FOOTNOTES

For the sake of brevity, when the term "System Manager" is used in this book, it will refer to a manager of either Digital's VAX or its Alpha AXP computer system.

goes down: Computer industry slang for "stops running".

For a detailed definition of disk see Chapter 1, The Physical Parts of a Disk. Or see disk in the Glossary.

Fragmentation is defined in detail in the Introduction. Or see fragmentation in the Glossary.

VMS, or VAX/VMS is the name of the operating system most commonly used on VAX computers. However, in 1993 Digital officially changed the name to OpenVMS. The two names are synonymous, and for the purposes of this book, we will use the term "OpenVMS."

Disk drive: This term is defined in Chapter 1, The Physical Parts of a Disk., Or see disk drive in the Glossary.

"Computer" definition excerpted from the Oxford American Dictionary.

multiprogramming: The capability of running two or more programs at the same time without interference.

bit: See the definition in Chapter 1. Or see bit in the Glossary.

32-bit architecture: The capability of processing 32 bits of information simultaneously in each cycle of the computer. Like a 32-lane highway, the path on which data flows through the computer carries 32 bits of data at once.

virtual address extension: The address of a location in a conceptual memory space that may or may not correspond to a location in the computer's physical memory, but which is translated by the computer in such a way as to make it appear that it does so correspond. This definition of VAX is fromThe Digital Dictionary, Second Edition, Digital Press, Bedford, Massachusetts, 1986.

RISC: Reduced Instruction Set Computer. By using simple sets of instructions, processing speed can be increased considerably.

"Disk" definition excerpted from The Digital Dictionary, Second Edition, Digital Press, Bedford, Massachusetts, 1986.

architecture: Design, character or style.

file structure: The standard format used for arranging files on a disk.

The concept of defining misunderstood words is more fully explained in The Basic Study Manual, by L. Ron Hubbard, Chapter Two: The Barriers to Study. Bridge Publications, Los Angeles, 1990.

system disk: That disk from which the operating system is loaded into memory when the computer is booted up.

CD-ROM: Compact Disc Read Only Memory. A disk identical in form to audio CDs.

RAM: Random Access Memory. Storage which can be accessed directly, without having to search through other storage locations.

memory: A part of the computer in which data can be stored and retrieved rapidly (in about one millionth of a second).

on-line: Directly connected to the computer system and available for use. The opposite, off-line, means that the object referred to is either not directly connected to the computer system or is otherwise inaccessible.

The term formatted is used to mean that the disk has certain marks that trigger reading and writing of data in particular spots, allowing storage and retrieval of data in groups of a particular size; in this case, 512-byte blocks.

megabyte, or MB: 1,048,576 bytes, or approximately one million bytes.

Before data can be stored on the disk, space for the data must be allocated from whatever remains available. One cluster is the minimum number of blocks that can be allocated.

cache: A very fast memory that can be used in combination with slower, large capacity memories.

microsecond: One one-millionth of a second.

millisecond: One one-thousandth of a second.

queue: A sequence of items waiting for service, like people in line at the checkout counter in a store.

I/O: Refers to the transfer of data between the CPU and a peripheral device, such as a disk device.

data structure: A design for the way data is laid out in a program.

Note: OpenVMS also uses the extent cache to allocate storage. The extent cache is defined in Chapter 1. Or see extent cache in the Glossary.

ODS-2: On-Disk Structure Level 2. The name of the file structure used on most OpenVMS disks.

"File" definition excerpted from The Digital Dictionary.

"Block" definition excerpted from The Digital Dictionary.

"Directory" definition excerpted The Digital Dictionary.

The phrase "under the head" is figurative. Sometimes the head is underneath the platter, so the data would have to be "over the head." Nevertheless, the phrase "under the head" is used to describe both conditions.

RPM: Revolutions Per Minute

The average access time is not exactly equal to half the sum of the rotational latency plus the seek time, as the rotation and the seek occur at the same time. The average access time, therefore, is somewhat less than the total.

Chunk: Due to the way OpenVMS disks are structured, disks in a stripeset are divided into "chunks" rather than clusters, chunks consisting (usually) of more blocks than clusters.

