Document revision date: 19 July 1999

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 8-1 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 8-1 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 8-2 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 8-2 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 8-3 identifies all reserved files, and indicates to which ODS level they pertain.

The files listed in Table 8-3 are in the master file directory (MFD), [000000]. Appendix A contains a description of each reserved file.

Table 8-3 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.

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

Table 8-4 compares specific characteristics of On-Disk Structure (ODS) Levels 1, 2, and 5.

Table 8-4 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 8-5 defines terms related to the ISO 9660 standard.

Table 8-5 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.

Extended File Specifications allows files with Windows 95 style or Windows NT style file names and attributes to reside on, and to be managed from, OpenVMS Alpha systems. Extended File Specifications support can also be selected on a per-volume basis.

Traditional and Extended File Names

In an Extended File Specifications environment, you can select either of the following naming styles for file specifications:

• Traditional file names are allowed on both ODS-2 and ODS-5 volumes.
• Extended file names are allowed on ODS-5, but not on ODS-2 volumes.

For detailed definitions of traditional and extended file names as well as other introductory information about Extended File Specifications, refer to the OpenVMS Guide to Extended File Specifications.

The following sections describe the current levels of disk, mixed-version, mixed-architecture, and network support for Extended File Specifications on OpenVMS systems.

8.1.2.1 System and User Disk Support

Restrictions and suggestions for using Extended File Specifications on your system are as follows:

• You can mount an ODS-5 volume only on an OpenVMS VAX or an Alpha Version 7.2 or later system. Most applications can access only traditionally named files on a VAX system.
• Compaq does not support creating the system disk as (or changing it to) an ODS-5 volume.

Users on OpenVMS Version 7.2 and later systems can take advantage of Extended File Specifications capabilities. In contrast, systems running prior versions of OpenVMS cannot mount ODS-5 volumes or correctly handle extended file names.

The following sections describe support on OpenVMS Version 7.2 and on prior versions of OpenVMS in a mixed-version cluster.

Users on OpenVMS Version 7.2 Systems

Users on OpenVMS Version 7.2 systems can continue to access pre-Version 7.2 files and directories. For example, they can perform all of the following actions:

• Create and access deep directory structures on ODS-2 volumes.
• Read a BACKUP save set created on an earlier version of OpenVMS.
• Use DECnet to copy a file with an ODS-5 name to a file with an ODS-2 name on a system running an earlier version of OpenVMS.

Users on Systems with Versions Prior to OpenVMS 7.2

On mixed-version clusters, some restrictions exist. Users on a version of OpenVMS prior to Version 7.2 have these limitations:

• Cannot access any files on an ODS-5 volume. This is true regardless of whether the volume is connected physically on a CI or SCSI, or by an MSCP or QIO server.
• Cannot successfully create or restore an ODS-5 image save set. However, these users can successfully restore ODS-2-compliant file names from an ODS-5 save set.

All Extended File Specifications capabilities are available on OpenVMS Alpha systems. Nearly all the current ODS-2 disk and file management functions remain the same on both VAX and Alpha Version 7.2 systems; however, extended file naming and parsing are not available on VAX systems.

The following sections describe support on OpenVMS VAX and Alpha systems in a mixed-architecture cluster.

Limited Extended File Specifications Capabilities on VAX Systems

In mixed-architecture OpenVMS Version 7.2 clusters, the following limited Extended File Specifications capabilities are available on OpenVMS VAX systems:

• Ability to mount and manage an ODS-5 disk
• Ability to write and manage ODS-2-compliant files on an ODS-5 disk

BACKUP Limitations

On an OpenVMS VAX system, users cannot successfully create or restore an ODS-5 image save set. However, these users can successfully restore ODS-2-compliant file names from an ODS-5 save set.

Users can also perform a disk-to-disk copy from ODS-5 to ODS-2 as long as they do not specify /IMAGE.

For more information, refer to the OpenVMS Guide to Extended File Specifications.

8.1.2.4 Network Support

For OpenVMS Version 7.2, the length of a file specification that can be sent over the network using DECnet is shorter than the maximum size of a file specification without the use of a network.

8.1.2.5 Enabling Extended File Specifications Features

To enable Extended File Specifications On-Disk Structure features for a volume on an OpenVMS Alpha system, you must perform one of the following actions:

Task	Reference
Initialize a new volume as ODS-5	Section 8.3.3
Convert an existing volume from ODS-2 to ODS-5	Section 8.5.5.1

Creating ODS-5 volumes allows you to take advantage of extended file names for Compaq Advanced Server for OpenVMS 7.2 clients; you can see and manage these names from OpenVMS Alpha systems.

The file storage system for magnetic tapes is based on the standard magnetic tape structure as defined by ISO 1001--1986, which is compatible with several national standards such as ANSI X3.27--1987.

For more information about tape concepts, refer to the Guide to OpenVMS File Applications.

Table 8-6 defines terms that apply to magnetic tapes.

Table 8-6 Terms Related to Magnetic Tapes

Term	Definition
Block	On magnetic tape, the size of a block is determined by the user. On ODS disks, a block is fixed at a size of 512 bytes.
bpi	Bits per inch; measure used for characters of data on tape on OpenVMS systems. Also called density.
IRG	Interrecord gap; interval of space between blocks.
Record blocking	Grouping of individual records into a block, thereby reducing wasted space.
Sequential	Organization of magnetic tape data; that is, data is organized in the order in which it is written to the tape.
Magnetic Tape Ancillary Control Process (MTACP)	Internal software process that interprets the logical format of standard labeled volumes.
Beginning-of-tape (BOT) marker and end-of-tape (EOT) marker	Pieces of photoreflective tape that appear on every volume to delimit the writable area on the volume. ANSI standards require that a minimum of 14 feet to a maximum of 18 feet of magnetic tape precede the BOT marker; a minimum of 25 feet to a maximum of 30 feet of tape, of which 10 feet must be writable, must follow the EOT marker. The EOT marker indicates the start of the end of the writable area of the tape, rather than the physical end of the tape. Therefore, data and labels can be written after the EOT marker.
Data record storage	Within tape files, data records are stored in variable-size data blocks. Each block contains one or more records. RMS provides management of records.
Header labels	Set of labels that the tape file system writes on the tape immediately preceding the data blocks when you create a file on tape. These labels contain information such as the user-supplied file name, creation date, and expiration date. To access a file on magnetic tape by the file name, the file system searches the tape for the header label set that contains the specified file name.
Trailer labels	Set of labels that the tape file system writes on the tape following the file.
Multivolume file	Additional volume on which a file is continued when the data blocks of a file or related files do not physically fit on one volume (a reel of magnetic tape).
Volume set	The volumes on which a set of files is recorded.

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