26 August 2015

How Hadoop distributed file system HDFS really works

What is HDFS in Hadoop - HDFS is optimized to support high-streaming read performance, and this comes at the expense of random seek performance. This means that if an application is reading from HDFS, it should avoid (or at least minimize) the number of seeks. Sequential reads are the preferred way to access HDFS files.

HDFS supports only a limited set of operations on files — writes, deletes, appends, and reads, but not updates. It assumes that the data will be written to the HDFS once, and then read multiple times.

HDFS does not provide a mechanism for local caching of data. The overhead of caching is large enough that data should simply be re-read from the source, which is not a problem for applications that are mostly doing sequential reads of large-sized data files.

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