HDFS的副本摆放策略旨在提高数据可靠性、可用性和网络带宽利用率。通常，第一个副本放置在本地节点，第二个副本放在不同rack的节点，第三个副本放在同一远程rack的另一节点。这减少了跨rack写入流量，提升写性能，同时保证了数据可靠性和读取效率。对于大于3的复制因子，后续副本随机放置，但确保每个rack的副本数量不超过上限。NameNode还考虑了存储类型和策略进行副本放置。
Replica Placement: The First Baby Steps
The placement of replicas is critical to HDFS reliability and performance. Optimizing replica placement distinguishes HDFS from most other distributed file systems. This is a feature that needs lots of tuning and experience. The purpose of a rack-aware replica placement policy is to improve data reliability, availability, and network bandwidth utilization. The current implementation for the replica placement policy is a first effort in this direction. The short-term goals of implementing this policy are to validate it on production systems, learn more about its behavior, and build a foundation to test and research more sophisticated policies.
Large HDFS instances run on a cluster of computers that commonly spread across many racks. Communication between two nodes in different racks has to go through switches. In most cases, network bandwidth between machines in the same rack is greater than network bandwidth betwee
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