Hadoop进入安全模式有三个条件具体源码如下
private boolean needEnter() {
// TODO-ZH DataNode汇报block状态为complete状态数量
return (threshold != 0 && blockSafe < blockThreshold) ||
// TODO-ZH 默认存活DataNode节点小于 datanodeThreshold（默认为0）
(datanodeThreshold != 0 && getNumLiveDataNodes() < datanodeThreshold) ||
// TODO-ZH 元数据存储磁盘空间是不充足
(!nameNodeHasResourcesAvailable());
}
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条件一：threshold != 0 && blockSafe < blockThreshold；DataNode汇报过来的complete的block个数占block总数是否达到设定阈值占比（默认0.99）,若未达到此阈值则进入安全模式，计算complete状态数量如下
// TODO-ZH 计算阈值 this.blockThreshold = (int) (blockTotal * threshold);上述计算过程中blockTotal为Hadoop集群中block块总个数，threshold为参数配置阈值，默认配置阈值为0.99f
this.threshold = conf.getFloat(DFS_NAMENODE_SAFEMODE_THRESHOLD_PCT_KEY, DFS_NAMENODE_SAFEMODE_THRESHOLD_PCT_DEFAULT);参数配置如下：
// block设置处于安全模式时的阈值占比，默认值为0.999f public static final String DFS_NAMENODE_SAFEMODE_THRESHOLD_PCT_KEY = "dfs.namenode.safemode.threshold-pct"; // block设置处于安全模式时的阈值占比的默认值 public static final float DFS_NAMENODE_SAFEMODE_THRESHOLD_PCT_DEFAULT = 0.999f;根据以上代码和配置可得知，只要DataNode汇报到NameNode的block块状态为complete占总block块占比小于0.99则Hadoop集群处于安全模式。
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条件二：datanodeThreshold != 0 && getNumLiveDataNodes() < datanodeThreshold；如果DataNode存活个数小于一定个数（默认为0）时则进入安全模式
datanodeThreshold通过参数配置获取，获取配置如下
this.datanodeThreshold = conf.getInt( DFS_NAMENODE_SAFEMODE_MIN_DATANODES_KEY, DFS_NAMENODE_SAFEMODE_MIN_DATANODES_DEFAULT);参数配置具体配置值如下
public static final String DFS_NAMENODE_SAFEMODE_MIN_DATANODES_KEY = "dfs.namenode.safemode.min.datanodes"; public static final int DFS_NAMENODE_SAFEMODE_MIN_DATANODES_DEFAULT = 0;getNumLiveDataNodes() 为获取存活DataNode存活数量，具体实现代码如下
public int getNumLiveDataNodes() { return getBlockManager().getDatanodeManager().getNumLiveDataNodes(); } public int getNumLiveDataNodes() { int numLive = 0; synchronized (datanodeMap) { for(DatanodeDescriptor dn : datanodeMap.values()) { if (!isDatanodeDead(dn) ) { numLive++; } } } return numLive; } /** Is the datanode dead? */ boolean isDatanodeDead(DatanodeDescriptor node) { return (node.getLastUpdateMonotonic() < (monotonicNow() - heartbeatExpireInterval)); }判定一个DataNode节点是否存活，是根据上次更新汇报心跳时间距现在时间超过一定时间，则判定此DataNode节点为不存活状态（即dead状态），判定逻辑如下：
/** Is the datanode dead? */ boolean isDatanodeDead(DatanodeDescriptor node) { return (node.getLastUpdateMonotonic() < (monotonicNow() - heartbeatExpireInterval)); }heartbeatExpireInterval为配置参数，即设定距离上次心跳更新时间超过此时间则判定此DataNode为不存活状态，设定计算如下
this.heartbeatExpireInterval = 2 * heartbeatRecheckInterval + 10 * 1000 * heartbeatIntervalSeconds; final int heartbeatRecheckInterval = conf.getInt( DFSConfigKeys.DFS_NAMENODE_HEARTBEAT_RECHECK_INTERVAL_KEY, DFSConfigKeys.DFS_NAMENODE_HEARTBEAT_RECHECK_INTERVAL_DEFAULT); // 5minutes public static final String DFS_NAMENODE_HEARTBEAT_RECHECK_INTERVAL_KEY = "dfs.namenode.heartbeat.recheck-interval"; public static final int DFS_NAMENODE_HEARTBEAT_RECHECK_INTERVAL_DEFAULT = 5*60*1000; final long heartbeatIntervalSeconds = conf.getLong( DFSConfigKeys.DFS_HEARTBEAT_INTERVAL_KEY, DFSConfigKeys.DFS_HEARTBEAT_INTERVAL_DEFAULT); public static final String DFS_HEARTBEAT_INTERVAL_KEY = "dfs.heartbeat.interval"; public static final long DFS_HEARTBEAT_INTERVAL_DEFAULT = 3;有上述参数和计算代码可得知，若DataNode节点超过10分钟30秒后，NameNode未接收到DataNode心跳则判定此DataNode为不存活状态。
