Set the data block storage size to 128M. When the HDFS client writes a file, when writing a 100M file, how much storage space is actually occupied?

Question

Options

• A128M
• B100M
• C64M
• D50M

• A
  3% (1)
• B
  83% (25)
• C
  3% (1)
• D
  10% (3)

Explanation

Option B is correct because HDFS does not pre-allocate the full block size on disk - a block only consumes the space actually occupied by real data. A 100M file fits within a single 128M block, but that block physically stores only 100M of data, not 128M.

Why the distractors fail:

• A (128M) is the most common wrong answer - it assumes HDFS works like a traditional filesystem where a full block is reserved regardless of content, but HDFS blocks are not fixed-size allocations.
• C (64M) has no basis in HDFS behavior; it may tempt those who confuse block size with some halving rule.
• D (50M) similarly has no relation to HDFS mechanics.

Memory tip: Think of an HDFS block as a maximum capacity container, not a pre-filled box. If your file is 100M and the container holds 128M, you only pay for the 100M you actually put in. The 128M block size only matters for splitting files larger than 128M into multiple blocks - it does not dictate minimum allocation.

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