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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 it as a volume.

The following sections use these terms to explain media concepts.

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.

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:

Disk and CD-ROM Terminology defines terms as they apply to disks and CD-ROMs.

Table 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.
Cluster1
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 blocks2, 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.

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.

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

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. Beginning with OpenVMS Version 7.2 on Alpha (and on I64) systems, you can specify ODS-5 to format disks as well.

The OpenVMS Alpha and I64 operating systems recognize all the file structures for disks and CD-ROMs shown in File Structure Options on OpenVMS Systems 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 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 among VAX, Alpha, and I64 with full compatibility; default disk structure of the OpenVMS operating system.
ODS-5
Both
Superset of ODS-2; use on Alpha and I64 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.) Reserved Files identifies all reserved files, and indicates to which ODS level they pertain.

The files listed in Reserved Files are in the master file directory (MFD), [000000]. The HP OpenVMS System Manager's Manual, Volume 2: Tuning, Monitoring, and Complex Systems contains a description of each reserved file.

Table 3   Reserved Files
Reserved File File Name Structure Level 13 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:

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

Comparison of ODS-1 (VAX only), ODS-2, and ODS-5 (Alpha and I64) Formats  

Comparison of ODS-1, ODS-2, and ODS-5 Formats compares specific characteristics of On-Disk Structure (ODS) Levels 1, 2, and 5.

Table 4   Comparison of ODS-1, ODS-2, and ODS-5 Formats
Characteristic ODS-1 (VAX only) ODS-2 ODS-5 (Alpha and I64)
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 the OpenVMS User's Manual .)
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 ordered4
Alpha: 255 directory levels5

VAX: 8 directory levels (with rooted logical, 16)
Alpha: 255 directory levels

VAX: 8 directory levels (with rooted logical, 16)
System disk
Cannot be an ODS-1 volume
Can be an ODS-2 volume
ODS-5 volume supported for Version 7.3-1 and higher
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


NoteFuture enhancements to OpenVMS software will be based primarily on Structure Levels 2 and 5; therefore, Structure Level 1 volumes might be further restricted in the future. However, HP does not intend for ODS-5 to be the default OpenVMS file structure. The principal function of ODS-5 is to enable an OpenVMS system to be a server for other systems (such as Windows NT®) that have extended file names.

The ISO 9660 Standard for CD-ROMs  

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

Table 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.

Extended File Specifications on OpenVMS Alpha and I64 Systems  

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 or I64 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:

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

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

System and User Disk Support  

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

Mixed-Version Support  

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:

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:

Mixed-Architecture Support  

All Extended File Specifications capabilities are available on OpenVMS Alpha and I64 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 or I64 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:

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 HP OpenVMS System Management Utilities Reference Manual: A--L.

Network Support  

For OpenVMS Version 7.2 and later, 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.

Enabling Extended File Specifications Features  

To enable Extended File Specifications On-Disk Structure features for a volume on an OpenVMS Alpha or I64 system, you must perform one of the following actions:

Task Reference
Initialize a new volume as ODS-5
Initializing a New Volume with ODS-5 Format
Convert an existing volume from ODS-2 to ODS-5
Converting from ODS-2 to ODS-5

Creating ODS-5 volumes allows you to take advantage of extended file names for HP Advanced Server for OpenVMS clients; you can see and manage these names from OpenVMS Alpha or I64 systems.


NoteExtended File Specifications are not available on systems running versions of OpenVMS Alpha prior to Version 7.2. On these systems, you cannot mount ODS-5 volumes nor take advantage of extended file names on an OpenVMS file system.

Tape Concepts  

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 .

Terms Related to Magnetic Tapes defines terms that apply to magnetic tapes.

Table 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.
<$nopage>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.

Record Blocking  

Record Blocking shows how record blocking can save space. 

Figure 2  Record Blocking  
Record

Assume that a 1600-bits-per-inch magnetic tape contains 10 records that are not grouped into a block. Each record is 160 characters long (0.1 inch at 1600 bits per inch) with a 0.6-inch IRG after each record, which uses 7 inches of tape. However, placing the same 10 records into one block uses only 1.6 inches of tape (1 inch for the data records and 0.6 inch for the IRG).

Record blocking also increases the efficiency of the flow of data into the computer. For example, 10 unblocked records require 10 separate physical transfers, while 10 records placed in a single block require only one physical transfer. Moreover, a shorter length of magnetic tape is traversed for the same amount of data; thus, the operation is completed in less time.

However, record blocking requires more buffer space to be allocated for your program. The greater the number of records in a block, the greater the buffer size requirements. You must determine the point at which the benefits of record blocking cease. Base this determination on the configuration of your computer system and your environment.

Multiple Tape Densities (Alpha and I64)  

In versions of OpenVMS Alpha prior to Version 7.2, the range of densities that users were able to set for magnetic tape devices was limited. On OpenVMS Version 7.2 and later Alpha systems and on I64 systems, that range includes any density that a specific tape drive supports. Because of this enhancement, exchanging tapes among tape drives with different default settings for density is much easier.

You can set densities using the following DCL commands:

Refer to the HP OpenVMS DCL Dictionary for details about using the /DENSITY qualifier with these DCL commands.