综上代码分析可知，当DataNode节点存活个数小于0时，Hadoop集群进入安全模式。
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条件三：nameNodeHasResourcesAvailable()；检查NameNode写元数据磁盘目录空间是否大于设定阈值（默认100M），若磁盘目录空间小于设定阈值则NameNodeHasResourceAvailable = false，此时HDFS进入安全模式，具体代码如下：
hasResourcesAvailable = nnResourceChecker.hasAvailableDiskSpace();hasAvailableDiskSpace()方法时获取存储元数据存盘目录是否有足够存储空间
return NameNodeResourcePolicy.areResourcesAvailable(volumes.values(), minimumRedundantVolumes);volumes里存放的就是需要检查的目录集合，具体设置如下：
private void addDirToCheck(URI directoryToCheck, boolean required) throws IOException { File dir = new File(directoryToCheck.getPath()); if (!dir.exists()) { throw new IOException("Missing directory "+dir.getAbsolutePath()); } // 一个目录就是一个checkVolume对象 CheckedVolume newVolume = new CheckedVolume(dir, required); CheckedVolume volume = volumes.get(newVolume.getVolume()); if (volume == null || !volume.isRequired()) { //volumes里面就会有多个目录 volumes.put(newVolume.getVolume(), newVolume); } }判定设定存储元数据存储目录是否有足够存储空间逻辑如下
static boolean areResourcesAvailable( Collection<? extends CheckableNameNodeResource> resources, int minimumRedundantResources) { // TODO: workaround: // - during startup, if there are no edits dirs on disk, then there is // a call to areResourcesAvailable() with no dirs at all, which was // previously causing the NN to enter safemode if (resources.isEmpty()) { return true; } int requiredResourceCount = 0; int redundantResourceCount = 0; int disabledRedundantResourceCount = 0; for (CheckableNameNodeResource resource : resources) { if (!resource.isRequired()) { redundantResourceCount++; if (!resource.isResourceAvailable()) { disabledRedundantResourceCount++; } } else { requiredResourceCount++; // TODO-ZH 判断磁盘资源是否充足 if (!resource.isResourceAvailable()) { // Short circuit - a required resource is not available. return false; } } } if (redundantResourceCount == 0) { // If there are no redundant resources, return true if there are any // required resources available. return requiredResourceCount > 0; } else { return redundantResourceCount - disabledRedundantResourceCount >= minimumRedundantResources; } } }判定逻辑
public boolean isResourceAvailable() { // TODO-ZH 获取当前磁盘目录空间大小 long availableSpace = df.getAvailable(); if (LOG.isDebugEnabled()) { LOG.debug("Space available on volume '" + volume + "' is " + availableSpace); } // TODO-ZH 如果磁盘空间大小小于100M则返回false if (availableSpace < duReserved) { LOG.warn("Space available on volume '" + volume + "' is " + availableSpace + ", which is below the configured reserved amount " + duReserved); return false; } else { return true; } }设定阈值获取如下
// TODO-ZH 检查资源是否充足阈值 duReserved = conf.getLong(DFSConfigKeys.DFS_NAMENODE_DU_RESERVED_KEY, DFSConfigKeys.DFS_NAMENODE_DU_RESERVED_DEFAULT);设定阈值如下
// 检查存储元数据磁盘低于多少的一个阈值，默认100M public static final String DFS_NAMENODE_DU_RESERVED_KEY = "dfs.namenode.resource.du.reserved"; // 元数据存储磁盘低于多少一个阈值默认值100M public static final long DFS_NAMENODE_DU_RESERVED_DEFAULT = 1024 * 1024 * 100; // 100 MB根据上述参数设定和代码计算可知，当存储元数据磁盘目录小于100M时，Hadoop集群进入安全模式