DECram for OpenVMS User's Manual

DECram for OpenVMS User's Manual

January 1999

This manual describes the features of the DECram for OpenVMS Device Driver (MDDRIVER). It includes information on how to determine which files should be stored on the device, how to configure the device, and how to use the device driver.

Revision/Update Information This document supersedes the DECram for VMS User's Manual Version 2.2.

Operating System and Version: OpenVMS VAX Version 6.2 or higher, OpenVMS Alpha Version 6.2 or higher

Software Version: DECram for OpenVMS Version 2.3

Compaq Computer Corporation
Houston, Texas

January 1999

Compaq Computer Corporation makes no representations that the use of its products in the manner described in this publication will not infringe on existing or future patent rights, nor do the descriptions contained in this publication imply the granting of licenses to make, use, or sell equipment or software in accordance with the description.

Possession, use, or copying of the software described in this publication is authorized only pursuant to a valid written license from Compaq or an authorized sublicensor.

Compaq conducts its business in a manner that conserves the environment and protects the safety and health of its employees, customers, and the community.

© Compaq Computer Corporation 1999. All rights reserved.

The following are trademarks of Compaq Computer Corporation: Alpha, Compaq, DECdirect, DIGITAL, OpenVMS, VAX, VAX DOCUMENT, VAXcluster, VMS, and the Compaq logo.

All other trademarks and registered trademarks are the property of their respective holders.


The DECram documentation set is available on CD-ROM.

Contents Index


This manual describes the DECram for OpenVMS Device Driver (MDDRIVER). You will find information on how to determine which files should be stored on the device, how to configure the device, and how to use the device driver.

Intended Audience

This manual is intended for system managers who want to take advantage of the increased performance that results from using the DECram driver. You should be familiar with managing the OpenVMS VAX and the OpenVMS Alpha operating systems before reading this manual.

Document Structure

This manual consists of three chapters and two appendixes, as follows:

Related Documents

See the following documents for information that is relevant to configuring and programming DECram disks:

The following documents contain information that will assist you in identifying files that can be stored on a DECram disk:

For additional information on the Open Systems Software Group (OSSG) products and services, access the following OpenVMS World Wide Web address: 

Reader's Comments

Compaq welcomes your comments on this manual.

Print or edit the online form SYS$HELP:OPENVMSDOC_COMMENTS.TXT and send us your comments by:
Fax 603 884-0120, Attention: OSSG Documentation, ZKO3-4/U08
Mail Compaq Computer Corporation
OSSG Documentation Group, ZKO3-4/U08
110 Spit Brook Rd.
Nashua, NH 03062-2698

How To Order Additional Documentation

Use the following World Wide Web address to order additional documentation: 

If you need help deciding which documentation best meets your needs, call 800-DIGITAL (800-344-4825).


The following conventions are used in this manual:
Ctrl/ x A sequence such as Ctrl/ x indicates that you must hold down the key labeled Ctrl while you press another key or a pointing device button.
PF1 x A sequence such as PF1 x indicates that you must first press and release the key labeled PF1 and then press and release another key or a pointing device button.
[Return] In examples, a key name enclosed in a box indicates that you press a key on the keyboard. (In text, a key name is not enclosed in a box.)

In the HTML version of this document, this convention appears as brackets, rather than a box.

... A horizontal ellipsis in examples indicates one of the following possibilities:
  • Additional optional arguments in a statement have been omitted.
  • The preceding item or items can be repeated one or more times.
  • Additional parameters, values, or other information can be entered.
A vertical ellipsis indicates the omission of items from a code example or command format; the items are omitted because they are not important to the topic being discussed.
( ) In command format descriptions, parentheses indicate that you must enclose the options in parentheses if you choose more than one.
[ ] In command format descriptions, brackets indicate optional elements. You can choose one, none, or all of the options. (Brackets are not optional, however, in the syntax of a directory name in an OpenVMS file specification or in the syntax of a substring specification in an assignment statement.)
[|] In command format descriptions, vertical bars separating items inside brackets indicate that you choose one, none, or more than one of the options.
{ } In command format descriptions, braces indicate required elements; you must choose one of the options listed.
bold text This text style represents the introduction of a new term or the name of an argument, an attribute, or a reason.
italic text Italic text indicates important information, complete titles of manuals, or variables. Variables include information that varies in system output (Internal error number), in command lines (/PRODUCER= name), and in command parameters in text (where dd represents the predefined code for the device type).
UPPERCASE TEXT Uppercase text indicates a command, the name of a routine, the name of a file, or the abbreviation for a system privilege.
Monospace text Monospace type indicates code examples and interactive screen displays.

