My main background is z/VSE but now I have to manage a bunch of z/OS
sites, including one that recently converted from z/VSE to z/OS.
On z/VSE, share-option 4 means that VSAM will prevent any read or write
integrity exposures when multiple tasks are accessing the same VSAM file.
z/VSE VSAM will internally lock any CI that is being updated so that
nobody else can update the CI. This ENQ/DEQ is handled by the IBM
provided VSAM IO routines at the task level. Additionally, VSAM will
flush all update buffers after a write or update. And, it will not
buffer reads when reading a share-option 4 file. (I am being somewhat
general in the descriptions, so the details are a little more
complicated.) All this to make sure that the records on disk and the
records in buffers match.
Now, with z/OS, my reading of the VSAM Demystified RedBook leads me to
the following:
1) Share-option 4 allows multiple open for update, but expects the
program, not the VSAM subsystem, to perform the ENQ/DEQs.
2) If a program does not perform ENQ/DEQs, then data integrity is lost
as multiple tasks can update the same record concurrently.
3) VSAM/RLS is one way to protect the data, but that is another can of
worms.
Am I understanding the z/OS side correctly?
--
Tony Thigpen
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