PDP-11: Programmable Data Processor-11. A 16-bit computer on which the VAX (originally, the VAX-11/780) was based. The PDP-11 introduced virtual memory to the Digital world, although early versions of the PDP-11 lacked this capability.

PDP-8: Programmable Data Processor-8. A 12-bit computer preceding the PDP-11. The PDP-8 is widely regarded as the first minicomputer which, at that time, was simply defined as a computer which cost less than $100,000. Considering inflation, the cost of such a computer would be closer to $1 million today.

RSX-11: Real-time Resource Sharing eXecutive for the PDP-11 computer.

field: An area reserved for data.

Bucket: A storage structure of 1 to 32 disk blocks that is used to store and transfer data in files. Unlike a block, a bucket can contain only entire records.

INDEXF.SYS: The name of the file that contains the information necessary for OpenVMS to identify and access the disk and the files contained on it. INDEXF stands for index file. The file type .SYS indicates that this is a system file, reserved for use by the OpenVMS operating system.

Volume: In a volume set, there is an INDEXF.SYS file on each disk in the set.

Mounted: The term mounted is used to describe a volume that has been introduced to the system and made available for use.

Zillion: A technical term meaning "a lot."

initialize: The process by which the computer prepares a . . . disk for handling user information. This process erases any information that was on the disk. (Digital Dictionary)

Disk device designation: In OpenVMS, a disk is referred to by a unique code consisting of letters and numbers that indicate exactly which disk is meant. In the example, "D" means it is a disk device, "U" indicates the type of disk, "A" indicates that it is the first controller for disks of that type on this computer (the second would be "B", the third "C" and so on), and "1" indicates that it is the second disk drive associated with that controller (the first would be "0").

"holds seven pointers": While seven is the default number of pointers, a file can be opened using "cathedral" windows that can accommodate a great many more pointers.

bottleneck:The severity of the performance bottleneck due to window turns is largely dependent upon the type of file being accessed. It is most severe for indexed files, minor for sequential files and rare for direct (random) access files.

Digital has released a product, DECps, which replaces SPM and VPA, incorporating the functionality of both products.

Disk Analysis Utility: For a free copy of the Disk Analysis Utility, write to Executive Software at 701 North Brand Boulevard, 6th Floor, P.O Box 29077, Glendale, California 91209-9077 USA or call 1-800-829-4357. It can also be downloaded from our website for OpenVMS or Windows NT.

Total Free Space Size: If the user does not have system privileges, the Total Free Space Size can be misleading because it omits free space in the extent cache in other nodes of a cluster.

compatibility mode: Some VAXes have the ability to run PDP-11 programs as if the VAX were a PDP-11 computer running the RSX-11M operating system. When operating in this way, the VAX is said to be in compatibility mode.

* After you initialize the disk, however, make very sure you use the /NOINITIALIZE qualifier on the BACKUP command. Otherwise, BACKUP will reinitialize the disk using its default settings.

"...off-line defragmenters.": Watch out, however, for supposed "on-line" defragmenters that are really off-line defragmenters modified to run with users active on the system. Such products suffer from the same drawbacks described for off-line defragmenters.

RMS: Record Management Services. An OpenVMS facility to aid in storing and retrieving file data.

common system disk: A system (OpenVMS-resident) disk that is used by more than one VAX (or Alpha AXP) simultaneously as the system disk for that VAX. Two comparable VAXes using one common system disk doubles the I/O to that disk. Three VAXes would triple the I/O, and so on.

subdirectory: A directory within a directory. Seven levels of subdirectories are allowed in OpenVMS.

system software: That software which is intended solely for the benefit of the computer system, including its environment and operational management. It is differentiated from application software, which has the purpose of providing a specific service or result to users of the computer system.

RSX: Resource Sharing eXecutive. The name of an earlier operating system for Digital's PDP-11 model computer.

DCU: Disk Compression Utility. An off-line style defragmenter for RSX systems.

An n-squared order problem is one that squares in difficulty for each increment in size of the problem. For example, a problem of size 2 has a difficulty of 2 x 2 = 4, while a problem of size 3 has a difficulty of 3 x 3 = 9. As the numbers grow large, the difficulty of the problem snowballs. A problem of size 20, though only ten times as large as the 2 problem, has a difficulty of 20 x 20 = 400, or 100 times the difficulty of the 2 problem.

