One block is used by Hadoop. This does not mean that memory capacity will be consumed in an equivalent way.
The output when watching HDFS from the Internet is as follows:
filename1 file 48.11 KB 3 128 MB 2012-04-24 18:36
filename2 file 533.24 KB 3 128 MB 2012-04-24 18:36
filename3 file 303.65 KB 3 128 MB 2012-04-24 18:37
You see that the size of each file is smaller than the block size, which is 128 MB. These files are in KB. HDFS capacity is consumed based on the actual file size, but a block is consumed for each file.
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