hadoop-hdfs-safemode

在啓動過程中，NAMENODE從FSIMAGE和編輯日誌文件加載文件系統狀態。然後，它等待數據塊報告它們的塊，以便即使在集羣中已經存在足夠的副本，也不會過早地開始複製這些塊。在此期間，NAMENODE停留在Safemode。NAMENODE的安全模式本質上是HDFS集羣的只讀模式，它不允許對文件系統或塊進行任何修改。通常，NAMENODE在數據報已經報告大多數文件系統塊可用之後自動離開SaveMode。如果需要，HDFS可以使用bin/hdfs dfsadmin -safemode命令顯式地放置在SaaMaMeDE中。NAMENODE首頁顯示Safemode是否打開或關閉。更詳細的描述和配置被保持爲JavaDoc用於setSafeMode().

相關操作

public interface SafeMode {
  /**
   * Check safe mode conditions.
   * If the corresponding conditions are satisfied,
   * trigger the system to enter/leave safe mode.
   */
  public void checkSafeMode();

  /** Is the system in safe mode? */
  public boolean isInSafeMode();

  /**
   * Is the system in startup safe mode, i.e. the system is starting up with
   * safe mode turned on automatically?
   */
  public boolean isInStartupSafeMode();

  /** Check whether replication queues are being populated. */
  public boolean isPopulatingReplQueues();

  /**
   * Increment number of blocks that reached minimal replication.
   * @param replication current replication 
   */
  public void incrementSafeBlockCount(int replication);

  /** Decrement number of blocks that reached minimal replication. */
  public void decrementSafeBlockCount(Block b);
}