重启hdfs集群的时候，报大量的gc问题。

问题现象：

2019-03-11 12:30:52,174 INFO org.apache.hadoop.util.JvmPauseMonitor: Detected pause in JVM or host machine (eg GC): pause of approximately 7653ms
GC pool 'ConcurrentMarkSweep' had collection(s): count=1 time=7692ms
2019-03-11 12:31:00,573 INFO org.apache.hadoop.util.JvmPauseMonitor: Detected pause in JVM or host machine (eg GC): pause of approximately 7899ms
GC pool 'ConcurrentMarkSweep' had collection(s): count=1 time=7951ms
2019-03-11 12:31:08,952 INFO org.apache.hadoop.util.JvmPauseMonitor: Detected pause in JVM or host machine (eg GC): pause of approximately 7878ms
GC pool 'ConcurrentMarkSweep' had collection(s): count=1 time=7937ms
2019-03-11 12:31:17,405 INFO org.apache.hadoop.util.JvmPauseMonitor: Detected pause in JVM or host machine (eg GC): pause of approximately 7951ms
GC pool 'ConcurrentMarkSweep' had collection(s): count=1 time=8037ms
2019-03-11 12:31:26,611 INFO org.apache.hadoop.util.JvmPauseMonitor: Detected pause in JVM or host machine (eg GC): pause of approximately 8705ms
GC pool 'ConcurrentMarkSweep' had collection(s): count=1 time=8835ms
2019-03-11 12:31:35,009 INFO org.apache.hadoop.util.JvmPauseMonitor: Detected pause in JVM or host machine (eg GC): pause of approximately 7897ms
GC pool 'ConcurrentMarkSweep' had collection(s): count=1 time=8083ms
2019-03-11 12:31:43,806 INFO org.apache.hadoop.util.JvmPauseMonitor: Detected pause in JVM or host machine (eg GC): pause of approximately 8296ms
GC pool 'ConcurrentMarkSweep' had collection(s): count=1 time=8416ms
2019-03-11 12:31:52,317 INFO org.apache.hadoop.util.JvmPauseMonitor: Detected pause in JVM or host machine (eg GC): pause of approximately 8010ms
GC pool 'ConcurrentMarkSweep' had collection(s): count=1 time=8163ms
2019-03-11 12:32:00,680 INFO org.apache.hadoop.util.JvmPauseMonitor: Detected pause in JVM or host machine (eg GC): pause of approximately 7862ms

　　

gc一段时间后出现：

2019-03-11 12:27:15,820 ERROR org.apache.hadoop.hdfs.server.namenode.NameNode: Failed to start namenode.
java.lang.OutOfMemoryError: Java heap space
        at java.lang.StringCoding$StringEncoder.encode(StringCoding.java:300)
        at java.lang.StringCoding.encode(StringCoding.java:344)
        at java.lang.String.getBytes(String.java:918)
        at java.io.UnixFileSystem.getBooleanAttributes0(Native Method)
        at java.io.UnixFileSystem.getBooleanAttributes(UnixFileSystem.java:242)
        at java.io.File.exists(File.java:819)
        at sun.misc.URLClassPath$FileLoader.getResource(URLClassPath.java:1282)
        at sun.misc.URLClassPath.getResource(URLClassPath.java:239)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:365)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:362)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:361)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
        at org.apache.hadoop.hdfs.server.namenode.JournalSet.close(JournalSet.java:244)
        at org.apache.hadoop.hdfs.server.namenode.FSEditLog.close(FSEditLog.java:400)
        at org.apache.hadoop.hdfs.server.namenode.FSEditLogAsync.close(FSEditLogAsync.java:112)
        at org.apache.hadoop.hdfs.server.namenode.FSImage.close(FSImage.java:1408)
        at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.loadFSImage(FSNamesystem.java:1079)
        at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.loadFromDisk(FSNamesystem.java:681)
        at org.apache.hadoop.hdfs.server.namenode.NameNode.loadNamesystem(NameNode.java:666)
        at org.apache.hadoop.hdfs.server.namenode.NameNode.initialize(NameNode.java:728)
        at org.apache.hadoop.hdfs.server.namenode.NameNode.<init>(NameNode.java:953)
        at org.apache.hadoop.hdfs.server.namenode.NameNode.<init>(NameNode.java:932)
        at org.apache.hadoop.hdfs.server.namenode.NameNode.createNameNode(NameNode.java:1673)
        at org.apache.hadoop.hdfs.server.namenode.NameNode.main(NameNode.java:1741)
2019-03-11 12:27:15,827 INFO org.apache.hadoop.util.ExitUtil: Exiting with status 1: java.lang.OutOfMemoryError: Java heap space
2019-03-11 12:27:15,830 INFO org.apache.hadoop.hdfs.server.namenode.NameNode: SHUTDOWN_MSG:

　　

