Analyze the Synonym Chain Analysis
report to understand how efficiently the root segments are distributed
by the randomizing routine (randomizer).
Before you begin
Ensure that you can access the Synonym Chain Analysis report
for the subject DEDB area.
Not all utilities of IMS Fast Path Solution Pack can produce
this report. Use the appropriate utilities and runtime options to generate these reports. For
details about utility options and reports that can be generated, see the topic "Generating the analysis reports" in the IMS Fast Path Solution Pack: IMS High Performance Fast Path Utilities
User's Guide.
Before you start, consider reviewing the database
record profile to obtain the average length of database records, the
average number of root I/Os, and the average number of database record
I/Os even if you have not received exceptions that are related to
the average database record length, root I/O, or record I/O (see Analyzing database record profile). These figures
are useful indicators to complete the steps in this topic.
About this task
One of the most frequently overlooked aspects of the database
design and implementation process is the selection of the randomizer
that best matches the key values of the database records. Using a
randomizer that does not match the key values can significantly affect
the performance characteristics of a database.
In
this task, you will first check the synonym chain distribution, then
check the synonym chain effect on root I/Os.
By
completing this task, you will be able to evaluate whether the current
randomizing logic matches the current distribution of database record
key values. If the logic does not match, you will be able to evaluate
how this mismatch affects the efficiency of root I/Os.
Procedure
Complete the following steps to
determine the randomizing efficiency:
Check the synonym chain distribution.
Browse
the Roots per RAP Distribution Graph in the Synonym Chain Analysis
report. The following figure shows an example of the Roots per RAP
Distribution Graph in the Synonym Chain Analysis report.
Figure 1. Roots per RAP Distribution Graph of Synonym Chain Analysis report
ROOTS PER RAP DISTRIBUTION GRAPH
1
1 2 3 4 5 6 7 8 9 0
0 0 0 0 0 0 0 0 0 0 % RAPS
--------------------------------------------------
# 0-|* (234)
1-|*** (963)
R 2-|******* (2009)
O 3-|********* (2834)
O 4-|********** (2954)
T 5-|******** (2493)
S 6-|****** (1812)
7-|*** (995)
/ 8-|** (569)
9-|* (258)
R 10-|* (124)
A 11-|* (32)
P 12-|* (16)
13-|* (7)
14-|
SYNONYM CHAIN STATISTICS
SYN CHAIN LONGEST: 13 RAP RBA: 006C4000
SYN CHAIN MAX I/O: 2 RAP RBA: 006C4000
SYN CHAIN LENGTH : AVG: 4.49
NO. ROOTS PER RAP: AVG: 4.27
Analyze this graph and the synonym chain statistics below
the graph to determine the following information:
Whether the general distribution of RAP synonym chain lengths
is acceptable.
In each row, the number on the left represents the number of root
segments that are assigned to a RAP. The number of asterisks (*) represents
the percentage of the RAPs to which the same number of root segments
are assigned. The numbers on the right (in parentheses) represent
the number of RAPs. For example, the third row (where the number on
the left is 2) represents the percentage and the number of the RAPs
to which two root segments are assigned.
The average length of RAP synonym chains is shown on
the SYN CHAIN LENGTH line in the Synonym Chain Statistics section.
Generally, if the peak in the graph is near the
bottom, the graph suggests that many long synonym chains exist.
Whether an exceptionally long synonym chain exists.
Existence
of such synonym chain might indicate that the randomizer does not
match the distribution of root sequence key values.
Whether the percentage of unused RAPs, which is indicated by the
first row in the graph, is high.
If this number is high
and the value for the average length of RAP synonym chains is also
high, the current randomizer might not match the distribution of root
sequence key values.
Check the synonym chain effect on root I/Os.
Browse
the synonym chain analysis section in the Synonym Chain Analysis
report.
This section of the report shows statistics about the
placement of root segments in RAA BASE, DOVF, and IOVF sections of the area.
The report also shows the effect on the number of physical I/Os that
would be required to read a root segment in a synonym chain for each
synonym chain length.
The first three columns are tabular representations
of the Roots per RAP Distribution Graph.
The
next six columns indicate, for each RAP synonym chain length, the
following statistics about the root segment occurrences in each section
of the area:
The average number (AVG.)
The maximum number (MAX)
The minimum number (MIN)
The standard deviation (SDEV)
The first two columns are for the RAA BASE section, the next two columns
are for the DOVF section, and the last two columns are for the IOVF
section.
AVG./SDEV
The AVG./SDEV columns (the first column for each section) indicate
the average number of root segment occurrences over all the synonym
chains of that chain length are displayed with the standard deviation
from the average.
MAX/MIN
The MAX/MIN columns (the second column for each section) indicate
the maximum and the minimum number of root segment occurrences among
all the synonym chains of that chain length are displayed.
For each section, a low standard deviation indicates
that, within a group of synonym chains that have the same length,
the number of root segments that occur in a synonym chain of that
group and that reside in that section tends to be very close to the
average number, over all synonym chains in that group, of root segment
occurrences in that section. A high standard deviation indicates that
the root segment occurrences are spread out over a large range of
values.
The last four columns indicate
the statistics for physical I/Os as follows:
AVG.
The AVG. column indicates the average number of I/Os that are
required to read a root segment in the synonym chain of that chain
length. The average is calculated based on all the synonym chains
that have the same length.
SDEV
The SDEV column indicates the standard deviation from the average
number.
MAX and MIN
The MAX and the MIN columns indicate the maximum and the minimum
number of root I/Os among all the synonym chains that have the same
length.
In this section, pay attention to the existence
of synonym chains that make the average number of I/Os high or that
have an exceptionally high (maximum) number of I/Os.
The
following figure shows the synonym chain analysis section in the Synonym
Chain Analysis report.
Figure 2. Synonym chain analysis section of the Synonym Chain Analysis
report
After you identify the cause of the exceptions, the next
step is to determine the actions to tune the subject area. The actions
might include tuning of the randomizer to make the database fit the
current characteristics of database records and application usage.
See Determining actions to resolve exceptions to determine
the appropriate tuning tasks.