In the C programming language, monospace type in text identifies the following elements: keywords, the names of independently compiled external functions and files, syntax summaries, and references to variables or identifiers introduced in an example.

- A hyphen at the end of a command format description, command line, or code line indicates that the command or statement continues on the following line.
numbers All numbers in text are assumed to be decimal unless otherwise noted. Nondecimal radixes---binary, octal, or hexadecimal---are explicitly indicated.

Chapter 1
DECram Disks

This chapter describes the DECram disk, which is a disk device created in physical memory. The operating system can read from and write to a DECram disk, using standard OpenVMS disk I/O operations, at access times much greater than those for standard hardware disks. DECram disks use the DECram for OpenVMS device driver (MDDRIVER).

1.1 Characteristics of a DECram Disk

The DECram driver allows you to create a disk in physical memory and to read and write to that disk using standard OpenVMS disk I/O operations. This provides high-speed access to read-only data such as libraries, fonts, and command files. Additionally, the DECram disk can be used to hold temporary or scratch files that may be required by an application.

Other characteristics include the following:


Because DECram disk data is resident in main memory, the data will be lost if the host system fails or is shut down. Therefore, Compaq recommends using the DECram disk to store small, frequently accessed files such as temporary (scratch) files, or read-only files such as commonly used image files that reside permanently on a conventional disk.

1.1.1 Comparing a DECram Disk to a Conventional Disk

Table 1-1 compares the features of a DECram disk to conventional disk features.

Table 1-1 DECram Disk Versus Conventional Disk
DECram Features Conventional Disk Features
Type of Storage Device
A virtual disk that operates like a physical disk device. Disk device.
Device Setup
Set up once each time the system is rebooted. Set up at installation time. There is rarely, if ever, a need to set up the device again.
Models Available
Only one model, although the size can vary. Upgrading is not a consideration. Many models, each fixed in size. Upgrading is always an important consideration. Options include price/performance, size of the disk, density, and seek time.
Use as a Storage Device
Typically holds small files. Expansibility is limited; cannot be used for offline storage. Holds files of all sizes. Expansible to several orders of magnitude more than the amount of data that can be stored on a DECram disk. Some models have removable packs for secure offline storage.
Write Operations
Stored data can be lost if system fails or is shut down. Writable, permanent files should not be placed on the DECram disk. Suitable for writable, permanent file storage.
Data Access Performance
Extremely fast, limited only by CPU power and memory bandwidth, which are electronic in nature. Speed limited by mechanical considerations, that is, seek time and rotation speed. Performance is also limited by other factors, such as interconnect bandwidth and controller features.

1.1.2 Comparing a DECram Disk to Disk Caches

Both DECram disks and disk caches improve system performance by providing faster access to data. However, they differ in how they function and in how they are used by the system. Table 1-2 describes these differences.

Table 1-2 DECram Disk Versus Disk Caches
DECram Features Disk Cache Features
Type of Storage Device
Operates like a disk device. Operates as memory.
Device Setup
Requires preloading files and using logical names to access these files on the DECram disk. A disk cache is easily configured. The cache is usually transparent to the user.
Use as a Storage Device
Typically holds entire files. Typically holds only portions of files.
Due to their fixed size, cannot hold large files. Maximum disk capacity is 524,280 blocks. Capable of holding entire databases.
Both data blocks and file system blocks can be held. Consequently, opening and closing files is faster. Disk caches often migrate to holding data blocks, as opposed to file system blocks.
Type of Data
Choice of files to be held on the disk; subject to size constraints. Choice of files to be held on disk caches not usually allowed.
Write Operations
Not intended for permanent data storage. No write-back of data to permanent storage media. Write operations write-back data to permanent storage media.
Generally more efficient than a write to a disk cache; scratch files are created relatively quickly. Writing to a disk cache is slower than writing to a DECram disk. Creating scratch files is a relatively slow process.
DECram disk files cannot survive a system shutdown. Data blocks survive a system shutdown.
Data Access Performance
Always reliable. Files are under the user's control and are readily accessible. Unpredictable. Access varies with disk size and system load. (However, access is always more efficient than on comparable systems without disk caches.)
Recommendations for Use
Used for small, frequently accessed files such as application scratch files, system images, libraries, and DCL procedures. Disk caches should be used for databases, general user work files, and files that increase in size.

Some disk caches use a protocol to write-back data where the software is informed a write operation is complete before ensuring the data is actually stored on the disk. Although write-back by a disk cache increases write performance to a level comparable to a write to a DECram disk, data can be lost if the system shuts down before the modified data is written back to disk. Users should understand that the write-back characteristics of a disk cache can result in lost data.

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