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[TEP-0135] Update Affinity Assistant documentation
Part of [tektoncd#6740][tektoncd#6740]. This commit updates the Affinity Assistant related documentation to reflect recent changes in the Affinity Assistant modes. This commit also adds a chart summarizing how to configure the Affinity Assistant modes using `coschedule` and `disabled-affinity-assistant` feature flags. /kind documentation [tektoncd#6740]: tektoncd#6740
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| <!-- | ||
| --- | ||
| linkTitle: "Affinity Assistants" | ||
| weight: 405 | ||
| --- | ||
| --> | ||
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| # Affinity Assistants | ||
| Affinity Assistant is a feature to coschedule `PipelineRun` `pods` to the same node | ||
| based on [kubernetes pod affinity](https://kubernetes.io/docs/concepts/scheduling-eviction/assign-pod-node/#inter-pod-affinity-and-anti-affinity) so that it possible for the taskruns to execute parallel while sharing volume. | ||
| Available Affinity Assistant Modes are **coschedule workspace**, **coschedule pipelineruns**, | ||
| **isolate pipelinerun** and **disabled**. | ||
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| > :seedling: **coschedule pipelineruns** and **isolate pipelinerun** modes are [**alpha features**](./additional-configs.md#alpha-features). | ||
| > **coschedule workspaces** is a **stable feature** | ||
| * **coschedule workspace** - When a `PersistentVolumeClaim` is used as volume source for a `Workspace` in a `PipelineRun`, | ||
| all `TaskRun` pods within the `PipelineRun` that share the `Workspace` will be scheduled to the same Node. | ||
| **Note:** Only one pvc-backed workspace can be mounted to each TaskRun in this mode. | ||
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| * **coschedule pipelineruns** - All `TaskRun` pods within the `PipelineRun` will be scheduled to the same Node. | ||
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| * **isolate pipelinerun** - All `TaskRun` pods within the `PipelineRun` will be scheduled to the same Node, | ||
| and only one PipelineRun is allowed to run on a node at a time. | ||
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| * **disabled** - The Affinity Assistant is disabled. No pod coscheduling behavior. | ||
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| This means that Affinity Assistant is incompatible with other affinity rules | ||
| configured for the `TaskRun` pods (i.e. other affinity rules specified in custom [PodTemplate](pipelineruns.md#specifying-a-pod-template) will be overwritten by Affinity Assistant). | ||
| If the `PipelineRun` has a custom [PodTemplate](pipelineruns.md#specifying-a-pod-template) configured, the `NodeSelector` and `Tolerations` fieldswill also be set on the Affinity Assistant pod. The Affinity Assistant | ||
| is deleted when the `PipelineRun` is completed. | ||
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| Currently, the Affinity Assistant Modes can be configured by the `disable-affinity-assistant` and `coschedule` feature flags. In 9 months, the `disable-affinity-assistant` feature flag will be deprecated, and the Affinity Assistant Modes will be only determined by the `coschedule` feature flag. | ||
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| The following chart summarizes the Affinity Assistant Modes with different combinations of the `disable-affinity-assistant` and `coschedule` feature flags during migration (when both feature flags are present) and after the migration (when only the `coschedule` flag is present). | ||
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| <table> | ||
| <thead> | ||
| <tr> | ||
| <th>disable-affinity-assistant</th> | ||
| <th>coschedule</th> | ||
| <th>behavior during migration</th> | ||
| <th>behavior after migration</th> | ||
| </tr> | ||
| </thead> | ||
| <tbody> | ||
| <tr> | ||
| <td>false (default)</td> | ||
| <td>disabled</td> | ||
| <td>N/A: invalid</td> | ||
| <td>disabled</td> | ||
| </tr> | ||
| <tr> | ||
| <td>false (default)</td> | ||
| <td>workspaces (default)</td> | ||
| <td>coschedule workspaces</td> | ||
| <td>coschedule workspaces</td> | ||
| </tr> | ||
| <tr> | ||
| <td>false (default)</td> | ||
| <td>pipelineruns</td> | ||
| <td>N/A: invalid</td> | ||
| <td>coschedule pipelineruns</td> | ||
| </tr> | ||
| <tr> | ||
| <td>false (default)</td> | ||
| <td>isolate-pipelinerun</td> | ||
| <td>N/A: invalid</td> | ||
| <td>isolate pipelinerun</td> | ||
| </tr> | ||
| <tr> | ||
| <td>true</td> | ||
| <td>disabled</td> | ||
| <td>disabled</td> | ||
| <td>disabled</td> | ||
| </tr> | ||
| <tr> | ||
| <td>true</td> | ||
| <td>workspaces (default)</td> | ||
| <td>disabled</td> | ||
| <td>coschedule workspaces</td> | ||
| </tr> | ||
| <tr> | ||
| <td>true</td> | ||
| <td>pipelineruns</td> | ||
| <td>coschedule pipelineruns</td> | ||
| <td>coschedule pipelineruns</td> | ||
| </tr> | ||
| <tr> | ||
| <td>true</td> | ||
| <td>isolate-pipelinerun</td> | ||
| <td>isolate pipelinerun</td> | ||
| <td>isolate pipelinerun</td> | ||
| </tr> | ||
| </tbody> | ||
| </table> | ||
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| **Note:** For users who previously accepted the default behavior (`disable-affinity-assistant`: `false`) but now want one of the new features, you need to set `disable-affinity-assistant` to "true" and then turn on the new behavior by setting the `coschedule` flag. For users who previously disabled the affinity assistant but want one of the new features, just set the `coschedule` flag accordingly. | ||
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| **Note:** Affinity Assistant use [Inter-pod affinity and anti-affinity](https://kubernetes.io/docs/concepts/scheduling-eviction/assign-pod-node/#inter-pod-affinity-and-anti-affinity) | ||
| that require substantial amount of processing which can slow down scheduling in large clusters | ||
| significantly. We do not recommend using the affinity assistant in clusters larger than several hundred nodes | ||
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| **Note:** Pod anti-affinity requires nodes to be consistently labelled, in other words every | ||
| node in the cluster must have an appropriate label matching `topologyKey`. If some or all nodes | ||
| are missing the specified `topologyKey` label, it can lead to unintended behavior. | ||
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| **Note:** Any time during the execution of a `pipelineRun`, if the node with a placeholder Affinity Assistant pod and | ||
| the `taskRun` pods sharing a `workspace` is `cordoned` or disabled for scheduling anything new (`tainted`), the | ||
| `pipelineRun` controller deletes the placeholder pod. The `taskRun` pods on a `cordoned` node continues running | ||
| until completion. The deletion of a placeholder pod triggers creating a new placeholder pod on any available node | ||
| such that the rest of the `pipelineRun` can continue without any disruption until it finishes. |
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