IBM 1130 User Manual page 95

Know Your Data
Before starting into a long discussion of how to
protect disk data, let us review the various types
of data fields in your disk records and determine
which, if any, are worth protecting. Naturally,
you don't want any of your data lost, but certain
items are more important than others, since they
are much more difficult to replace.
Take a typical payroll file, where there is a
record for each employee:
1. Employee number
2. Name, address, city and state
3. Indicators - marital status, sex, number
of dependents, etc.
4. Pay rate
5. Year-to-date dollar figures - gross,
taxes, etc.
6. Quarter-to-date dollar figures - gross,
taxes, etc.
7. Miscellaneous cumulative - days vacation,
sick leave taken, etc.
Section Subsections Page
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10
The first four items are comparatively static,
seldom changing, but the latter three probably
change every pay period.
01
If an accident occurs (you should assume the
worst possible case), the entire record for every
employee is lost. How would you reconstruct your
data file? The first four items are easy - the
latest information probably exists in the form of
a card deck and can simply be reloaded onto the
disk. That is how it got there in the first place.
However, the last three items present a different
picture - they change each pay period. When you
write the updated disk record, this week's total is
written over last week's total, and last week's
total disappears from the disk. Unless you take
definite steps to save it before writing on top of
it, last week's total will completely cease to exist.
Some disk data fields, therefore, are more
critical than others - particularly those that
change often, are modified on the basis of previous
data (for example, year-to-date gross), or are
not kept in duplicate copies.