Record Lengths And Sector Utilization - IBM 1130 User Manual

RE CORD LENGTHS AND SECTOR UTILIZATION
Remember, the disk is physically composed of
sectors, each containing 320 words. A symbolic
record may not cross the boundary between two
physical sectors; in other words, a record must
lie entirely within one sector of 320 words. This
means that a record cannot exceed 320 words in
length. (Actually, it is possible to have records
longer than 320 words, using a trick covered in a
later subsection.)
It
does not mean that only one
record may occupy a sector; it is possible that
many records will be placed on one sector. For
example, if your record size is twelve words, you
may place 26 records onto each sector (26x12
=
312
words), with eight words (320-312) words remaining.
These eight words will not be used for two-thirds of
the 27th reco.rd, since that would violate the rule
spelled out above. The remaining eight words will
not be used, and are inaccessible to the FORTRAN
programmer.
It goes without saying that you will gain the most
efficient use of your disk if you utilize all 320 words
of every sector. As the previous example shows,
however, this may not always occur. Figure 80.3
shows the relationship between record size and
sector utilization.
Clearly, certain record lengths result in very
poor disk utilization. Take a 65-word record, for
example.
It
will allow four records per sector,
using 4x65 or 260 words, but leaving the remaining
60 words (about 20% of the sector) unused.
On
the
other hand, if you could reduce the length of that
record by one word, to 64, you could fit five records
in a sector, using 5x64 or 320 words, and wasting
none.
Inefficient use of the disk can have two major
effects on your overall system:
1. A given number of records may require more
space than is available on the disk. If you have 800
employee records at two per sector, you need 400
sectors or 50 cylinders, fully 25% of the disk. If
you could fit three records per sector, your total
sector requirements would drop to 234, or 30
cylinders.
It
is entirely possible that there are 30
cylinders available on a particular disk, but not 50.
Section Subsections
Page
80 40
I
00
01
In this case you either have to abandon the job, delete
something else from the disk, or shorten the record
size.
2. Even
if
50 cylinders are available, you can-
not escape the fact that you are using them ineffi-
ciently. If your 800 employee records are spread
out over 50 cylinders, rather than 30, you will spend
proportionately more time in disk arm movement.
Your records will be 67% further apart, and your
disk arm seek time will be about the same percent-
age greater.
Thus you have two incentives to make your disk
records as short as possible. Several techniques
for doing so are given in the subsection 80.60.00.
For long records (46 words or more), you should
inspect your record size to determine whether it is
at or slightly above a boundary, or break point --
46, 54, 65, 81, 107, or 161 words. (See Figure
80.3.) If this is the case, it is worth considerable
effort to shorten this record enough to increase the
"packing factor" by one.
For medium records (19 to 45 words in length) the
record size is always near a boundary or break point,
so the packing factor can be increased by one or two
with a small reduction in record length. With records
of this length, however, it becomes more difficult
to find ways to shorten the records.
For short records (9 to 18 words in length), even
greater improvements are theoretically possible,
but are proportionately more difficult to obtain.
NOTE: When shortening disk record lengths,
always keep future needs in mind.
Record Records Record Records Record Records Record Records
Length per Sector Length
per Sector
Length per Sector Length per Sector
320 10 32 19 - 20
16
41 - 45
160
11 29 21
15 46- 53
106 12 26
22
14 54- 64
80 13 24 23 - 24
13
65 - 80
64
14
22
25 - 26
12 81 -106
53
15
21 27 - 29 11 107 - 160
45 16 20 30- 32 10 161 - 320
40 17 18 33- 35 cannot exceed 320
35 18 17 36 - 40
Figure 80.3.