HP OpenVMS Systems Documentation

This chapter discusses the following subjects:

• Storage media terms and concepts
• Volumes and volume sets
  Tasks related to setting up both disks and magnetic tape drives: initializing and mounting
• Disks
  Tasks related to disk maintenance: the management of disk space and methods for detecting and repairing disk errors

Information Provided in This Chapter

This chapter describes the following tasks:

This chapter explains the following concepts:

The following list contains concepts related to storage media in general:

The following sections use these terms to explain media concepts.

9.1.1 Disk and CD-ROM Concepts

This section defines terms related to disks and CD-ROMs. It also compares on-disk file structures and explains the ISO 9660 standard.

9.1.1.1 Disk Terminology

Disks are physical media on which files reside. On--Disk Structure (ODS) refers to a logical structure given to information stored on a disk; it is a hierarchical organization of files, their data, and the directories needed to gain access to them. The OpenVMS file system implements the ODS and provides access control to the files located on the disk.

Compact disc read-only memory (CD-ROM) discs and readers used with computers are very similar to the CD-ROMs used for audio applications. The major differences are that CD-ROM drives have a digital (rather than an audio) interface, and that CD-ROM discs employ additional layers of error correction and formatting to encode data blocks.

The advantages of storing data on CD-ROMs rather than on tape or other removable media are:

• You can access data directly, which you cannot do with tape.
• CD-ROMs are much less expensive than other direct-access media. They are typically used for publishing and distribution.
• CD-ROMs have exceptional storage space capability. A CD-ROM can hold approximately 650 megabytes of data.

Table 9-1 defines terms as they apply to disks and CD-ROMs.

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.

Figure 9-1 On-Disk Structure Hierarchy of a File

OpenVMS File Structure Options

On-Disk Structures include Levels 1, 2, and 5. (Levels 3 and 4 are internal names for ISO 9660 and High Sierra CD formats.) ODS-1 and ODS-2 structures have been available on OpenVMS systems for some time. With OpenVMS Version 7.2 on Alpha systems, you can now specify ODS-5 to format disks as well.

The OpenVMS Alpha operating system recognizes all the file structures for disks and CD-ROMs shown in Table 9-2 except ODS-1. On VAX systems, you can mount ODS-5-enabled volumes, but you cannot access ODS-5-specific features. You can use one or more formats to incorporate a volume and file structure that is compatible with the input/output processing used by the system.

When you create a file, you normally specify a file name to OpenVMS Record Management Services (RMS), which assigns this name to the file on an on-disk volume. RMS places the file name and file ID associated with the newly created file in a directory, which contains an entry defining the location for each file.

When you access a file, you supply the file name, which supplies a path to the file identifier through the directory entry. The file identifier, in turn, points to the location of the file header, which contains a listing of the extent or extents that locate the actual data.

Reserved Files on OpenVMS Systems

Reserved files control the structure of a On-Disk Structure (ODS) Levels 2 and 5 volumes. (Only five of these files are used for a Level 1 volume.) Table 9-3 identifies all reserved files, and indicates to which ODS level they pertain.

The files listed in Table 9-3 are in the master file directory (MFD), [000000]. Appendix A contains a description of each reserved file.

Limits of Storage and Index File Bitmaps

In previous versions of OpenVMS, both storage and index file bitmaps were limited to 255 blocks. This size, in turn, limited a volume to approximately one million allocation units, or clusters. Larger disks were required to have a larger cluster factor to accommodate the limit; for example, a 9 GB disk required a cluster factor of 18.

Beginning with OpenVMS Version 7.2, the limits of storage and index file bitmaps have been increased as follows:

The increased bitmap limits have the following advantages:

• They allow you to use space more efficiently with small files.
• They increase the number of files allowed on a volume to the architectural maximum of approximately 16 million.

The behaviors of the INITIALIZE and BACKUP commands reflect the larger bitmap sizes. Refer to the OpenVMS DCL Dictionary for INITIALIZE command details and the OpenVMS System Management Utilities Reference Manual for BACKUP utility details.

Message Displayed with Earlier Versions

The following message is displayed on pre-Version 7.2 systems when you attempt to access a disk that was initialized on a Version 7.2 or later system:

%MOUNT-F-FILESTRUCT, unsupported file structure level

The increased size of the BITMAP field is incompatible with earlier versions of OpenVMS.

Check the size of BITMAP.SYS on the disk you want to access. If the size is 256 or greater and you want to access the disk from a pre-Version 7.2 system, you must copy the data to a disk with a BITMAP.SYS that is less than 256 blocks. If you use the DCL command BACKUP/IMAGE, be sure to use the /NOINITIALIZE qualifier.

9.1.1.3 Comparison of ODS-1, ODS-2, and ODS-5 Formats

Table 9-4 compares specific characteristics of On-Disk Structure (ODS) Levels 1, 2, and 5.

The OpenVMS implementation of On-Disk Structure conforms to the file structures defined by the ISO 9660 standard. Table 9-5 defines terms related to the ISO 9660 standard.

Two aspects of an implementation are required to support ISO 9660 file structures in an OpenVMS environment: partially recorded data blocks and data interleaving.

• Partially recorded data blocks
  ISO 9660 files are recorded in the data area of the media, using extents that consist of blocks. A block might not be filled with data but also might not be the final block of a file. The OpenVMS implementation of Files--11 ensures that the system does not treat an unfilled block as the end of the file unless it actually is the final file block. This is not visible to the user.
• Data interleaving
  Within an extent, data is recorded using file units separated by an interleave gap, which consists of a specified number of blocks. Data interleaving allows you to control the speed of access to file data.