A file system is a tree-structured hierarchy of directories and files. Some file systems, such as the UNIX File System (UFS), reside on block-oriented devices. File systems are created by mkfs(1M) and newfs(1M).
When an application issues a read(2) or write(2) system call to an ordinary file on the UFS file system, the file system may call the device driver strategy(9E) entry point for the block device on which the file resides. The file system code may call strategy(9E) several times for a single read(2) or write(2) system call.
It is the file system code that determines the logical device address, or logical block number, for each block and builds a block I/O request in the form of a buf(9S) structure. The driver strategy(9E) entry point then interprets the buf(9S) structure and completes the request.