Fixed block size is a feature of a computer file system that determines the maximum number of bytes or characters that can be stored in a single data block or cluster of the file. This size, which is specific to a system and remains the same regardless of the amount of data stored in it, is called fixed block size.
In a nutshell, a data block or cluster is essentially a unit of storage, and fixed block size sets the capability of how much data can be stored in that unit.
The primary advantages of a fixed block size are that all file systems have a predictable allocation and storage approach. This makes them more efficient for reads and writes, and oftentimes reduces the number of disk reads and writes required for a given operation.
Fixed block size can also limit the size of a file. This means that even if you have lots of free disk space, having a large file may cause the file to be split into multiple blocks, which reduces performance and increases overhead.
At the same time, fixed block size allows for faster file access since all data can be found within a certain size of block without having to search through a lot of blocks to locate all the data.
For a deeper understanding, you can check out this article: https://www.lifewire.com/fixed-block-size-2625881.
