Technical data

Managing Storage Media
9.1 Understanding Storage Media Concepts
Table 9–1 Disk and CD–ROM Terminology
Term Definition
Sector Uniquely addressable unit. Each sector on a CD–ROM comprises a
sequence of 2048 8-bit bytes; on most disks, a sector is equivalent to a
block (512 bytes).
Logical block Organizational unit of volume space containing 512 8-bit bytes. A
CD–ROM sector comprises 4 blocks.
Logical block
numbering
Logical blocks are numbered from 0 to n-1.
Cluster
1
Logical grouping of blocks; basic unit by which disk (not CD–ROM)
space is allocated.
Extent Contiguous logical blocks allocated to a particular file.
File Array of consecutive virtual blocks
2
, numbered 1 to n, plus a set of
attributes with values. A file is either a data file or a directory file.
Directories can contain both data files and directory files.
Volume Disk that has been prepared for use by placing a new file structure on
it.
Volume set Combination of several volumes; has the appearance of one large
volume.
1
This usage of cluster does not refer to a set of nodes that form an OpenVMS Cluster environment; it
refers only to disks.
2
A logical block resides at an absolute address on a disk; a virtual block, on the other hand, resides at
an address relative to a file.
Information on a disk or CD–ROM can be accessed as logical blocks on the disk
or as virtual blocks of files on the disk.
9.1.1.2 Disk and CD–ROM File Structures
On-Disk Structure (ODS) refers to a logical structure given to information
stored on a disk or CD–ROM. It is a hierarchical organization of files, their data,
and the directories needed to gain access to them. The OpenVMS file system
implements the On-Disk Structure and provides access control to the files located
on the disk.
Figure 9–1 shows the hierarchy of blocks, clusters, extents, and files in the On-
Disk Structure. The number of blocks in any one extent is a multiple of the
cluster size. The figure assumes a cluster size of 2 blocks.
Managing Storage Media 9–3