Expanding OpenVMS Memory

When you expand OpenVMS memory, there are a number of other parameters you may wish to revisit. If you increase your memory dramatically, you will certainly have to change these SYSGEN parameters. You can also look each parameter up using HELP:


(The parameter SYS is short for SYS_PARAMETERS.)

Some parameters to consider changing are the following:

  • GBLPAGES. If you don’t increase this, you’ll be getting warning messages when you try to take advantage of all that memory. In short, this parameter sets the amount of memory that the kernel can keep track of; if you use too much this parameter is a limiting factor.
  • GBLPAGFIL. The page file needs to be able to take all of the pages that it might be called upon to reserve; increase this parameter.
  • VCC_CACHE_MAX. If you’ve not tuned your cache (XFC) then you’ll find half of your memory to be taken by the cache. This is almost certainly not what you want; modify this parameter to reduce the amount of memory your cache is allowed to take. Even so, do remember that your cache will decrease and increase dynamically in any case – but if you scale it back, then you’re not wasting memory so much.
  • MAXPROCESSCNT. This sets the maximum number of process slots – in essence, the maximum process count (which is what the parameter is called, after all). If you have a lot more memory, you’ll want to use it to run more, right? That’s not any good if you use too many processes and can’t run any more.
  • BALSETCNT. If you set MAXPROCESSCNT, you should set BALSETCNT to the same amount minus two – and never higher.

These changes can be made in the SYS$SYSTEM:MODPARAMS.DAT file and then use the AUTOGEN command to configure the sysetm. The MODPARAMS.DAT file uses a simple format; for our purposes, you can use something like this:


In place of ADD_* you can also use MAX_* or MIN_*. You can see more examples in HELP AUTOGEN MODPARAMS.DAT. AUTOGEN is described in the HELP; be careful using it! You don’t want to muck up the system so bad you have to reboot or to reinstall.

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