Document revision date: 30 March 2001

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:

Task	Section
Allocating and deallocating disk and tape drives	Section 9.2
Initializing volumes	Section 9.3
Protecting volumes	Section 9.4
Mounting volumes	Section 9.5
Setting up disk volume sets	Section 9.6
Mounting ISO 9660 volume sets and groups	Section 9.7
Mounting tape volume sets	Section 9.8
Dismounting volumes and volume sets	Section 9.9
Using command procedures for media setup	Section 9.10
Managing disk space	Section 9.11
Using the Analyze/Disk_Structure utility to check and repair disks	Section 9.12
Using mount verification for recovery	Section 9.13
Using Interrupt Priority Level C (IPC)	Section 9.14
Using the Bad Block Locator utility to detect media errors	Section 9.15

This chapter explains the following concepts:

Concept	Section
Disks and CD-ROMs	Section 9.1.1
Extended File Specifications on OpenVMS Alpha systems	Section 9.1.2
Magnetic tape	Section 9.1.3
Public and private disk volumes	Section 9.1.4
Tape and disk volume protection	Section 9.4
Disk volume sets	Section 9.6.1
Disk quotas	Section 9.11.1
Mount verification	Section 9.13.1

9.1 Understanding Storage Media Concepts

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

Term	Definition
Device (or Drive)	Hardware that allows access to storage media.
Media	Physical items on which you can store data.
Volume	Logical unit of data storage; one or more media units. A disk or tape must be mounted on a device for the operating system to recognize itas a volume.

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.

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.

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.

Table 9-2 File Structure Options on OpenVMS Systems

Structure	Disk or CD	Description
ODS-1	Both	VAX only; use for RSX compatibility: RSX--11M, RSX--11D, RSX--11M--PLUS, and IAS operating systems.
ODS-2	Both	Use to share data between VAX and Alpha with full compatibility; default disk structure of the OpenVMS operating system.
ODS-5	Both	Superset of ODS-2; use on Alpha systems when working with systems like NT that need expanded character sets or deeper directories than ODS-2.
ISO 9660 CD	CD	ISO format files: use for platform-independent publishing and distribution; supported by other systems.
High Sierra	CD	A variant of ISO 9660.
Dual format	CD	Single volume with both ISO 9660 CD and ODS formats. You can use both formats to access files whose directories might point to the same data.
Foreign	Both	Unknown file structure. When you specify a foreign structure, you make the contents of a volume known to the system, but the system makes no assumptions about its file structure. The application is responsible for supplying a structure.

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.

Table 9-3 Reserved Files

Reserved File	File Name	+Structure Level 1	Structure Levels 2 and 5
Index file	INDEXF.SYS;1	X	X
Storage bit map file	BITMAP.SYS;1	X	X
Bad block file	BADBLK.SYS;1	X	X
Master file directory	000000.DIR;1	X	X
Core image file	CORIMG.SYS;1	X	X
Volume set list file	VOLSET.SYS;1		X
Continuation file	CONTIN.SYS;1		X
Backup log file	BACKUP.SYS;1		X
Pending bad block	BADLOG.SYS;1		X
Volume security profile	SECURITY.SYS;1		X

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:

Type of Bitmap	Limit
Storage bitmap	65535 blocks
Index file bitmap	4095 blocks

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.

Table 9-4 Comparison of ODS-1, ODS-2, and ODS-5 Formats

Characteristic	ODS-1 (VAX only)	ODS-2	ODS-5
File name length	9.3	39.39	238 bytes, including the dot and the file type. For Unicode, 236 bytes is 119 characters, including the dot and the file type.
Character set	Uppercase alphanumeric	Uppercase alphanumeric plus hyphen (-), dollar sign ($), and underscore (_)	ISO Latin-1, Unicode (refer to the description in OpenVMS Guide to Extended File Specifications.)
File versions	32,767 limit; version limits are not supported	32,767 limit; version limits are supported.	32,767 limit; version limits are supported.
Directories	No hierarchies of directories and subdirectories; directory entries are not ordered 1	Alpha: 255 2 VAX: 8 (with rooted logical, 16)	Alpha: 255 VAX: 8 (with rooted logical, 16)
System disk	Cannot be an ODS-1 volume	Can be an ODS-2 volume	ODS-5 volume not supported for Version 7.2
Page file, swap file, dump file, parameter (.PAR) file, and other system files	Supported	Supported	Not supported
OpenVMS Cluster access	Local access only; files cannot be shared across a cluster	Files can be shared across a cluster	Files can be shared across a cluster. However, only computers running OpenVMS Version 7.2 or later can mount ODS-5 disks. VAX computers running Version 7.2 or later can see only files with ODS-2 style names.
Disk	Unprotected objects	Protected objects	Protected objects
Disk quotas	Not supported	Supported	Supported
Multivolume files and volume sets	Not supported	Supported	Supported
Placement control	Not supported	Supported	Supported
Caches	No caching of file identification slots or extent entries	Caching of file header blocks, file identification slots, and extent entries	Caching of file header blocks, file identification slots, and extent entries
Clustered allocation	Not supported	Supported	Supported
Backup home block	Not supported	Supported	Supported
Protection code E	E means "extend" for the RSX--11M operating system but is ignored by OpenVMS	E means "execute access"	E means "execute access"
Enhanced protection features (for example, access control lists)	Not supported	Enhanced protection features supported	Enhanced protection features supported

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.

Table 9-5 ISO 9660 Terms

Term	Description
Volume space	Set of all logical blocks on a volume containing information about the volume.
System area	One of two divisions of CD-ROM volume space; includes logical sectors 0 through 15; reserved for system use.
Data area	One of two divisions of CD-ROM volume space; includes the remaining volume space, beginning with logical sector 16.

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.

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