Redefine section on sizing data nodes (#90274)

Now that we have the estimated field mappings heap overhead
in nodes stats, we can refer to them in the guide for sizing
data nodes appropriately.

Relates to #86639

docs/reference/cat/nodes.asciidoc

@@ -6,8 +6,8 @@
 
 [IMPORTANT]
 ====
-cat APIs are only intended for human consumption using the command line or {kib} 
-console. They are _not_ intended for use by applications. For application 
+cat APIs are only intended for human consumption using the command line or {kib}
+console. They are _not_ intended for use by applications. For application
 consumption, use the <<cluster-nodes-info,nodes info API>>.
 ====
 
@@ -314,6 +314,12 @@ Time spent in suggest, such as `0`.
 
 `suggest.total`, `suto`, `suggestTotal`::
 Number of suggest operations, such as `0`.
+
+`mappings.total_count`, `mtc`, `mappingsTotalCount`::
+Number of mappings, including <<runtime,runtime>> and <<object,object>> fields.
+
+`mappings.total_estimated_overhead_in_bytes`, `mteo`, `mappingsTotalEstimatedOverheadInBytes`::
+Estimated heap overhead, in bytes, of mappings on this node, which allows for 1KiB of heap for every mapped field.
 --
 
 include::{es-repo-dir}/rest-api/common-parms.asciidoc[tag=help]

