Add note on the danger of closed indices (#61332)

Prior to #33888 it is dangerous to keep closed indices in your cluster
long-term: Elasticsearch does not maintain their shard copies so they
tend to get lost as the cluster migrates to new nodes. This risk isn't
documented today. This commit addresses that gap.

docs/reference/indices/open-close.asciidoc

@@ -7,6 +7,24 @@ for maintaining its metadata), and is blocked for read/write operations.
 A closed index can be opened which will then go through the normal
 recovery process.
 
+[WARNING]
+====
+To reduce the risk of data loss, avoid keeping closed indices in your cluster
+for long periods of time. When a node leaves your cluster, {es} does not
+rebuild the lost copies of the shards in any closed indices. As you replace the
+nodes in your cluster over time you will gradually lose the shard data for any
+closed indices.
+
+In managed environments such as https://www.elastic.co/cloud[Elastic Cloud]
+nodes may be automatically replaced at any time, which will result in the loss
+of data held in closed indices. You should not close any indices if your
+cluster is running in such a a managed environment.
+
+If you want to reduce the overhead of an index while keeping it available for
+occasional searches, <<frozen-indices,freeze>> the index instead. If you want
+to store an index outside of the cluster, use a <<modules-snapshots,snapshot>>.
+====
+
 The REST endpoint is `/{index}/_close` and `/{index}/_open`. For
 example:
 
@@ -30,8 +48,10 @@ Identifying indices via wildcards or `_all` can be disabled by setting the
 `action.destructive_requires_name` flag in the config file to `true`.
 This setting can also be changed via the cluster update settings api.
 
-Closed indices consume a significant amount of disk-space which can cause problems in managed environments. Closing indices can be disabled via the cluster settings
-API by setting `cluster.indices.close.enable` to `false`. The default is `true`.
+Closed indices consume a significant amount of disk-space which can cause
+problems in managed environments. Closed indices are enabled by default. You can
+disable closed indices by setting `cluster.indices.close.enable` to `false`
+using the <<cluster-update-settings,cluster settings API>>.
 
 IMPORTANT: Closed indices are ignored by many APIs. For instance, the shards of
 a closed index are not included in the output of the <<cat-shards>> API.

docs/reference/setup/install.asciidoc

@@ -3,10 +3,9 @@
 
 [float]
 === Hosted Elasticsearch
-Elasticsearch can be run on your own hardware or using our hosted
-Elasticsearch Service on https://www.elastic.co/cloud[Elastic Cloud], which is
-available on AWS and GCP. You can
-https://www.elastic.co/cloud/elasticsearch-service/signup[try out the hosted service] for free.
+{es} can be run on your own hardware or using our hosted {ess} on
+https://www.elastic.co/cloud[{ecloud}], which is available on AWS, GCP and
+Azure. You can {ess-trial}[try out the hosted service] for free.
 
 [float]
 === Installing Elasticsearch Yourself
@@ -76,4 +75,3 @@ include::install/rpm.asciidoc[]
 include::install/windows.asciidoc[]
 
 include::install/docker.asciidoc[]
-

server/src/main/java/org/elasticsearch/action/admin/indices/close/TransportCloseIndexAction.java

@@ -84,8 +84,9 @@ protected AcknowledgedResponse newResponse() {
     protected void doExecute(Task task, CloseIndexRequest request, ActionListener<AcknowledgedResponse> listener) {
         destructiveOperations.failDestructive(request.indices());
         if (closeIndexEnabled == false) {
-            throw new IllegalStateException("closing indices is disabled - set [" + CLUSTER_INDICES_CLOSE_ENABLE_SETTING.getKey() +
-                ": true] to enable it. NOTE: closed indices still consume a significant amount of diskspace");
+            throw new IllegalStateException("closing indices is forbidden since data in closed indices may be lost during migrations and " +
+                    "upgrades, and they also consume significant disk space; set [" + CLUSTER_INDICES_CLOSE_ENABLE_SETTING.getKey()
+                    + ": true] to permit indices to be closed");
         }
         super.doExecute(task, request, listener);
     }

server/src/test/java/org/elasticsearch/indices/state/CloseIndexDisableCloseAllIT.java

@@ -49,9 +49,9 @@ public void testCloseAllRequiresName() {
 
         IllegalStateException illegalStateException = expectThrows(IllegalStateException.class,
                 () -> client().admin().indices().prepareClose("test_no_close").get());
-        assertEquals(illegalStateException.getMessage(),
-                "closing indices is disabled - set [cluster.indices.close.enable: true] to enable it. NOTE: closed indices still " +
-                        "consume a significant amount of diskspace");
+        assertEquals(illegalStateException.getMessage(), "closing indices is forbidden since data in closed indices may be lost during " +
+                "migrations and upgrades, and they also consume significant disk space; set [cluster.indices.close.enable: true] to " +
+                "permit indices to be closed");
     }
 
     private void assertIndexIsClosed(String... indices) {