Filesystems

A “filesystem” is an organization of “files” into a system, often backed by some sort of long-term storage device. Before the kernel or userland can perform file operations, the filesystem has to be properly “mounted”. Modern operating systems support the mounting of multiple filesystems, and provide a virtual filesystem layer.

KUDOS is no exception.

KUDOS supports one filesystem, called the Trivial Filesystem. Filesystems are managed and accessed through a layer called the Virtual Filesystem layer which represents a union of all mounted filesystems.

The Trivial Filesystem supports only the most primitive filesystem operations and does not enable concurrent access to the filesystem. Only one request (read, write, create, open, close, etc.) is allowed to be in action at any given time. TFS enforces this restriction internally.

Filesystem Conventions

Files on filesystems are addressed by filenames. In KUDOS, filenames can have at most 15 alphanumeric characters. The full path to a file is called an absolute pathname and it must contain the volume (mount-point or filesystem) on which the file is, possibly a directory path, and finally, the name of the file within that directory.

An example of a valid filename is shell. A full absolute path to a shell might be [disk]shell or [disk]bin/shell. Here shell is the name of a file, disk is a volume name (you could also call it a disk, filesystem or mount-point). If directories are used, bin is a name of a directory. Directories have the same restrictions on filenames as files do (directory are really just files). Directory names in a path are separated by /.

Filesystem Layers

Typically a filesystem is located on a disk (but it can also be a network filesystem or even totally virtual). Disks are accessed through Generic lock Devices (see Device Drivers). At boot time, the system will try to mount all available filesystem drivers on all available disks through their GBDs. The mounting is done into a virtual filesystem.

Virtual Filesystem is a super-filesystem which contains all attached (mounted) filesystems. The same access functions are used to access disk, networked and fully virtual filesystems. An example of a “fully virtual filesystem” is the proc filesystem on Linux, which makes a range of process-related information available from under the /proc directory. The actual filesystem driver is recognized from the volume name part of a full absolute pathname provided to the access functions.

Files	Purpose
vfs.[hc]	Virtual Filesystem implementation
filesystems.[hc]	Available filesystems
tfs.[hc]	Trivial Filesystem implementation