Vnode

A vnode, short for virtual node, is the kernel’s in‑memory representation of a filesystem object in many BSD-derived operating systems. It provides an abstraction layer between the virtual file system switch (VFS) and the underlying filesystem implementation, allowing the kernel to treat files and directories in a uniform way regardless of the filesystem type.

Each vnode carries common metadata and a reference to its mount point, and it holds a pointer to filesystem‑specific data (often an inode‑like structure) in a field such as v_data. The VFS accesses filesystem objects through a set of vnode operations, or the vop vector, which defines actions like lookup, getattr, setattr, open, close, read, write, create, remove, and readdir. This design enables different filesystems to plug into the same interface.

Vnodes are reference‑counted objects. The kernel increments and decrements references with operations such as vget/vref and vrele. When no references remain, a vnode may be recycled or reclaimed via vgone or the filesystem’s reclaim operation. This lifecycle supports efficient caching of metadata and reduces repeated filesystem work.

The vnode abstraction is distinct from the on‑disk inode; the vnode maps to the filesystem’s internal data structures through v_data, allowing the VFS to operate without knowledge of the underlying format. It is central to BSD VFS design and is used in systems like macOS. In other Unix-like systems that use different VFS models (for example, Linux with inodes and dentries), analogous concepts exist but under different terminology.