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 format(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 can call the device driver strategy(9E) entry point for the block device on which the file system resides. The file system code can call strategy(9E) several times for a single read(2) or write(2) system call.
The file system code determines the logical device address, or logical block number, for each ordinary file block. A block I/O request is then built in the form of a buf(9S) structure directed at the block device. The driver strategy(9E) entry point then interprets the buf(9S) structure and completes the request.