Added additional entries for troubleshooting unhealthy cluster

Reordered "Re-enable shard allocation" because not as common as other causes

Added additional causes of yellow statuses

Changed watermark commadn to include high and low watermark so users can make their cluster operate once again.

Author: thekofimensah

docs/reference/troubleshooting/common-issues/red-yellow-cluster-status.asciidoc

@@ -74,35 +74,31 @@ A shard can become unassigned for several reasons. The following tips outline th
 most common causes and their solutions.
 
 [discrete]
-[[fix-cluster-status-reenable-allocation]]
-===== Re-enable shard allocation
+[[fix-cluster-status-only-one-node]]
+===== Single Node Cluster
 
-You typically disable allocation during a <<restart-cluster,restart>> or other
-cluster maintenance. If you forgot to re-enable allocation afterward, {es} will
-be unable to assign shards. To re-enable allocation, reset the
-`cluster.routing.allocation.enable` cluster setting.
+{es} will never assign a replica to the same node as the primary shard. If you only have one node it is expected for your cluster to indicate yellow. If you prefer it to be green, then change the <<dynamic-index-number-of-replicas,num_of_replicas>> on each index to be 0.
 
-[source,console]
-----
-PUT _cluster/settings
-{
-  "persistent" : {
-    "cluster.routing.allocation.enable" : null
-  }
-}
-----
-
-See https://www.youtube.com/watch?v=MiKKUdZvwnI[this video] for walkthrough of troubleshooting "no allocations are allowed".
+Similarly if the number of replicas is equal to or exceeds the number of nodes, then it will not be possible to allocate one or more of the shards for the same reason.
 
 [discrete]
 [[fix-cluster-status-recover-nodes]]
 ===== Recover lost nodes
 
 Shards often become unassigned when a data node leaves the cluster. This can
-occur for several reasons, ranging from connectivity issues to hardware failure.
+occur for several reasons.
+
+* If you manually restart a node, then it will temporarily cause an unhealthy cluster until the node has recovered.
+
+* If you have a node that is overloaded or has stopped operating for any reason, then it will temporarily cause an unhealthy cluster. Nodes may disconnect because of prolonged garbage collection (GC) pauses, which can result from "out of memory" errors or high memory usage due to intensive search operations. See <<fix-cluster-status-jvm,Reduce JVM memory pressure>> for more JVM related issues.
+
+* If nodes cannot reliably communicate due to networking issues, they may lose contact with one another. This can cause shards to become out of sync. You can often identify this issue by checking the logs for repeated messages about nodes leaving and rejoining the cluster.
+
 After you resolve the issue and recover the node, it will rejoin the cluster.
 {es} will then automatically allocate any unassigned shards.
 
+You can monitor this process by <<cluster-health,checking your cluster health>>. You will see that the number of unallocated shards progressively reduces until green status is reached.
+
 To avoid wasting resources on temporary issues, {es} <<delayed-allocation,delays
 allocation>> by one minute by default. If you've recovered a node and don’t want
 to wait for the delay period, you can call the <<cluster-reroute,cluster reroute
@@ -151,7 +147,7 @@ replica, it remains unassigned. To fix this, you can:
 
 * Change the `index.number_of_replicas` index setting to reduce the number of
 replicas for each primary shard. We recommend keeping at least one replica per
-primary.
+primary for high availability.
 
 [source,console]
 ----
@@ -162,7 +158,6 @@ PUT _settings
 ----
 // TEST[s/^/PUT my-index\n/]
 
-
 [discrete]
 [[fix-cluster-status-disk-space]]
 ===== Free up or increase disk space
@@ -183,6 +178,8 @@ If your nodes are running low on disk space, you have a few options:
 
 * Upgrade your nodes to increase disk space.
 
+* Add more nodes to the cluster.
+
 * Delete unneeded indices to free up space. If you use {ilm-init}, you can
 update your lifecycle policy to use <<ilm-searchable-snapshot,searchable
 snapshots>> or add a delete phase. If you no longer need to search the data, you
@@ -215,11 +212,34 @@ watermark or set it to an explicit byte value.
 PUT _cluster/settings
 {
   "persistent": {
-    "cluster.routing.allocation.disk.watermark.low": "30gb"
+    "cluster.routing.allocation.disk.watermark.low": "90%",
+    "cluster.routing.allocation.disk.watermark.high": "95%"
   }
 }
 ----
 // TEST[s/"30gb"/null/]
+**It is important to note that this is usually a temporary solution and may cause instability if the disk space is not freed up.**
+
+[discrete]
+[[fix-cluster-status-reenable-allocation]]
+===== Re-enable shard allocation
+
+You typically disable allocation during a <<restart-cluster,restart>> or other
+cluster maintenance. If you forgot to re-enable allocation afterward, {es} will
+be unable to assign shards. To re-enable allocation, reset the
+`cluster.routing.allocation.enable` cluster setting.
+
+[source,console]
+----
+PUT _cluster/settings
+{
+  "persistent" : {
+    "cluster.routing.allocation.enable" : null
+  }
+}
+----
+
+See https://www.youtube.com/watch?v=MiKKUdZvwnI[this video] for walkthrough of troubleshooting "no allocations are allowed".
 
 [discrete]
 [[fix-cluster-status-jvm]]
@@ -267,4 +287,4 @@ POST _cluster/reroute?metric=none
 // TEST[s/^/PUT my-index\n/]
 // TEST[catch:bad_request]
 
-See https://www.youtube.com/watch?v=6OAg9IyXFO4[this video] for a walkthrough of troubleshooting `no_valid_shard_copy`.
+See https://www.youtube.com/watch?v=6OAg9IyXFO4[this video] for a walkthrough of troubleshooting `no_valid_shard_copy`.