diff --git a/hadoop-hdds/common/src/main/java/org/apache/hadoop/hdds/fs/AbstractSpaceUsageSource.java b/hadoop-hdds/common/src/main/java/org/apache/hadoop/hdds/fs/AbstractSpaceUsageSource.java
index 2de8da7d020d..5f606f31b2e8 100644
--- a/hadoop-hdds/common/src/main/java/org/apache/hadoop/hdds/fs/AbstractSpaceUsageSource.java
+++ b/hadoop-hdds/common/src/main/java/org/apache/hadoop/hdds/fs/AbstractSpaceUsageSource.java
@@ -75,9 +75,14 @@ public String toString() {
     return path;
   }
 
+  /**
+   * Get available space, excluding system reserved space.
+   * See {@link File#getUsableSpace()} and {@link File#getFreeSpace()}.
+   * @return available space for data
+   */
   @Override
   public long getAvailable() {
-    return file.getFreeSpace();
+    return file.getUsableSpace();
   }
 
   @Override
diff --git a/hadoop-hdds/common/src/main/java/org/apache/hadoop/hdds/fs/DedicatedDiskSpaceUsage.java b/hadoop-hdds/common/src/main/java/org/apache/hadoop/hdds/fs/DedicatedDiskSpaceUsage.java
index 1d651cce332e..a468b762eda0 100644
--- a/hadoop-hdds/common/src/main/java/org/apache/hadoop/hdds/fs/DedicatedDiskSpaceUsage.java
+++ b/hadoop-hdds/common/src/main/java/org/apache/hadoop/hdds/fs/DedicatedDiskSpaceUsage.java
@@ -45,8 +45,14 @@ public long getUsedSpace() {
     return time(this::calculateUsedSpace, LOG);
   }
 
+  /**
+   * Calculate used space, all free space are counted out.
+   * Some space may not be used by us, but reserved for system usage.
+   * See {@link File#getUsableSpace()} and {@link File#getFreeSpace()}.
+   * @return used space
+   */
   private long calculateUsedSpace() {
-    return getCapacity() - getAvailable();
+    return getCapacity() - getFile().getFreeSpace();
   }
 
 }