the feature by default until extensive testing takes place and upstream | ||
announces beta or GA support for the feature. | ||
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For potential operational impacts of enabling swap, see [KEP-2400: Risks and | ||
Mitigations]. | ||
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#### Failure Modes | ||
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See [KEP-2400: Rollout, Upgrade and Rollback Planning] and [KEP-2400: | ||
Troubleshooting]. | ||
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[KEP-2400: Rollout, Upgrade and Rollback Planning]: https://github.com/kubernetes/enhancements/blob/master/keps/sig-node/2400-node-swap/README.md#rollout-upgrade-and-rollback-planning | ||
[KEP-2400: Troubleshooting]: https://github.com/kubernetes/enhancements/blob/master/keps/sig-node/2400-node-swap/README.md#troubleshooting | ||
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#### Support Procedures | ||
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See [KEP-2400: Production Readiness Review Questionnaire] and [KEP-2400: | ||
Troubleshooting]. | ||
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[KEP-2400: Production Readiness Review Questionnaire]: https://github.com/kubernetes/enhancements/blob/master/keps/sig-node/2400-node-swap/README.md#production-readiness-review-questionnaire | ||
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## Implementation History | ||
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- [Upstream alpha swap support] completed in Kubernetes 1.22. | ||
- [CRI-O support] added in 1.23.0. | ||
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[Upstream alpha swap support]: https://github.com/kubernetes/enhancements/issues/2400#issuecomment-884327938 | ||
[CRI-O support]: https://github.com/cri-o/cri-o/pull/5207 | ||
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## Drawbacks | ||
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See [KEP-2400: Drawbacks]. | ||
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[KEP-2400: Drawbacks]: https://github.com/kubernetes/enhancements/blob/master/keps/sig-node/2400-node-swap/README.md#drawbacks | ||
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## Alternatives | ||
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See [KEP-2400: Alternatives]. | ||
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[KEP-2400: Alternatives]: https://github.com/kubernetes/enhancements/blob/master/keps/sig-node/2400-node-swap/README.md#alternatives | ||
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## Infrastructure Needed [optional] | ||
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- We will need to configure periodic e2e tests on VMs with swap enabled. | ||
- We will need to enable swap on a [reliability cluster] to gauge long-term | ||
stability. | ||
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[reliability cluster]: https://issues.redhat.com/browse/OCPNODE-619 |