You can also set densities using the following system management utilities:

Refer to the MOUNT command in the HP OpenVMS DCL Dictionary and to the BACKUP utility in the HP OpenVMS System Management Utilities Reference Manual for details about using the /DENSITY qualifier. Also refer to Using BACKUP for details about using the /DENSITY qualifier with BACKUP.

Example

$ INITIALIZE/DENSITY=tk85 MKA500: TEST
The command in this example initializes the media in the MKA500: drive to tk85 density with a label of TEST.

Densities Supported

The following densities are valid in the command strings for DCL commands and system management utilities: 800, 1600, 6250, DAT, 833, DDS1, DDS2, DDS3, DDS4, TK50, TK70, TK85, TK86, TK87, TK88, TK89, 8200, 8500, 3480, 3490E, AIC, AIT, and DEFAULT.

Usage Notes

You cannot use multiple tape densities on one piece of media. In other words, one density applies to one piece of media. If you do not specify a density, the default density is used; the default is the highest density a particular drive supports.

Density changes can occur only at beginning-of-tape (BOT). Once media is initialized to a density, the media remains at that density until it is reinitialized to a different density.

If a density is not supported on a particular device, depending on the drive, the density field either remains the same or takes the default. If a drive does not support the density you select, the system displays an invalid density error. Some drives do not report the error and simply ignore your selection, leaving the media at the previous density.

When media is set to a specific density, the "density" field displayed when you enter $ SHOW DEVICE/FULL MKA300:, for example, displays the corresponding ASCII string for density.

Magtape JENSO3$MKA300:, device type TZ87, is online, file-oriented device, error
    logging is enabled, controller supports compaction (compaction, disabled),
    device supports fastskip.
 
    Error count                    0    Operations completed                   0
    Owner process                 ""    Owner UIC                       [SYSTEM]   
    Owner process ID        00000000    Dev Prot             S:RWPL,O:RWPL,G:R,W
    Reference count                0    Default buffer size                 2048
    Density                     TK85    Format                         Normal-11
 
Volume status: no-unload on dismount, position lost, odd parity.

Public and Private Disk Volumes  

A volume is one or more units of storage media that you can mount on a device. The volume is the largest logical unit of disk file structure.

This section explains the concepts of public and private volumes.

Public Disk Volumes  

A public volume is a file-structured disk volume that can contain both private and public files. Public volumes can be either of the following ones:

Type of Volume Description
System volumes
Available to all the users on a system
Group volumes
Available to all the users in a group

As long as file protections permit it, all users have access to public volumes and to the files on them.

One way to permit users to create and store files on a public volume is to create a default directory on the public volume for each authorized user. You control access to public files and volumes by the protection codes that you establish.

A user is free to create, write, and manipulate files on a public volume only under the following conditions:

The following sections contain guidelines for setting up and maintaining public files and volumes.

Planning Public Volumes

You must balance users' space needs with the system's available mass storage resources. These determinations depend, in part, on whether you have relatively small or large mass storage capability. A comparison of the two follows.

Configuration Characteristics
Small mass storage
Both system files and user files are on the same public volume. You might want to set disk quotas to ensure that user files do not exhaust the free space on the disk volume.
Large mass storage
Keep all system files on one disk volume (known as a system disk or a system volume), and keep all user files on separate volumes.

The system disk is kept active reading system images, paging and swapping, spooling files, maintaining system logs, and so forth.

The most common arrangement is to have one public volume with system files and the directories of privileged users, and other public volumes dedicated to user directories, databases, and applications required by your site.

Whichever arrangement you select, plan each public volume and monitor disk performance once the system is running:

You can often move system files off the system disk and use search lists or logical names to access them. See HP OpenVMS System Manager's Manual, Volume 2: Tuning, Monitoring, and Complex Systems for more information.

In large configurations, you can place secondary paging and swapping files on other devices to balance disk load. See HP OpenVMS System Manager's Manual, Volume 2: Tuning, Monitoring, and Complex Systems for more information. The OpenVMS Performance Management provides detailed information about redistributing system files and achieving a balanced disk load.

Private Disk Volumes  

A private volume is a file-structured volume that contains only private files.

Under some circumstances, users might want to perform their work on a device that unauthorized users cannot access. By creating a private volume and mounting it on a device allocated exclusively to a user's process, you ensure that users can perform their work without fear of interference from others.

Users can often prepare and manipulate their own private volumes. They might, however, need your assistance if the computer and its peripheral devices are off limits to or remotely located from them. Users requiring assistance can use the operator communication manager (OPCOM) to communicate with an operator. See Assisting Users in Mounting Volumes for instructions on answering users' requests for assistance.


Footnotes
1This usage of cluster does not refer to a set of nodes that form an OpenVMS Cluster environment; it refers only to disks.
2A 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.
3VAX specific
4RSX-11M, RSX-11D, RSX-11M-PLUS, and IAS systems do not support subdirectories and alphabetical directory entries.
5Prior to OpenVMS Version 7.2, RMS limited directory levels to 8 or 16, although PATHWORKS and other programs that did not use RMS could use unlimited directory depth.

( Number takes you back )


 
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