inodeid

Inodeid is a term used to describe the unique numeric identifier assigned to an inode within a filesystem. An inode is the on-disk data structure that stores metadata about a file or directory, including its ownership, permissions, size, timestamps, and pointers to its data blocks. The inodeid, often referred to as the inode number, serves as the primary handle the system uses to locate and access the inode's metadata.

Key characteristics

- Uniqueness and scope: The inodeid is unique within a single filesystem, enabling the kernel to distinguish between files stored in that filesystem. It does not imply global uniqueness across multiple filesystems.

- Lifespan: The inodeid persists for the lifetime of the inode. If a file is deleted and its inode is freed, the same numeric value may be reused for a future file, depending on the filesystem’s allocation policy.

- Stability and changes: Renaming a file or moving it within the same filesystem does not typically change its inodeid. Creating a new file yields a new inodeid, while hard links to an existing file share the same inodeid.

- Cross-filesystem references: In networked or portable references (such as certain file handles in NFS), the inodeid can be part of an identifier that helps locate a file. However, this reference is not portable across different filesystems or systems.

Limitations

- Inodeid is not portable: It is meaningful only within the specific filesystem where the inode resides. Moving or copying a file to another filesystem often results in a new inodeid.

- It is not a path: The inodeid identifies a file’s metadata location, not its human-readable name or path.

Relation to metadata

- The inodeid points to the inode, which contains metadata about the file and the pointers to the file’s data blocks. The inodeid itself is a numeric key used for fast lookup by the filesystem.