Logical Volumes

 

    The local file system of each machine is made up of a set of logical volumes. A logical volume is an abstraction of a physical volume such as a disk pack; a logical volume can actually be implemented as a set of subvolumes (pieces) of one or more physical volumes.

Each logical volume is a self-contained unit made up of a set of files and the directory entries that reference them. Before the files on a logical volume can be used, the logical volume has to be added to the file name space of the machine. This is done by `mounting' the logical volume, which identifies the root directory of that logical volume with some existing directory in the name space. While a logical volume is mounted, its directory hierarchy is a subhierarchy of the local name space.

  Even while it is mounted, a logical volume still maintains its autonomy. A hard link (created by the HardLink procedure, page ) in a directory residing on one logical volume cannot refer to a file residing on a different logical volume. And of course free space on one logical volume cannot be used to create or extend a file residing on a different logical volume. For these reasons, it is typical to create a single large logical volume occupying most of the space on all the physical volumes on a machine.

Taos note: Operations for manipulating logical volumes are described in Appendix A.5, page .

  Ultrix note: The Ultrix term for a logical volume is a file system. Ultrix allows a disk drive to be partitioned into several file systems, but doesn't allow a file system to span disk drives. The Ultrix implementation of OS does not support the manipulation of logical or physical volumes.