diff --git a/CHANGES.txt b/CHANGES.txt
index 135f59e5e3..d2292e1f9b 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -183,6 +183,7 @@
  * Add the ability to disable bulk loading of SSTables (CASSANDRA-18781)
  * Clean up obsolete functions and simplify cql_version handling in cqlsh (CASSANDRA-18787)
 Merged from 5.0:
+ * Don't allocate in ThreadLocalReadAheadBuffer#close() (CASSANDRA-20551)
  * Ensure RowFilter#isMutableIntersection() properly evaluates numeric ranges on a single column (CASSANDRA-20566)
  * SAI marks an index as non-empty when a partial partition/row modifications is flushed due to repair (CASSANDRA-20567)
  * SAI fails queries when multiple columns exist and a non-indexed column is a composite with a map (CASSANDRA-19891)
diff --git a/src/java/org/apache/cassandra/io/util/ThreadLocalReadAheadBuffer.java b/src/java/org/apache/cassandra/io/util/ThreadLocalReadAheadBuffer.java
index 824acaa8d8..bc92407bef 100644
--- a/src/java/org/apache/cassandra/io/util/ThreadLocalReadAheadBuffer.java
+++ b/src/java/org/apache/cassandra/io/util/ThreadLocalReadAheadBuffer.java
@@ -131,18 +131,21 @@ public final class ThreadLocalReadAheadBuffer
 
     public void clear(boolean deallocate)
     {
-        Block block = getBlock();
+        // avoid calling block() here to reduce unintended allocations
+        Block block = blockMap.get().get(channel.filePath());
+        if (block == null)
+            return;
+
         block.index = -1;
+        if (block.buffer == null)
+            return;
 
         ByteBuffer blockBuffer = block.buffer;
-        if (blockBuffer != null)
+        blockBuffer.clear();
+        if (deallocate)
         {
-            blockBuffer.clear();
-            if (deallocate)
-            {
-                FileUtils.clean(blockBuffer);
-                block.buffer = null;
-            }
+            FileUtils.clean(blockBuffer);
+            block.buffer = null;
         }
     }