Important Data Structures for a File System Implementation

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There are two important data structures used to represent information about a virtual file system, the vfs structure and the v-node. Each virtual file system has a vfs structure in memory that describes its type, attributes, and position in the file tree hierarchy. Each file object within that virtual file system can be represented by a v-node.

The vfs structure contains the following fields:

Item	Description
`vfs_flag`	Contains the state flags: VFS_DEVMOUNT Indicates whether the virtual file system has a physical mount structure underlying it. VFS_READONLY Indicates whether the virtual file system is mounted read-only.
`vfs_type`	Identifies the type of file system implementation. Possible values for this field are described in the /usr/include/sys/vmount.h file.
`vfs_ops`	Points to the set of operations for the specified file system type.
`vfs_mntdover`	Points to the mounted-over v-node.
`vfs_data`	Points to the file system implementation data. The interpretation of this field is left to the discretion of the file system implementation. For example, the field could be used to point to data in the kernel extension segment or as an offset to another segment.
`vfs_mdata`	Records the user arguments to the mount call that created this virtual file system. This field has a time stamp. The user arguments are retained to implement the mntctl call, which replaces the /etc/mnttab table.