[SPARK-29452][WEBUI] Improve Storage tab tooltip

### What changes were proposed in this pull request?
Added Tootips for each column in storage tab of Web UI.

### Why are the changes needed?
Tooltips will help users in understanding columns of storage tabs.

### Does this PR introduce any user-facing change?
Yes

### How was this patch tested?
Manually Tested.

Closes apache#26226 from iRakson/storage_tooltip.

Authored-by: root1 <[email protected]>
Signed-off-by: Sean Owen <[email protected]>

Author: iRakson

core/src/main/scala/org/apache/spark/ui/storage/StoragePage.scala

@@ -25,6 +25,7 @@ import scala.xml.Node
 import org.apache.spark.status.{AppStatusStore, StreamBlockData}
 import org.apache.spark.status.api.v1
 import org.apache.spark.ui._
+import org.apache.spark.ui.storage.ToolTips._
 import org.apache.spark.util.Utils
 
 /** Page showing list of RDD's currently stored in the cluster */
@@ -56,7 +57,8 @@ private[ui] class StoragePage(parent: SparkUITab, store: AppStatusStore) extends
             rddHeader,
             rddRow(request, _: v1.RDDStorageInfo),
             rdds,
-            id = Some("storage-by-rdd-table"))}
+            id = Some("storage-by-rdd-table"),
+            tooltipHeaders = tooltips)}
         </div>
       </div>
     }
@@ -72,6 +74,16 @@ private[ui] class StoragePage(parent: SparkUITab, store: AppStatusStore) extends
     "Size in Memory",
     "Size on Disk")
 
+  /** Tooltips for header fields of the RDD table */
+  val tooltips = Seq(
+    None,
+    Some(RDD_NAME),
+    Some(STORAGE_LEVEL),
+    Some(CACHED_PARTITIONS),
+    Some(FRACTION_CACHED),
+    Some(SIZE_IN_MEMORY),
+    Some(SIZE_ON_DISK))
+
   /** Render an HTML row representing an RDD */
   private def rddRow(request: HttpServletRequest, rdd: v1.RDDStorageInfo): Seq[Node] = {
     // scalastyle:off

core/src/main/scala/org/apache/spark/ui/storage/ToolTips.scala

@@ -0,0 +1,42 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.ui.storage
+
+private[ui] object ToolTips {
+
+  val RDD_NAME =
+    "Name of the persisted RDD"
+
+  val STORAGE_LEVEL =
+    "StorageLevel displays where the persisted RDD is stored, " +
+      "format of the persisted RDD (serialized or de-serialized) and" +
+      "replication factor of the persisted RDD"
+
+  val CACHED_PARTITIONS =
+    "Number of partitions cached"
+
+  val FRACTION_CACHED =
+    "Fraction of total partitions cached"
+
+  val SIZE_IN_MEMORY =
+    "Total size of partitions in memory"
+
+  val SIZE_ON_DISK =
+    "Total size of partitions on the disk"
+}
+

core/src/test/scala/org/apache/spark/ui/storage/StoragePageSuite.scala

@@ -20,6 +20,7 @@ package org.apache.spark.ui.storage
 import javax.servlet.http.HttpServletRequest
 
 import org.mockito.Mockito._
+import scala.xml.{Node, Text}
 
 import org.apache.spark.SparkFunSuite
 import org.apache.spark.status.StreamBlockData
@@ -74,7 +75,21 @@ class StoragePageSuite extends SparkFunSuite {
       "Fraction Cached",
       "Size in Memory",
       "Size on Disk")
-    assert((xmlNodes \\ "th").map(_.text) === headers)
+
+    val headerRow: Seq[Node] = {
+      headers.view.zipWithIndex.map { x =>
+        storagePage.tooltips(x._2) match {
+          case Some(tooltip) =>
+            <th width={""} class={""}>
+              <span data-toggle="tooltip" title={tooltip}>
+                {Text(x._1)}
+              </span>
+            </th>
+          case None => <th width={""} class={""}>{Text(x._1)}</th>
+        }
+      }.toList
+    }
+    assert((xmlNodes \\ "th").map(_.text) === headerRow.map(_.text))
 
     assert((xmlNodes \\ "tr").size === 3)
     assert(((xmlNodes \\ "tr")(0) \\ "td").map(_.text.trim) ===