The logical file system is the level of the file system at which users can request file operations by system call. This level of the file system provides the kernel with a consistent view of what may be multiple physical file systems and multiple file system implementations. As far as the logical file system is concerned, file system types, whether local, remote, or strictly logical, and regardless of implementation, are indistinguishable.
A consistent view of file system implementations is made possible by the virtual file system abstraction. This abstraction specifies the set of file system operations that an implementation must include in order to carry out logical file system requests. Physical file systems can differ in how they implement these predefined operations, but they must present a uniform interface to the logical file system.
Each set of predefined operations implemented constitutes a virtual file system. As such, a single physical file system can appear to the logical file system as one or more separate virtual file systems.
Virtual file system operations are available at the logical file system level through the virtual file system switch. This array contains one entry for each virtual file system, with each entry holding entry point addresses for separate operations. Each file system type has a set of entries in the virtual file system switch.
The logical file system and the virtual file system switch support other operating system file-system access semantics. This does not mean that only other operating system file systems can be supported. It does mean, however, that a file system implementation must be designed to fit into the logical file system model. Operations or information requested from a file system implementation need be performed only to the extent possible.
Logical file system can also refer to the tree of known path names in force while the system is running. A virtual file system that is mounted onto the logical file system tree itself becomes part of that tree. In fact, a single virtual file system can be mounted onto the logical file system tree at multiple points, so that nodes in the virtual subtree have multiple names. Multiple mount points allow maximum flexibility when constructing the logical file system view.
The logical file system is divided into the following components:
Implement services exported to users. System calls that carry out file system requests do the following:
Manage open file table entries and per-process file descriptors. An open file table entry records the authorization of a process's access to a file system object. A user can refer to an open file table entry through a file descriptor or by accessing the virtual memory to which the file was mapped. The logical file system routines are those kernel services, such as fp_ioctl and fp_select, that begin with the prefix fp_.
Provide system calls with a mechanism for local name resolution. Local name resolution allows the logical file system to access multiple file system implementations through a uniform name space.
Understanding Virtual Nodes (V-nodes).
Understanding Generic I-nodes (G-nodes).
Understanding the Virtual File System Interface.
Understanding Data Structures and Header Files for Virtual File Systems.
Configuring a Virtual File System.
List of Virtual File System Operations.
Logical File System Kernel Services.
