Clarify replica shard allocation step in rolling restart procedure (#1246)

Updates the restart documentation to specify that setting
`cluster.routing.allocation.enable` to `primaries` disables
**_replica_** shard allocation, not all shard allocation.

The phrase “disable shard allocation” seems to be a colloquial
shorthand. Those experienced with Elasticsearch may implicitly
understand it refers to replica shards during restarts, but this can be
unclear to newer users.

Historically (and TIL 💡), the restart procedures
([doc](https://www.elastic.co/guide/en/elasticsearch/reference/6.6/rolling-upgrades.html))
used the `none` setting to fully disable shard allocation, which matched
the “disable shard allocation” phrasing . In version 6.7, the
recommended setting changed to `primaries` to allow primary shard
allocation while avoiding unnecessary replica movement. However, the
descriptive text was not updated to reflect this change, leaving behind
a vestigial phrase from earlier behavior.

Clarifying this language helps align the documentation with the actual
behavior in the example API calls we provide and reduces the risk of
misinterpretation (e.g., mistakenly using "none" instead of
"primaries").

Author: rseldner

deploy-manage/maintenance/start-stop-services/full-cluster-restart-rolling-restart-procedures.md

@@ -17,8 +17,8 @@ Nodes exceeding the low watermark threshold will be slow to restart. Reduce the
 
 ## Full-cluster restart [restart-cluster-full]
 
-1. **Disable shard allocation.**
-   When you shut down a data node, the allocation process waits for `index.unassigned.node_left.delayed_timeout` (by default, one minute) before starting to replicate the shards on that node to other nodes in the cluster, which can involve a lot of I/O. Since the node is shortly going to be restarted, this I/O is unnecessary. You can avoid racing the clock by [disabling allocation](elasticsearch://reference/elasticsearch/configuration-reference/cluster-level-shard-allocation-routing-settings.md#cluster-routing-allocation-enable) of replicas before shutting down [data nodes](../../distributed-architecture/clusters-nodes-shards/node-roles.md#data-node-role):
+1. **Disable replica shard allocation.**
+   When you shut down a data node, the allocation process waits for `index.unassigned.node_left.delayed_timeout` (by default, one minute) before starting to replicate the shards on that node to other nodes in the cluster, which can involve a lot of I/O. Since the node is shortly going to be restarted, this I/O is unnecessary. You can avoid racing the clock by [disabling allocation of replicas](elasticsearch://reference/elasticsearch/configuration-reference/cluster-level-shard-allocation-routing-settings.md#cluster-routing-allocation-enable) before shutting down [data nodes](../../distributed-architecture/clusters-nodes-shards/node-roles.md#data-node-role):
 
    ```console
    PUT _cluster/settings
@@ -91,8 +91,8 @@ Nodes exceeding the low watermark threshold will be slow to restart. Reduce the
    When a node joins the cluster, it begins to recover any primary shards that are stored locally. The [`_cat/health`](https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-cat-health) API initially reports a `status` of `red`, indicating that not all primary shards have been allocated.
    Once a node recovers its local shards, the cluster `status` switches to `yellow`, indicating that all primary shards have been recovered, but not all replica shards are allocated. This is to be expected because you have not yet re-enabled allocation. Delaying the allocation of replicas until all nodes are `yellow` allows the master to allocate replicas to nodes that already have local shard copies.
 
-8. **Re-enable allocation.**
-   When all nodes have joined the cluster and recovered their primary shards, re-enable allocation by restoring `cluster.routing.allocation.enable` to its default:
+8. **Re-enable replica shard allocation.**
+   When all nodes have joined the cluster and recovered their primary shards, re-enable replica allocation by restoring `cluster.routing.allocation.enable` to its default:
 
     ```console
     PUT _cluster/settings
@@ -103,7 +103,7 @@ Nodes exceeding the low watermark threshold will be slow to restart. Reduce the
     }
     ```
 
-   Once allocation is re-enabled, the cluster starts allocating replica shards to the data nodes. At this point it is safe to resume indexing and searching, but your cluster will recover more quickly if you can wait until all primary and replica shards have been successfully allocated and the status of all nodes is `green`.
+   Once replica allocation is re-enabled, the cluster starts allocating replica shards to the data nodes. At this point it is safe to resume indexing and searching, but your cluster will recover more quickly if you can wait until all primary and replica shards have been successfully allocated and the status of all nodes is `green`.
    You can monitor progress with the [`_cat/health`](https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-cat-health) and [`_cat/recovery`](https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-cat-recovery) APIs:
 
     ```console
@@ -123,8 +123,8 @@ Nodes exceeding the low watermark threshold will be slow to restart. Reduce the
 
 ## Rolling restart [restart-cluster-rolling]
 
-1. **Disable shard allocation.**
-   When you shut down a data node, the allocation process waits for `index.unassigned.node_left.delayed_timeout` (by default, one minute) before starting to replicate the shards on that node to other nodes in the cluster, which can involve a lot of I/O. Since the node is shortly going to be restarted, this I/O is unnecessary. You can avoid racing the clock by [disabling allocation](elasticsearch://reference/elasticsearch/configuration-reference/cluster-level-shard-allocation-routing-settings.md#cluster-routing-allocation-enable) of replicas before shutting down [data nodes](../../distributed-architecture/clusters-nodes-shards/node-roles.md#data-node-role):
+1. **Disable replica shard allocation.**
+   When you shut down a data node, the allocation process waits for `index.unassigned.node_left.delayed_timeout` (by default, one minute) before starting to replicate the shards on that node to other nodes in the cluster, which can involve a lot of I/O. Since the node is shortly going to be restarted, this I/O is unnecessary. You can avoid racing the clock by [disabling allocation of replicas](elasticsearch://reference/elasticsearch/configuration-reference/cluster-level-shard-allocation-routing-settings.md#cluster-routing-allocation-enable) before shutting down [data nodes](../../distributed-architecture/clusters-nodes-shards/node-roles.md#data-node-role):
 
     ```console
     PUT _cluster/settings
@@ -187,8 +187,8 @@ Nodes exceeding the low watermark threshold will be slow to restart. Reduce the
     GET _cat/nodes
     ```
 
-7. **Reenable shard allocation.**
-   For data nodes, once the node has joined the cluster, remove the `cluster.routing.allocation.enable` setting to enable shard allocation and start using the node:
+7. **Re-enable replica shard allocation.**
+   For data nodes, once the node has joined the cluster, remove the `cluster.routing.allocation.enable` setting to enable replica shard allocation and start using the node:
 
     ```console
     PUT _cluster/settings