osdocs-21632:adds hostPrefix docs 5.0 - #118400
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@JoeAldinger: This pull request references osdocs-21632 which is a valid jira issue. Warning: The referenced jira issue has an invalid target version for the target branch this PR targets: expected the epic to target either version "5.1.0." or "openshift-5.1.0.", but it targets "openshift-5.0" instead. DetailsIn response to this:
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🤖 Tue Sep 08 20:14:44 - Prow CI generated the docs preview: |
| .. Verify node readiness across the cluster. | ||
| .. Verify critical workloads and pod scheduling behavior. | ||
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| [NOTE] |
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knobunc
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Great stuff! A few minor suggestions, none required.
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| The `hostPrefix` value determines the subnet size assigned to each node from the `spec.clusterNetwork[].cidr` range. For example, with a cluster network CIDR of `172.28.0.0/16`, changing `hostPrefix` from `23` to `20` changes each node allocation from a `/23` subnet to a `/20` subnet. The `/20` subnet provides a larger per-node subnet than the `/23` subnet, and therefore provides more pod IP addresses per node. | ||
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| By default, a `/23` per-node subnet provides 512 addresses, with approximately 510 pod IP addresses available per node. Increasing the subnet size can provide more addresses per node; however, node-level pod density remains dependent on cluster scalability limits, node resources, workload behavior, and operational constraints. |
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Does it make sense to explain the math? A /23 allows the node to allocate 2 ^ (32 - 23) addresses, so 2 ^ 9 or 512.
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I don't think explaining the math is going to clarify the concept. Ok to skip, imo.
| = Planning for a cluster network hostPrefix increase | ||
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| Before you increase `hostPrefix`, verify address capacity, workload disruption tolerance, and operational sequencing requirements. Increasing the per-node subnet size reduces the total number of node subnets available from the cluster CIDR. |
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"Increasing the per-node subnet size reduces the total number of node subnets available from the cluster CIDR."
Maybe:
"Increasing the per-node subnet size reduces the total number of nodes that can be part of the cluster."
Not sure if it helps to explain the math here too. If your clusternetwork is a /16 CIDR and you have a hostPrefix of 23 then you have allocated 32 - 16 bits (16) to the cluster to use. Of those 16 bits it can use, the hostPrefix of 23 leaves 32-23 (9) bits for the nodes to use and 23-16 (7) to address the nodes themselves. So with that configuration you get 128 nodes with a max of 512 pods per node. 128 * 512 = 65536 = 2^16.
Maybe a picture would make it clear too?
32 bit IP address
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| Part to identify the cluster (16 bits) | Part to identify the node (7 bits) | Part to identify the pod (9 bits |
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I don't think an image is inside scope for this work and here again I'm not sure showing the math will help either. Your rephrasing suggestion is good, thank you.
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| ==== | ||
| * Changing the `hostPrefix` value does not change the support for increased pod density or the number of pods that can be supported per node. Red{nbsp}Hat supports tested cluster maximums rather than absolute cluster maximums.You must still adhere to the cluster scalability limits, node resources, workload behavior, and operational constraints. See "Planning your environment according to object maximums". |
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LGTM |
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LGTM |
7 participants
Version(s):
4.18
Issue:
https://redhat.atlassian.net/browse/OSDOCS-21632
Link to docs preview:
QE review:
Additional information:
Update to existing docs for xrefs, etc: #118396