private boolean checkFileSystem() {
    if (this.fsOk) {
      try {
        FSUtils.checkFileSystemAvailable(this.fs);
        FSUtils.checkDfsSafeMode(this.conf);
      } catch (IOException e) {
        services.abort("Shutting down HBase cluster: file system not available", e);
        this.fsOk = false;
      }
    }
    return this.fsOk;
  }
Checks to see if the file system is still accessible. If not, sets closed
@return false if file system is not available
hbase-server/src/main/java/org/apache/hadoop/hbase/master/MasterWalManager.java