HBASE-27805 The chunk created by mslab may cause memory fragement and lead to fullgc

hbase-common/src/main/resources/hbase-default.xml

@@ -755,8 +755,12 @@ possible configurations would overwhelm and obscure the important.
   </property>
   <property>
     <name>hbase.hregion.memstore.mslab.chunksize</name>
-    <value>2097152</value>
-    <description>The maximum byte size of a chunk in the MemStoreLAB. Unit: bytes</description>
+    <value>2096128</value>
+    <description>
+      The maximum byte size of a chunk in the MemStoreLAB.
+      Use 2047k as the default because 2m may cause memory fragmentation in some cases.
+      Unit: bytes
+    </description>
   </property>
   <property>
     <name>hbase.regionserver.offheap.global.memstore.size</name>

hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/MemStoreLAB.java

@@ -51,7 +51,7 @@ public interface MemStoreLAB {
   String MSLAB_CLASS_NAME = "hbase.regionserver.mslab.class";
 
   String CHUNK_SIZE_KEY = "hbase.hregion.memstore.mslab.chunksize";
-  int CHUNK_SIZE_DEFAULT = 2048 * 1024;
+  int CHUNK_SIZE_DEFAULT = 2047 * 1024;
   String INDEX_CHUNK_SIZE_PERCENTAGE_KEY = "hbase.hregion.memstore.mslab.indexchunksize.percent";
   float INDEX_CHUNK_SIZE_PERCENTAGE_DEFAULT = 0.1f;
   String MAX_ALLOC_KEY = "hbase.hregion.memstore.mslab.max.allocation";

src/main/asciidoc/_chapters/offheap_read_write.adoc

@@ -213,7 +213,8 @@ of off-heap memory that should be used by MSLAB (e.g. `25` would result in 25MB
 MaxDirectMemorySize property (see <<hbase.offheapsize>> for more on _HBASE_OFFHEAPSIZE_). The default value of
 `hbase.regionserver.offheap.global.memstore.size` is 0 which means MSLAB uses onheap, not offheap, chunks by default.
 
-`hbase.hregion.memstore.mslab.chunksize` controls the size of each off-heap chunk. Default is `2097152` (2MB).
+`hbase.hregion.memstore.mslab.chunksize` controls the size of each off-heap chunk. Default is `2096128` (that's 2047KB, why not 2MB?
+see link:https://issues.apache.org/jira/browse/HBASE-27805[HBASE-27805] for more detail).
 
 When a Cell is added to a MemStore, the bytes for that Cell are copied into these off-heap buffers (if `hbase.regionserver.offheap.global.memstore.size` is non-zero)
 and a Cell POJO will refer to this memory area. This can greatly reduce the on-heap occupancy of the MemStores and reduce the total heap utilization for RegionServers