VSAM LDS used in DB2

[Chandra Shekar G R]

Recently I learnt that DB2 uses a VSAM's Linear DS for managing the tablespace. Is this true? If True then KSDS,ESDS,RRDS are known to be a far better VSAM dataset against LDS why can't DB2 use them instead of LDS?


Regards,
Chandra

[Veera]

Chandra,

For DB2 tablespace why LDSS vsam is used because of Extent consolidation i guess.

At the physical level, in DB2 each tablespace is stored in a pageset, which
consists of one or multiple VSAM ESDS data sets or LDSs.

Extent consolidation
z/OS consolidates adjacent extents for VSAM SMS managed data sets when extending on the same volume. VSAM extent consolidation is automatic and requires no action on your part. If the extents are adjacent, the new extent is incorporated into the previous extent. This means that first extent tracks as seen on ISPF 3.4 Data set level listing shows allocations larger than the primary when extents are adjacent to the primary.

There is a DFSMSdfp (Data Facility Program) limit of 123 extents per volume.

Extent consolidation continues even when an LDS exceeds what would have been 123 extents.

Its more on SYSTEM INTERNALS and related to MEMORY MANAGEMENT , some
system programers should able to explain it better.

Thanks,
Veera.