VSAM terminology and concepts

VSAM terminology and concepts
Logical record - A logical record is a unit of information used to store data in a VSAM data set.
Ways to identify logical records
In VSAM there are three ways to identify a logical record:
  • Key field - An important field in the logical record is the key. Its contents can be used to retrieve the specific logical record.
  • Relative byte address (RBA)
The RBA of a logical record is the offset of its first byte from the beginning of the file. Then, the first logical record in a VSAM file has an RBA of zero; the second logical record has an RBA equal to the length of the first record, and so on. VSAM returns to the application the RBA of the stored logical record. The RBA of a logical record in VSAM includes the free space and control information stored before this logical record. RBAs might change when records are added, deleted, or changed in size. With compressed files, the RBAs for compressed records are not predictable. Therefore, access by RBA is not suggested for normal use. There are two important RBA values, kept in the Catalog:
      High used RBA (HURBA) is the RBA of the first available byte for inclusion at end of the VSAM file.
      High allocated RBA (HARBA) is the RBA of the last byte in the VSAM file
  • Relative Record Number (RRN)
It is the relative number of a logical record in a VSAM specific organization (RRDS). If a logical record has an RRN of 3 means that it is the fourth logical record in the file.


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