HP 3000 Manuals HP 3000 Manuals
Disc File Storage
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).