Inode vs Vnode Difference

Question

I had some doubts regarding an Inode vs a Vnode. As far as my understanding goes, inode is the representation of a file that is used by the Virtual File System. Whereas vnodes are file system specific. Is this correct?

Also, I am confused whether inode is a kernel data structure i.e whether it is an in-memory data structure or a data structure that exists on blocks in an actual disk?

Answer 1

To add something to this: The best explanation I could find for a vnode is here on the FreeBSD docs. For the more academically minded there is also the original paper that introduced the concept, [Vnodes: An Architecture for Multiple File System Types in Sun UNIX], which provides a much more in depth resource.

That said vnodes were originally created for FreeBSD because of the proliferation of different types of filesystems that needed to be used like UFS, NFS, etc. Vnodes are meant to provide an abstraction across all possible filesystems so that the OS can interface with them and so that kernel functions don't have to specifically support every filesystem under the sun; they only have to have knowledge of how to interact with the file's vnode.

Back to your original question vnodes, as @Allen Luce mentioned, are in memory abstractions and are not filesystem specific. They can be used interchangeably in UFS, ext4, and whatever else. In contrast, inodes are stored on disk and are specific to the exact filesystem being used. inodes contain metadata about the file like size, owner, pointers to the block addresses among other things. Vnodes contain some data about the file but only attributes that do not change over the file's lifetime so an inode would be the location to reference if you wanted the most information possible about a file. If you're more interested in inodes I would suggest you check out wikipedia which has a good article on them.

Answer 2

Typically (like for Linux and BSD mainstream filesystems), an inode is first an on-disk structure that describes the storage of a file in terms of that disk (usually in blocks). A vnode is an in-memory structure that abstracts much of what an inode is (an inode can be one of its data fields) but also captures things like operations on files, locks, etc. This lets it support non-inode based filesystems, in particular networked filesystems.

Answer 3

It depends on your OS. On a Linux system for instance there is no v-node, only a generic i-node struct inode which although is conceptually similar to a v-node is implemented differently.

For BSD-derived and UNIX kernels, the v-node points to an i-node structure specific to the filesystem, along with some additional information including pointers to functions that operate on the file and metadata not included in the inode. A major difference is the inode is files system while the vnode is not. (In Linux as mentioned above there is both a system-independent inode and a file system-dependent inode)

An inode is not a kernel data structure, the vnode/generic inode is however, being an in-kernel representation of the inode.

Answer 4

The concept of vnode differs to a degree depending on what system you're under, everyone just kind of took the name and ran with it.

Under Linux and Unix, you can consider the abstraction as follow. Suppose there exists

f.tmp

You want f to stick around while your program is running, because you're accessing it, but if your program ends or crashes, you want to be sure it goes away.

You can do this by opening f, and then unlink() it. You will still retain a reference to f, even though its inode now has 0 directory entries, and so has been marked free. The operating system is still retaining where the file started and its allocation state, until your program ends. This "virtualization" of the inode that no longer exists is a vnode.

Another common situation for this is when you're reading a resource that disappears out from under you. Suppose you're watching a movie, while it is streaming to a temporary location. When the movie is completely downloaded, it will be relocated to another volume for storage. Somehow you can continue watching and scrubbing through the movie so long as it remains open. In this case even though there are again no links, since there is a vnode, this inode can't be cleaned up yet.