[SPARK-26700][CORE] enable fetch-big-block-to-disk by default

## What changes were proposed in this pull request?

This is a followup of #16989

The fetch-big-block-to-disk feature is disabled by default, because it's not compatible with external shuffle service prior to Spark 2.2. The client sends stream request to fetch block chunks, and old shuffle service can't support it.

After 2 years, Spark 2.2 has EOL, and now it's safe to turn on this feature by default

## How was this patch tested?

existing tests

Closes #23625 from cloud-fan/minor.

Authored-by: Wenchen Fan <wenchen@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

core/src/main/scala/org/apache/spark/internal/config/package.scala

@@ -699,17 +699,19 @@ package object config {
   private[spark] val MAX_REMOTE_BLOCK_SIZE_FETCH_TO_MEM =
     ConfigBuilder("spark.maxRemoteBlockSizeFetchToMem")
       .doc("Remote block will be fetched to disk when size of the block is above this threshold " +
-        "in bytes. This is to avoid a giant request takes too much memory. We can enable this " +
-        "config by setting a specific value(e.g. 200m). Note this configuration will affect " +
-        "both shuffle fetch and block manager remote block fetch. For users who enabled " +
-        "external shuffle service, this feature can only be worked when external shuffle" +
-        "service is newer than Spark 2.2.")
+        "in bytes. This is to avoid a giant request takes too much memory. Note this " +
+        "configuration will affect both shuffle fetch and block manager remote block fetch. " +
+        "For users who enabled external shuffle service, this feature can only work when " +
+        "external shuffle service is at least 2.3.0.")
       .bytesConf(ByteUnit.BYTE)
       // fetch-to-mem is guaranteed to fail if the message is bigger than 2 GB, so we might
       // as well use fetch-to-disk in that case.  The message includes some metadata in addition
       // to the block data itself (in particular UploadBlock has a lot of metadata), so we leave
       // extra room.
-      .createWithDefault(Int.MaxValue - 512)
+      .checkValue(
+        _ <= Int.MaxValue - 512,
+        "maxRemoteBlockSizeFetchToMem cannot be larger than (Int.MaxValue - 512) bytes.")
+      .createWithDefaultString("200m")
 
   private[spark] val TASK_METRICS_TRACK_UPDATED_BLOCK_STATUSES =
     ConfigBuilder("spark.taskMetrics.trackUpdatedBlockStatuses")

docs/configuration.md

@@ -626,19 +626,6 @@ Apart from these, the following properties are also available, and may be useful
     You can mitigate this issue by setting it to a lower value.
   </td>
 </tr>
-<tr>
-  <td><code>spark.maxRemoteBlockSizeFetchToMem</code></td>
-  <td>Int.MaxValue - 512</td>
-  <td>
-    The remote block will be fetched to disk when size of the block is above this threshold in bytes.
-    This is to avoid a giant request that takes too much memory.  By default, this is only enabled
-    for blocks > 2GB, as those cannot be fetched directly into memory, no matter what resources are
-    available.  But it can be turned down to a much lower value (eg. 200m) to avoid using too much
-    memory on smaller blocks as well. Note this configuration will affect both shuffle fetch
-    and block manager remote block fetch. For users who enabled external shuffle service,
-    this feature can only be used when external shuffle service is newer than Spark 2.2.
-  </td>
-</tr>
 <tr>
   <td><code>spark.shuffle.compress</code></td>
   <td>true</td>
@@ -1519,6 +1506,17 @@ Apart from these, the following properties are also available, and may be useful
     you can set larger value.
   </td>
 </tr>
+<tr>
+  <td><code>spark.maxRemoteBlockSizeFetchToMem</code></td>
+  <td>200m</td>
+  <td>
+    Remote block will be fetched to disk when size of the block is above this threshold
+    in bytes. This is to avoid a giant request takes too much memory. Note this
+    configuration will affect both shuffle fetch and block manager remote block fetch.
+    For users who enabled external shuffle service, this feature can only work when
+    external shuffle service is at least 2.3.0.
+  </td>
+</tr>
 </table>
 
 ### Scheduling