UnderReplicatedBlocks處理流程

hdfs處理block副本不足、分佈不合理的流程以下：

 

優先級解釋：

• QUEUE_HIGHEST_PRIORITY: the blocks that must be replicated first. That is blocks with only one copy, or blocks with zero live copies but a copy in a node being decommissioned. These blocks are at risk of loss if the disk or server on which they remain fails.
• QUEUE_VERY_UNDER_REPLICATED: blocks that are very under-replicated compared to their expected values. Currently that means the ratio of the ratio of actual:expected means that there is less than 1:3.These blocks may not be at risk, but they are clearly considered "important". 
• QUEUE_UNDER_REPLICATED: blocks that are also under replicated, and the ratio of actual:expected is good enough that they do not need to go into the QUEUE_VERY_UNDER_REPLICATED queue.
• QUEUE_REPLICAS_BADLY_DISTRIBUTED: there are as least as many copies of a block as required, but the blocks are not adequately distributed. Loss of a rack/switch could take all copies off-line.
• QUEUE_WITH_CORRUPT_BLOCKS This is for blocks that are corrupt and for which there are no-non-corrupt copies (currently) available. The policy here is to keep those corrupt blocks replicated, but give blocks that are not corrupt higher priority.