Every file system implements the notion of block (unit of persistent storage) where a file is split into one or more blocks.
The block size is the smallest unit of data that a file system can store.
If you store a file that's 1k or 4b with a block size of 4kb, it'll take up one block.
There is one request per block
OS File System
The block of a operating system file system is known as a cluster.