HADOOP-9974: document setting mvn heap size

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1524650 13f79535-47bb-0310-9956-ffa450edef68

BUILDING.txt

@@ -4,8 +4,8 @@ Build instructions for Hadoop
 Requirements:
 
 * Unix System
-* JDK 1.6
-* Maven 3.0
+* JDK 1.6+
+* Maven 3.0 or later
 * Findbugs 1.3.9 (if running findbugs)
 * ProtocolBuffer 2.5.0
 * CMake 2.6 or newer (if compiling native code)
@@ -149,6 +149,21 @@ Create a local staging version of the website (in /tmp/hadoop-site)
 
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+Handling out of memory errors in builds
+
+----------------------------------------------------------------------------------
+
+If the build process fails with an out of memory error, you should be able to fix
+it by increasing the memory used by maven -which can be done via the environment
+variable MAVEN_OPTS. 
+
+Here is an example setting to allocate between 256 and 512 MB of heap space to
+Maven
+
+export MAVEN_OPTS="-Xms256m -Xmx512m"
+
+----------------------------------------------------------------------------------
+
 Building on OS/X
 
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