cdh5 HA涉及的JournalNodes介紹

官網介紹：

• There must be at least three JournalNode daemons, since edit log modifications must be written to a majority of JournalNodes. This will allow the system to tolerate the failure of a single machine. You can also run more than three JournalNodes, but in order to actually increase the number of failures the system can tolerate, you should run an odd number of JournalNodes, (three, five, seven, etc.) Note that when running with N JournalNodes, the system can tolerate at most (N - 1) / 2 failures and continue to function normally. If the requisite quorum is not available, the NameNode will not format or start, and you will see an error similar to this:

• 12/10/01 17:34:18 WARN namenode.FSEditLog: Unable to determine input streams from QJM to [10.0.1.10:8485, 10.0.1.10:8486, 10.0.1.10:8487]. Skipping.
  java.io.IOException: Timed out waiting 20000ms for a quorum of nodes to respond.

• 選擇奇數個主機（至少三個）做爲JournalNodes。 JournalNodes應託管在與NameNodes具備類似硬件規格的主機上。建議您將JournalNode分別放置在與主動和備用NameNode相同的主機上，以及相似硬件上的第三個JournalNode，例如JobTracker。
• 參考資料鏈接：
• http://www.cloudera.com/documentation/manager/5-0-x/Cloudera-Manager-Managing-Clusters/cm5mc_hdfs_hi_avail.html