ABCs of z/OS System Programming Volume 3 - IBM Redbooks

4.35 VSAM: Processing a KSDS cluster
Index
Component
Data
Component
H
D
R
Application: GET record with key = 23
7 11 14 21 26 38
2 5 7
8 9
12 13 14
15 16 19
22 23 26
31 35 38
HDR 38 67
Control
Interval Control Area Control Area Control Area
Figure 4-45 Processing an indexed VSAM cluster: Direct access
H
D
R
43 50 54 57 64 67
39 41 43
44 45 46
51 53 54
55 56 57
58 61 62
65 66 67
HDR 67 95
Application
Processing a KSDS cluster
A KSDS has an index that relates key values to the relative locations in the data set. This
index is called the prime index. It has two uses:
► Locate the collating position when inserting records
► Locate records for retrieval
When initially loading a KSDS data set, records must be presented to VSAM in key sequence.
This loading can be done through the IDCAMS VSAM utility named REPRO. The index for a
key-sequenced data set is built automatically by VSAM as the data set is loaded with records.
When a data CI is completely loaded with logical records, free space, and control information,
VSAM makes an entry in the index. The entry consists of the highest possible key in the data
control interval and a pointer to the beginning of that control interval.
When accessing records sequentially, VSAM refers only to the sequence set. It uses a
horizontal pointer to get from one sequence set record to the next record in collating
sequence.
Sequence
Set
Request for data direct access
When accessing records directly, VSAM follows vertical pointers from the highest level of the
index down to the sequence set to find vertical pointers to the requested logical record.
Figure 4-45 shows how VSAM searches the index when an application issues a GET for a
logical record with key value 23.
H
D
R
HDR 95
71 75 78 85 92 95
68 69
72 73 74
76 77 78
79 80 85
86 89
93 94 95
Index
Set
Chapter 4. Storage management software 167