I know that DB2 does not use VSAM KSDS - only for very limited purposes,
IIRC - some logfile directories or boot strap data sets, don't recall
the details.
The tablespace data sets are VSAM LDS - linear data sets.
Anyway: there are some improvements done to VSAM for DB2, AFAIK ...
I believe some improvements to enable list prefetch or other things I don't
recall at the moment ... and I recall only vaguely a paper from ORACLE where
they tell that they benefit from these VSAM improvements, too.
Kind regards
Bernd
Am 21.04.2014 16:59, schrieb Joel C. Ewing:
I'm pretty sure the original motivation behind VSAM was to provide a
more efficient keyed-access alternative to an earlier ISAM (Indexed
Sequential Access Method) keyed-access method --ISAM had abysmal
performance when the number of inserts became large. But, just as a
clarification, while it is true that VSAM under IBM Operating systems
includes support for keyed-access data sets in the forms of KSDS (Keyed
Sequential) and RRDS (Relative Record), VSAM also includes support for
non-keyed-access data set types: ESDS (Entry-Sequenced) and LDS
(Linear).
The LDS data sets on z/OS do not have any organizational structure
imposed upon them by VSAM other than being a stream of consecutive bytes
physically organized in 4KiB, pageable chunks; and DB2 uses these LDS
data sets for its table spaces. So, it is incorrect to think of DB2 as
somehow using or building upon VSAM keyed-access techniques to locate
table rows. DB2 has it own table structures, indexing organization, and
key search techniques built on top of the very rudimentary Linear Data
Set structure.
DB2 required greater flexibility and control to achieve its performance
and data consistency goals than was possible with VSAM keyed-access
support. A very cursory look at "Experiences Installing Oracle
Database 10g on z/OS" would suggest Oracle on z/OS makes use of VSAM LDS
for table storage as well.
Joel C Ewing
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