In our installation, we use VTOC(0,1,2024) INDEX(135,0,405) for mod27
This is not a big issue unless you need the space, but your choice of
405 tracks for the VTOCIX is grossly oversized. In the ICKDSF manual
there are formulas for calculating the VTOCIX size but they are awkward
since they depend on knowing the pattern of dataset names on the
volume. Similarly named datasets take up much less room in the VTOCIX
than disparate names. But anyways there is a table of the max size of
the VTOC and VTOCIX by volume size. For 3390-27 it shows 9828 for the
VTOC and 491 for the VTOCIX. These are both based on an entire volume
of single track datasets with names that don't compress well in the
VTOCIX.
Since your VTOC is only 2024 tracks, the VTOCIX only needs to be about
5% of that, or 100 tracks, and could probably be less. But there is no
penalty for making the VTOCIX too large, since it only uses the tracks
it needs to store the data it has. Extra tracks are unused.
It is the opposite in the VTOC, where every track is formatted with
Format 0 (unused) DSCBs, and many software products need to read the
entire VTOC. So oversizing the VTOC can impact the performance of
products like backup software, reporting tools, etc.
We just dealt with a customer who made every -27 the max size VTOC of
9828 tracks, but they had a good case. They found that they had many
very small datasets on these volumes and they had a number of "VTOC
full" conditions, so they just made all the VTOCs the max size.
We get calls from customers rather frequently asking this exact same
question. I think it is time for IBM to provide some more detailed
guidance on VTOC and VTOCIX sizes.
--
Bruce Black
Senior Software Developer
Innovation Data Processing
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