In the traditional UNIX file-system model, data is stored on disk in physical…

Concept

A physical disk block is the smallest hardware-addressable unit that a disk controller reads or writes. It is distinct from a file system’s logical allocation block, which may combine several physical blocks.

Application

Ritchie and Thompson’s original UNIX description states that a file-system device is divided into 512-byte blocks. Among the listed sizes, 512 bytes is therefore the physical-block value intended by the question.

Contrast

• 1024 bytes equals one kibibyte and is associated with some logical-block or file-reporting conventions.

• 2048 bytes is a common data-sector size for optical media rather than the traditional magnetic-disk sector.

• 256 bytes is smaller than the historical disk sector assumed in this UNIX convention.

• 512 bytes is the historical sector-sized physical unit used in the traditional UNIX convention.

Cross-check

Historical UNIX disk-usage reporting also uses 512-byte units. That independent convention is consistent with the physical-sector interpretation of the stem.

Result

The intended physical block size is 512 bytes.