或者出现下面的错误：

2019-03-11 11:09:16,124 ERROR org.apache.hadoop.hdfs.server.namenode.NameNode: Failed to start namenode.
java.lang.OutOfMemoryError: GC overhead limit exceeded
        at com.google.protobuf.CodedInputStream.<init>(CodedInputStream.java:573)
        at com.google.protobuf.CodedInputStream.newInstance(CodedInputStream.java:55)
        at com.google.protobuf.AbstractParser.parsePartialFrom(AbstractParser.java:199)
        at com.google.protobuf.AbstractParser.parsePartialDelimitedFrom(AbstractParser.java:241)
        at com.google.protobuf.AbstractParser.parseDelimitedFrom(AbstractParser.java:253)
        at com.google.protobuf.AbstractParser.parseDelimitedFrom(AbstractParser.java:259)
        at com.google.protobuf.AbstractParser.parseDelimitedFrom(AbstractParser.java:49)
        at org.apache.hadoop.hdfs.server.namenode.FsImageProto$INodeSection$INode.parseDelimitedFrom(FsImageProto.java:10867)
        at org.apache.hadoop.hdfs.server.namenode.FSImageFormatPBINode$Loader.loadINodeSection(FSImageFormatPBINode.java:233)
        at org.apache.hadoop.hdfs.server.namenode.FSImageFormatProtobuf$Loader.loadInternal(FSImageFormatProtobuf.java:250)
        at org.apache.hadoop.hdfs.server.namenode.FSImageFormatProtobuf$Loader.load(FSImageFormatProtobuf.java:176)
        at org.apache.hadoop.hdfs.server.namenode.FSImageFormat$LoaderDelegator.load(FSImageFormat.java:226)
        at org.apache.hadoop.hdfs.server.namenode.FSImage.loadFSImage(FSImage.java:937)
        at org.apache.hadoop.hdfs.server.namenode.FSImage.loadFSImage(FSImage.java:921)
        at org.apache.hadoop.hdfs.server.namenode.FSImage.loadFSImageFile(FSImage.java:794)
        at org.apache.hadoop.hdfs.server.namenode.FSImage.loadFSImage(FSImage.java:724)
        at org.apache.hadoop.hdfs.server.namenode.FSImage.recoverTransitionRead(FSImage.java:322)
        at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.loadFSImage(FSNamesystem.java:1052)
        at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.loadFromDisk(FSNamesystem.java:681)
        at org.apache.hadoop.hdfs.server.namenode.NameNode.loadNamesystem(NameNode.java:666)
        at org.apache.hadoop.hdfs.server.namenode.NameNode.initialize(NameNode.java:728)
        at org.apache.hadoop.hdfs.server.namenode.NameNode.<init>(NameNode.java:953)
        at org.apache.hadoop.hdfs.server.namenode.NameNode.<init>(NameNode.java:932)
        at org.apache.hadoop.hdfs.server.namenode.NameNode.createNameNode(NameNode.java:1673)
        at org.apache.hadoop.hdfs.server.namenode.NameNode.main(NameNode.java:1741)
2019-03-11 11:09:16,127 INFO org.apache.hadoop.util.ExitUtil: Exiting with status 1: java.lang.OutOfMemoryError: GC overhead limit exceeded

　　

解决：

打开hadoop-env.sh文件，找到HADOOP_HEAPSIZE=  和HADOOP_NAMENODE_INIT_HEAPSIZE=  调整这两个参数，具体调整多少，视情况而定，默认是1000m，也就是一个g，我这里调整如下：

export HADOOP_HEAPSIZE=32000
export HADOOP_NAMENODE_INIT_HEAPSIZE=16000                  这两个参数去掉前面的#号，两台namenode节点都要调整

　　

接着重新启动hdfs，如果还不行，打开hadoop-env.sh文件，找到HADOOP_NAMENODE_OPTS

export HADOOP_NAMENODE_OPTS="-Dhadoop.security.logger=${HADOOP_SECURITY_LOGGER:-INFO,RFAS} -Dhdfs.audit.logger=${HDFS_AUDIT_LOGGER:-INFO,NullAppender}  $HADOOP_NAMENODE_OPTS"    ----这是系统默认值
调整如下：
export HADOOP_NAMENODE_OPTS="-Dhadoop.security.logger=${HADOOP_SECURITY_LOGGER:-INFO,RFAS} -Dhdfs.audit.logger=${HDFS_AUDIT_LOGGER:-INFO,NullAppender}  -Xms6000m -Xmx6000m -XX:+UseCompressedOops -XX:+UseParNewGC -XX:+UseConcMarkSweepGC -XX:+CMSClassUnloadingEnabled -XX:+UseCMSCompactAtFullCollection -XX:CMSFullGCsBeforeCompaction=0 -XX:+CMSParallelRemarkEnabled -XX:+DisableExplicitGC -XX:+UseCMSInitiatingOccupancyOnly -XX:CMSInitiatingOccupancyFraction=75 -XX:SoftRefLRUPolicyMSPerMB=0 $HADOOP_NAMENODE_OPTS"

　　

接着重新启动hdfs，如果还是报上面的错误，那就继续调大上面

HADOOP_HEAPSIZE和

HADOOP_NAMENODE_INIT_HEAPSIZE  的值