lowerdirlowerupperdirupperworkdirworkinodeonly

lowerdirlowerupperdirupperworkdirworkinodeonly is a directive used in certain operating system contexts, particularly within overlay file systems like OverlayFS. It specifies how directories and their contents are organized and managed when creating an overlay mount. The directive is a combination of four separate path specifications: lowerdir, upperdir, workdir, and inodeonly.

The lowerdir component refers to one or more read-only directories that form the base of the overlay. Changes are not made to these directories. The upperdir is a read-write directory where modifications, such as creating new files or altering existing ones, are stored. When a file is modified in the upperdir, it is often "copied up" from the lowerdir to the upperdir before the modification occurs.

The workdir is a temporary directory that OverlayFS uses internally to facilitate operations like copy-up and renaming. It must be on the same file system as the upperdir. The inodeonly flag, when set, indicates that inode information should only be considered from the upper directory. This can affect how the file system handles certain metadata operations and is often used to optimize performance or behavior in specific scenarios.

Together, these components define the structure and operational characteristics of an overlay mount, enabling efficient storage and management of layered file systems.