UNIX Filesystems: Evolution, Design, and Implementation

The HP-UX VFS Architecture

HP-UX has a long and varied history. Although originally derived from System III UNIX, the HP-UX 1.0 release, which appeared in 1986, was largely based on SVR2. Since that time, many enhancements have been added to HP-UX from SVR3, SVR4, and Berkeley versions of UNIX. At the time of writing, HP-UX is still undergoing a number of new enhancements to make it more scalable and provide cleaner interfaces between various kernel components.

The HP-UX Filesystem-Independent Layer

HP-UX maintains the mapping between file descriptors in the user area through the system file table to a vnode, as with other VFS/vnode architectures. File descriptors are allocated dynamically as with SVR4.

The file structure is similar to its BSD counterpart in that it also includes a vector of functions so that the user can access the filesystem and sockets using the same set of file-related system calls. The operations exported through the file table are fo_rw(), fo_ioctl(), fo_select(), and fo_close().

The HP-UX VFS/Vnode Layer

Readers familiar with the SVR4 VFS/vnode architecture will find many similarities with the HP-UX implementation of vnodes.

The vfs structure, while providing some additional fields, retains most of the original fields of the original Sun implementation as documented in [KLEI86]. The VFS operations more resemble the SVR4 interfaces but also provide additional interfaces for quota management and enabling the filesystem to export a freeze/thaw capability.