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The Pains of Small Files in Hadoop

Shashank Naik and Bhagya Gummalla explain why having a large number of small data files in Hadoop is a problem:

When a NameNode restarts, it must load the filesystem metadata from local disk into memory. This means that if the namenode metadata is large, restarts will be slower. The NameNode must also track changes in the block locations on the cluster. Too many small files can also cause the NameNode to run out of metadata space in memory before the DataNodes run out of data space on disk. The datanodes also report block  changes to the NameNode over the network; more blocks means more changes to report over the network.

More files mean more read requests that need to be served by the NameNode, which may end up clogging NameNode’s capacity to do so. This will increase the RPC queue and processing latency, which will then lead to degraded performance and responsiveness. An overall RPC workload of close to 40K~50K RPCs/s is considered high.

There are a few reasons you want to pack data into large files on Hadoop and this explains them well.