docs/reference/how-to/size-your-shards.asciidoc

@@ -222,31 +222,128 @@ GET _cat/shards?v=true
 
 [discrete]
 [[field-count-recommendation]]
-==== Data nodes should have at least 1kB of heap per field per index, plus overheads
-
-The exact resource usage of each mapped field depends on its type, but a rule
-of thumb is to allow for approximately 1kB of heap overhead per mapped field
-per index held by each data node. In a running cluster, you can also consult the
-<<cluster-nodes-stats,Nodes stats API>>'s `mappings` indices statistic, which
-reports the number of field mappings and an estimation of their heap overhead.
-
-Additionally, you must also allow enough heap for {es}'s
-baseline usage as well as your workload such as indexing, searches and
-aggregations. 0.5GB of extra heap will suffice for many reasonable workloads,
-and you may need even less if your workload is very light while heavy workloads
-may require more.
-
-For example, if a data node holds shards from 1000 indices, each containing
-4000 mapped fields, then you should allow approximately 1000 × 4000 × 1kB = 4GB
-of heap for the fields and another 0.5GB of heap for its workload and other
-overheads, and therefore this node will need a heap size of at least 4.5GB.
-
-Note that this rule defines the absolute maximum number of indices that a data
-node can manage, but does not guarantee the performance of searches or indexing
-involving this many indices. You must also ensure that your data nodes have
-adequate resources for your workload and that your overall sharding strategy
-meets all your performance requirements. See also <<single-thread-per-shard>>
-and <<each-shard-has-overhead>>.
+==== Allow enough heap for field mappers and overheads
+
+Mapped fields consume some heap memory on each node, and require extra
+heap on data nodes.
+Ensure each node has enough heap for mappings, and also allow
+extra space for overheads associated with its workload. The following sections
+show how to determine these heap requirements.
+
+[discrete]
+===== Mapping metadata in the cluster state
+
+Each node in the cluster has a copy of the <<cluster-state-api-desc,cluster state>>.
+The cluster state includes information about the field mappings for
+each index. This information has heap overhead. You can use the
+<<cluster-stats,Cluster stats API>> to get the heap overhead of the total size of
+all mappings after deduplication and compression.
+
+[source,console]
+----
+GET _cluster/stats?human&filter_path=indices.mappings.total_deduplicated_mapping_size*
+----
+// TEST[setup:node]
+
+This will show you information like in this example output:
+
+[source,console-result]
+----
+{
+  "indices": {
+    "mappings": {
+      "total_deduplicated_mapping_size": "1gb",
+      "total_deduplicated_mapping_size_in_bytes": 1073741824
+    }
+  }
+}
+----
+// TESTRESPONSE[s/"total_deduplicated_mapping_size": "1gb"/"total_deduplicated_mapping_size": $body.$_path/]
+// TESTRESPONSE[s/"total_deduplicated_mapping_size_in_bytes": 1073741824/"total_deduplicated_mapping_size_in_bytes": $body.$_path/]
+
+[discrete]
+===== Retrieving heap size and field mapper overheads
+
+You can use the <<cluster-nodes-stats,Nodes stats API>> to get two relevant metrics
+for each node:
+
+* The size of the heap on each node.
+
+* Any additional estimated heap overhead for the fields per node. This is specific to
+data nodes, where apart from the cluster state field information mentioned above,
+there is additional heap overhead for each mapped field of an index held by the data
+node. For nodes which are not data nodes, this field may be zero.
+
+[source,console]
+----
+GET _nodes/stats?human&filter_path=nodes.*.name,nodes.*.indices.mappings.total_estimated_overhead*,nodes.*.jvm.mem.heap_max*
+----
+// TEST[setup:node]
+
+For each node, this will show you information like in this example output:
+
+[source,console-result]
+----
+{
+  "nodes": {
+    "USpTGYaBSIKbgSUJR2Z9lg": {
+      "name": "node-0",
+      "indices": {
+        "mappings": {
+          "total_estimated_overhead": "1gb",
+          "total_estimated_overhead_in_bytes": 1073741824
+        }
+      },
+      "jvm": {
+        "mem": {
+          "heap_max": "4gb",
+          "heap_max_in_bytes": 4294967296
+        }
+      }
+    }
+  }
+}
+----
+// TESTRESPONSE[s/"USpTGYaBSIKbgSUJR2Z9lg"/\$node_name/]
+// TESTRESPONSE[s/"name": "node-0"/"name": $body.$_path/]
+// TESTRESPONSE[s/"total_estimated_overhead": "1gb"/"total_estimated_overhead": $body.$_path/]
+// TESTRESPONSE[s/"total_estimated_overhead_in_bytes": 1073741824/"total_estimated_overhead_in_bytes": $body.$_path/]
+// TESTRESPONSE[s/"heap_max": "4gb"/"heap_max": $body.$_path/]
+// TESTRESPONSE[s/"heap_max_in_bytes": 4294967296/"heap_max_in_bytes": $body.$_path/]
+
+[discrete]
+===== Consider additional heap overheads
+
+Apart from the two field overhead metrics above, you must additionally allow
+enough heap for {es}'s baseline usage as well as your workload such as indexing,
+searches and aggregations. 0.5GB of extra heap will suffice for many reasonable
+workloads, and you may need even less if your workload is very light while heavy
+workloads may require more.
+
+[discrete]
+===== Example
+
+As an example, consider the outputs above for a data node. The heap of the node
+will need at least:
+
+* 1 GB for the cluster state field information.
+
+* 1 GB for the additional estimated heap overhead for the fields of the data node.
+
+* 0.5 GB of extra heap for other overheads.
+
+Since the node has a 4GB heap max size in the example, it is thus sufficient
+for the total required heap of 2.5GB.
+
+If the heap max size for a node is not sufficient, consider
+<<avoid-unnecessary-fields,avoiding unnecessary fields>>,
+or scaling up the cluster, or redistributing index shards.
+
+Note that the above rules do not necessarily guarantee the performance of
+searches or indexing involving a very high number of indices. You must also
+ensure that your data nodes have adequate resources for your workload and
+that your overall sharding strategy meets all your performance requirements.
+See also <<single-thread-per-shard>> and <<each-shard-has-overhead>>.
 
 [discrete]
 [[avoid-node-hotspots]]

server/src/main/java/org/elasticsearch/rest/action/cat/RestNodesAction.java

@@ -312,7 +312,7 @@ public class RestNodesAction extends AbstractCatAction {
         table.addCell("mappings.total_count", "alias:mtc,mappingsTotalCount;default:false;text-align:right;desc:number of mappings");
         table.addCell(
             "mappings.total_estimated_overhead_in_bytes",
-            "alias:mteoi,mappingsTotalEstimatedOverheadInBytes;default:false;text-align:right;desc:estimated"
+            "alias:mteo,mappingsTotalEstimatedOverheadInBytes;default:false;text-align:right;desc:estimated"
                 + " overhead in bytes of mappings"
         );