The File System physically stores a file on disc by breaking it into
extents and maintaining pointers to where each extent resides.
It logically stores a file based on where the file reference
indicates the file belongs in the directory structure.
Both concepts are discussed in Chapter 1, "Overview."
The group specified for a file in the standard file reference
format (filename.groupname.acctname) determines the volume set
on which the file resides.
You can select the volume set, volume name,
and class for a file by using the HPFOPEN intrinsic.
Most common uses of files do not require specification
of volume sets or classes, but it is sometimes useful to restrict
a file to a particular volume or group of volumes.
If a file resides on only one volume, then damage to other discs
will not destroy the file.
However, when a volume specification is given,
the File System distributes extents throughout a volume
set to prevent one physical disc from running out of space.
Disc storage is logically partitioned into:
Volume, which is a physical disc and a member of a volume set. It may also be a member of one or more volume classes. It is removable while the system is running, is automatically recognized and mounted by the system, and the operator is notified when an unmounted volume is requested.
Free space is tracked by a separate free space bit map
on each individual disc volume.
To find space to put a new disc file, several tables may be
checked.
You can display disc space information by running
DISCFREE.PUB.SYS.
Volume set, which consists of a maximum of 255 volumes or volume classes. It provides a way of partitioning user data into removable entities and are used only for permanent storage space.
The directory structure is spread across a volume set. Each volume has a label object, which contains the file labels for files. The directory has no arbitrary limit to the number of files per volume. The system disc is usually configured to be LDEV 1.
Volume class, which can contain a maximum of 255 volumes and is optional within a volume set. You cannot mount a disc by a volume class designation; it is used exclusively to allocate disc storage space.
You can specify disc volume restrictions in the following ways:
Use the volume name option or the volume class
option of HPFOPEN or the device parameter of
FOPEN to specify either a volume name or a volume class
name. This restricts the placement of your file s extents to either the
specified volume or the specified volume class within the volume
set, thus facilitating file portability.
Using FOPEN, you can restrict the placement of
a file extent to a specified volume or a specified volume
class within the volume set assigned to the group in
which the file will be created.
Using FOPEN, you can specify a volume name or a volume
class in a way that maintains FOPEN compatibility with
MPE V/E-based systems.
For example, MPE/iX translates
a logical device number (LDEV) passed to FOPEN
into the volume name currently mounted on the disc drive and
places the volume name in the file label.
For more information on specifying disc volume restrictions,
refer to
Accessing Files Programmer's Guide (32650-90017).
