Content deleted Content added
m →Overview: name= |
→VSAM files: control intervals, control areas |
||
Line 24:
|url=http://www.cedix.de/VorlesMirror/Band1/VSAM01.pdf |title=VSAM Demystified}}</ref>
==
The physical organization of VSAM data sets differs considerably from the organizations used by other access methods, as follows.
A VSAM file is defined as a cluster of VSAM components, e.g., for KSDS a DATA component and an INDEX component.
===Control Intervals and Control Areas===
VSAM components consist of fixed length physical blocks grouped into fixed length control intervals<ref name=VSAM.CI1/><ref name=VSAM.CI2/> (CI) and control areas (CA). The size of the CI and CA is determined by the [[IBM mainframe utility programs#IDCAMS|Access Method Services]] (AMS), and the way in which they are used is normally not visible to the user. There will be a fixed number of control intervals in each control area.
A control interval normally contains multiple records. The records are stored within the control interval starting from the low address upwards. Control information is stored at the other end of the control interval, starting from the high address and moving downwards. The space between the records and the control information is free space. The control information comprises two types of entry: a control interval descriptor field (CIDF) which is always present, and record descriptor fields (RDF) which are present when there are records within the control interval and describe the length of the associated record. Free space within a CI is always contiguous.
Line 37 ⟶ 38:
You can use three types of record-orientated file organization with VSAM (the contents of linear data sets have no record structure):
===
{{further|Entry Sequenced Data Set}}
Line 50 ⟶ 51:
|title=ABCs of z/OS System Programming Volume 3}}</ref> Records are loaded irrespective of their contents and their byte addresses cannot be changed.
===
{{further|Key Sequenced Data Set}}
A KSDS has two parts: the index component and the data component. These may be stored on separate disk volumes.
Line 63 ⟶ 64:
|title=What is VSAM?}}</ref>
===
{{further|Relative Record Data Set}}
| |||