V Block 1
Segment 2 Segment 1
~~
Figure 52. Spanned Variable-Length Records
thus be longer than a block, and may in fact span several blocks. As a resul t, block length can be defined in such a way as to optimize the use of auxiliary
storage, without regard to record length.
No special progra~ng is necessary to make use of spanned records, but programs can be written to process either complete records or record segments. Each segment contains a field that defines i t length and its relative position within the record. The field indicates whether the segment is the first segment, the last segment, or a
middle segment, or whether the segment is a record complete in itself.
One or several related blocks of data separated by gaps form a data set or, as i t is often called~ a data file. Each set of data represents an organized body of
related information, such as all of the information concerning a series or group of sales transactions. Data sets are usually independent of one another, both logically and physically. They may, for example, be stored in different auxiliary storage volumes.
A volume is a section or unit of
auxiliary storage space that is serviced by a single read/write mechanism whose
operation is entirely independent of any other read/write mechanism. In an
operating system installation, a volume may be:
• A reel of tape.
• A disk pack..
• A data cell.
• A drum.
• A section of an IBM 2302 Disk Storage (Model 3 or 4) serviced by a single read/write mechanism.
They are called volumes because, like collections of books, they store related sets of information (data sets).
Sometimes, like books, they can be moved from place to place (disk pack and tape reel volumes, for example). Often the volume containing a particular set of data
Segment n
~
' - - - y - - - '
Block 3 Block n+ 1
is located by searching through a series of indexes, in much the same way as a book in a library is found. Also, each direct access volume contains a table of contents defining sets of data contained in the volume and tells where they are located.
Library Reference System
In a medium or large scale data processing organization, keeping track of data can be a formidable undertaking. This is true in organizations that attempt to take full advantage of the benefits offered by direct access storage devices.
At anyone time a great many data sets or files may exist within the organization.
The bulk of these are usually stored on magnetic tape or in direct access storage.
Usually each programmer must keep track of the tape and direct access storage volumes on which his data sets are stored. For direct access storage volumes, he must also keep track of exactly where on a volume his data sets are located.
Furthermore, the operations staff at an installation must assign space for storing data sets. They may assign removable tape and disk pack volumes to programmers and, in the cas,e of direct access storage volumes, they may also assign specific areas of storage on particular volumes. In any event, the operations staff must
maintain up-to-date records on the
assignment of space for storing data and must systematically control its use and disposition.
Over a period of time, as new data sets are created and old data sets are
abandoned, the problem of managing the data mass becomes more severe. This happens not only because of an increased number of data sets (both current and obsolete) but also because human beings become too much
involved in the mechanics of managing data and more subject to error. As a result,
Information Management 69
data sets may be destroyed at times because of mixups i.n space assignments, and
programmers may abandon installation procedure::;, maint.ain their own private library of removable volumes, .and avoid sharing storage space and data with one another.
Another proble!m facing many
installatiClns is the problem of using operations staff to reorganize and
consolidatE! the (lata sets on the vol urnes can be a difficult undertaking involving many individual users.
Because of such problems, the operating system cont:ains 2l built-in library
reference system that is used to classify searching through the corresponding major class index and each corresponding subclass index, in turn, the control p:['ogram can automatically find and allocate unused space on the volume f or the 'temporary or only distinction among the different types of data is in the nature of the data itself and the way in which i t is used.
Most of the data sets that make up the operating system and that are used by the operating system in performing its work are cataloged within the library reference system. Once the operating system is generated, a system programmer can extend the catalog using system utility programs specifically provided for that purpose.
For example, he can construct a catalog example, to reflect the organizational structure of an engineering department that uses the system.
Figure 53.
~~
See Figure 56
To Other Indexes
Generation Data Groups
simplified Diagram of Catalog System for Locating a Volume
Information Management 71
Although the highest level index of the catalog is always stored on the direct access volume containing the operating system control program, branches of the catalog can be stored on other direct access volumes, including removable volumes. For example, the index named DESIGN in Figure 53, as well as any subclass indexes, could be stored on a removable disk pack that is assigned for exclusive use by 1:he design dl~partment.
In addition to cataloging single data sets in the library reference system,
several successive generations (updates) of a data set (called a generation data group) can be cataloged.3 This method can be used to catalog a data set, such as a
year-to-date earnings data set, that is updated weekly by a payroll program. Each generation of the data set ma~i have the same name and be identified rl~lative to the current genE~ration of the data set. With this method of cataloging,thl~ system can automatically keep track of the generations that have bE~en created., and delete obsolete generations as new ones are created.
A special type of direct access data set, called a partitioned data set can also be cataloged (Figure 54).3..0 This type of