W dniu 24.05.2024 o 04:32, Phil Smith III pisze:
I'm curious whether any of you old-timers can explain why we have both VTOCs
and catalogs. I'm guessing it comes down to (a) VTOCs
came first and catalogs were added to solve some problem (what?) and/or (b)
catalogs were added to save some I/O and/or memory, back
when a bit of those mattered. But I'd like to understand. Anyone?
Yes, your guess is correct - it is matter of timeline and *compatibility*.
However it is not like "catalogs replaced VTOC".
IMHO the crucial information kept in VTOC is disk map, physical location
of extents.
For compatibility reasons the VTOC also keeps parameters of nonVSAM
datasets, like RECFM, LRECL, BLKSIZE, etc.
However new thing like VSAM had parameters which could not be kept in
old existing VTOC fields. Change VTOC could mean incompatibility, so
they decided to use another structure called VVDS. Note - VVDS is "per
volume", so it is more like VTOC extension.
The "catalog" usually understood as BCS is not tied 1:1 to volume, it is
kind of database linking dsname withe the volume *and* further
information residing in VTOC/VVDS.
--
Radoslaw Skorupka
Lodz, Poland
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