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PDB maxUnavailable rounding logic 

Clarify the implications of rounding-up in combination with using a percent value and maxUnavailable.
pull/39670/head
Alexander Idelberger 2023-02-25 15:32:19 +01:00
parent e244bf9113
commit ced4a3e31c
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1 changed files with 8 additions and 4 deletions
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content/en/docs/tasks/run-application/configure-pdb.md
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@ -86,12 +86,14 @@ Values for `minAvailable` or `maxUnavailable` can be expressed as integers or as
- When you specify an integer, it represents a number of Pods. For instance, if you set `minAvailable` to 10, then 10 - When you specify an integer, it represents a number of Pods. For instance, if you set `minAvailable` to 10, then 10
Pods must always be available, even during a disruption. Pods must always be available, even during a disruption.
- When you specify a percentage by setting the value to a string representation of a percentage (eg. `"50%"`), it represents a percentage of - When you specify a percentage by setting the value to a string representation of a percentage (eg. `"50%"`), it represents a percentage of
total Pods. For instance, if you set `maxUnavailable` to `"50%"`, then only 50% of the Pods can be unavailable during a total Pods. For instance, if you set `minAvailable` to `"50%"`, then at least 50% of the Pods remain available during a
disruption. disruption.
When you specify the value as a percentage, it may not map to an exact number of Pods. For example, if you have 7 Pods and When you specify the value as a percentage, it may not map to an exact number of Pods. For example, if you have 7 Pods and
you set `minAvailable` to `"50%"`, it's not immediately obvious whether that means 3 Pods or 4 Pods must be available. you set `minAvailable` to `"50%"`, it's not immediately obvious whether that means 3 Pods or 4 Pods must be available.
Kubernetes rounds up to the nearest integer, so in this case, 4 Pods must be available. You can examine the Kubernetes rounds up to the nearest integer, so in this case, 4 Pods must be available. When you specify the value
`maxUnavailable` as a percentage, Kubernetes rounds up the number of Pods that may be disrupted. Thereby a disruption
can exceed your defined `maxUnavailable` percentage. You can examine the
[code](https://github.com/kubernetes/kubernetes/blob/23be9587a0f8677eb8091464098881df939c44a9/pkg/controller/disruption/disruption.go#L539) [code](https://github.com/kubernetes/kubernetes/blob/23be9587a0f8677eb8091464098881df939c44a9/pkg/controller/disruption/disruption.go#L539)
that controls this behavior. that controls this behavior.
@ -129,8 +131,10 @@ of the number of desired replicas are healthy.
Example 3: With a `maxUnavailable` of 5, evictions are allowed as long as there are at most 5 Example 3: With a `maxUnavailable` of 5, evictions are allowed as long as there are at most 5
unhealthy replicas among the total number of desired replicas. unhealthy replicas among the total number of desired replicas.
Example 4: With a `maxUnavailable` of 30%, evictions are allowed as long as no more than 30% Example 4: With a `maxUnavailable` of 30%, evictions are allowed as long as the number of
of the desired replicas are unhealthy. unhealthy replicas does not exceed 30% of the total number of desired replica rounded up to
the nearest integer. If the total number of desired replicas is just one, the only one replica
is still allowed for disruption, leading to an effective unavailability of 100%.
In typical usage, a single budget would be used for a collection of pods managed by In typical usage, a single budget would be used for a collection of pods managed by
a controller—for example, the pods in a single ReplicaSet or StatefulSet. a controller—for example, the pods in a single ReplicaSet or StatefulSet.