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etcdctl: Documment downgrade commands

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Marek Siarkowicz 2022-02-22 16:29:35 +01:00
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etcdctl/README.md
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@ -1085,7 +1085,29 @@ echo ${transferee_id}
### DOWNGRADE \<subcommand\>
Downgrade provides commands to downgrade cluster version.
NOTICE: Downgrades is an experimental feature in v3.6 and is not recommended for production clusters.
Downgrade provides commands to downgrade cluster.
Normally etcd members cannot be downgraded due to cluster version mechanism.
After initial bootstrap, cluster members agree on the cluster version. Every 5 seconds, leader checks versions of all members and picks lowers minor version.
New members will refuse joining cluster with cluster version newer than theirs, thus preventing cluster from downgrading.
Downgrade commands allow cluster administrator to force cluster version to be lowered to previous minor version, thus allowing to downgrade the cluster.
Downgrade should be is executed in stages:
1. Verify that cluster is ready be downgraded by running `etcdctl downgrade validate <TARGET_VERSION>`
2. Start the downgrade process by running `etcdctl downgrade enable <TARGET_VERSION>`
3. For each cluster member:
1. Ensure that member is ready for downgrade by confirming that it wrote `The server is ready to downgrade` log.
2. Replace member binary with one with older version.
3. Confirm that member has correctly started and joined the cluster.
4. Ensure that downgrade process has succeeded by checking leader log for `the cluster has been downgraded`
Downgrade can be canceled by running `etcdctl downgrade cancel` command.
In case of downgrade being canceled, cluster version will return to its normal behavior (pick the lowest member minor version).
If no members were downgraded, cluster version will return to original value.
If at least one member was downgraded, cluster version will stay at the `<TARGET_VALUE>` until downgraded members are upgraded back.
### DOWNGRADE VALIDATE \<TARGET_VERSION\>