DECUS: VAX Special Interest Group Question and Answer session held at the DECUS European Symposium at Cannes, France. September 1985.

PAGESWAPPER (newsletter of the VAX Special Interest Group within the Digital Equipment Computer Users Society), February 1986.

VMS Systems Dispatch, August 1989, Digital Equipment Corporation

"...time consuming": By forcing a disk out of service, the backup-and-restore process can also present a serious financial hardship to the organization that depends on the data on that disk for its operations.

"...accessed.": OpenVMS does keep a revision date for files, but this tells you only when the file was last written. No information is kept about reads of files.

"...accessed at all.": There is an archiving mechanism used for determining when a disk volume or the files on it should be archived, but this mechanism does not properly serve our purpose and, worse, imposes a high amount of overhead.

system root directory: A top-level directory that contains system files and subdirectories. All files used by the operating system are in this directory or subdirectories under it. System root directories are named SYS0, SYS1, SYS2, and so on.

Murphy's Law: "If anything can go wrong, it will."

It sometimes occurs that a file will contain a disk block that is supposed to be a part of some other file. Such a block might even appear to be in both files. This type of error is referred to as an allocation error and opens the door to further data corruption when the file is deleted, such as after being copied to a new location by a defragmenter.

incremental backup: A backup in which the only files saved are those which have been created or modified since the last backup. The OpenVMS BACKUP utility optionally records the backup date in the header of each file saved. This gives a later incremental backup the ability to determine whether or not a file needs to be saved to have a complete copy of all files without having to save copies of the same file repeatedly.

Saveset: A container file holding the data saved during backup.

"...file": Changing the extent size and location data in the header requires changing two additional pieces of administrative information. The header contains a count of the number of map area words and this must be changed when a defragmenter reduces the number of extents. There is also a special number used for data integrity purposes and this must be updated whenever there is any change to the header.

Volume retention dates are used by OpenVMS to determine when a file on the volume "expires." When a file is created, its expiration date is set to the current date and time plus the specified maximum retention time. Each time the file is accessed, the current time is added to the minimum time. If the sum is greater than the expiration date, the expiration date is recomputed. This allows the System Manager to determine which files are not being used and so might be candidates for archiving.

"...lost performance": Lest anyone complain that electronic disks are so fast that any argument about optimization or defragmentation is moot, let me point out that "fast" is a relative concept. I know of at least one application that requires a large array of electronic disks with striping, data caching and defragmentation to achieve the necessary performance. File placement adds absolutely nothing to the speed of this application.

HSC: Hierarchical Storage Controller. A disk and tape management facility physically separate from the computer(s) it serves.

MSCP: Mass Storage Control Protocol. A software system that allows computers to access disks on other nodes in a VAXcluster.

LAVC: Local Area VAXcluster. A VAXcluster that communicates over Ethernet rather than specialized VAXcluster hardware.

See Chapter 1, HOW A DISK WORKS for an explanation of QIO. Or see QIO in the Glossary.

installed image: An executable program that is kept in an "always ready" state. Part of the program is kept in memory and the program image file is kept open so the program can be started faster than it could be otherwise.

The home block contains the disk volume label, protection information about the volume and the location of the INDEXF.SYS file. There are several copies of the home block in addition to the original.

"Backup and Restore": It is worth noting that the file INDEXF.SYS is created by the DCL INITIALIZE command with a default size of 16 blocks, then extended a few blocks at a time as needed, even when INDEXF.SYS is created by OpenVMS BACKUP. It is highly likely that these numerous small extensions will cause the file to be badly fragmented, ruining free space consolidation on your disk. I recommend that you initialize your disks using the /HEADERS=n qualifier and specifying a value for n that is a bit higher than the total number of files you expect to store on that disk. A recent full backup log will tell you how many files are on the disk now.

An alternate pagefile is a second or third file that can be used for paging. Swapfiles can have alternates as well.

"...not installed": Page and swap files are installed using the OpenVMS SYSGEN utility. See the OpenVMS documentation set for further information.

The idea for this memo layout is taken from the essay Completed Staff Work (CSW) - How to Get Approval of Actions and Projects, by L. Ron Hubbard.