[NO ISSUE][EXT] Change default read buffer size to 8K

- user model changes: no
- storage format changes: no
- interface changes: no

Details:
- Change default read buffer size for external datasets to 8K.

Change-Id: I5f6108e07080c0a67f6f9496d90a4b49a14b081b
Reviewed-on: https://asterix-gerrit.ics.uci.edu/c/asterixdb/+/10663
Integration-Tests: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
Tested-by: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
Reviewed-by: Michael Blow <mblow@apache.org>
diff --git a/asterixdb/asterix-external-data/src/main/java/org/apache/asterix/external/util/ExternalDataConstants.java b/asterixdb/asterix-external-data/src/main/java/org/apache/asterix/external/util/ExternalDataConstants.java
index 1e3ab45..fd5b269 100644
--- a/asterixdb/asterix-external-data/src/main/java/org/apache/asterix/external/util/ExternalDataConstants.java
+++ b/asterixdb/asterix-external-data/src/main/java/org/apache/asterix/external/util/ExternalDataConstants.java
@@ -24,6 +24,8 @@
 import java.util.function.LongSupplier;
 import java.util.function.Supplier;
 
+import org.apache.hyracks.util.StorageUtil;
+
 public class ExternalDataConstants {
 
     private ExternalDataConstants() {
@@ -257,7 +259,7 @@
     /**
      * Size default values
      */
-    public static final int DEFAULT_BUFFER_SIZE = 4096;
+    public static final int DEFAULT_BUFFER_SIZE = StorageUtil.getIntSizeInBytes(8, StorageUtil.StorageUnit.KILOBYTE);
     public static final float DEFAULT_BUFFER_INCREMENT_FACTOR = 1.5F;
     public static final int DEFAULT_QUEUE_SIZE = 64;
     public static final int MAX_RECORD_SIZE = 32000000;