大家说让我比较一下AvatarNode HA实现和Hadoop with DRBD and Linux HA.这两种方法都是需要主节点写事务日志到一个共享的硬件。不同点是，Standby AvatarNode是一个热备而DRBD-LinuxHA是一种冷备份。AvatarNode对于5亿文件的failover时间是1分钟，但是DRBD-LinuxHA可能需要一个小时。AvatarNode可以热备的原因就是它封装了一个NameNode的实例并且这个实例会从DataNode接受信息，这使得metadata的状态是最新的。
代码参见HDFS-976. (前提条件HDFS-966)
2. 最新代码参见：
https://github.com/facebook/hadoop-20-warehouse/tree/master/src/contrib/highavailability
3. Dhruba回答的部分问题
Dhruba关于NameNode启动过程所需要时间的一个参考：
our clyster has 2000 nodes. The fsimage is about 12 GB(70 million files and directories). The cluster has a total of around 90 milliosn blocks. It takes about 6 minutes to read and process the fsimage file. Then it takes another 35 minutes to process block reports from all datanodes.
AvatarNode和ZooKeeper整合(解决VIP问题)
We are in the process of open-sourcing the AvatarNode integration with zookeeper. We will post this patch as part of HDFS-976 very soon.
有关NFS延迟
I agree that NFS is not the fastest way to read/access that transaction log. However, it is not really NFS but rather the NFS implementation that could be an issue. We use a NetApp NFS Filer and the filer's uptime and latencies are hard to beat! Also, we depend on NFS close-to-open cache coherency semantics: