Technical data

Managing Storage Media
9.1 Understanding Storage Media Concepts
Figure 9–2 Record Blocking
IRGIRGIRGIRGIRGIRGIRGIRGIRGIRG
Record
Without Record Blocking
With Record Blocking
All 10 Records
ZK−0741−GE
Record Record Record Record
Record Record Record Record Record
IRG
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.
9.1.3.2 Multiple Tape Densities (Alpha Only)
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, 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:
• $ INITIALIZE
• $ SET MAGTAPE (for tapes only)
Refer to the 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:
• $ MOUNT (with /FOREIGN qualifier for mounted tapes)
• $ BACKUP
9–12 Managing Storage Media