It seems you may be misunderstanding what I said. Let me try again.

The three things that may need a lot of memory are the file system buffers, the SGA, and Oracle server processes. If you use raw or direct file system I/O on Solaris then the file system buffers can be small, and a good starting point would be to allow up to 70% of physical memory for the SGA and about 20% for the Oracle server processes. If you use buffered I/O on Solaris, then a good starting point would be to allow about 30% of physical memory for file system buffers, up to 40% for the SGA and about 20% for Oracle server processes. These are NOT rules. They are reasonable starting points.

Yes, if you have more than one instance sharing a domain, then these starting point guidelines can be applied to the sum of the SGAs, and processes.

We're planning to re-org our Solaris E10K, possibly breaking it up into more domains than at present and I'm trying to figure out the effect on the various oracle databases.

I've been reading Steve Adams' IXORA site about memory and he suggests I should limit the Oracle processes to 30% of physical memory. We will be limited to a max of 4Gb memory on some domains.

Does all the above mean that the maximum SGA for all the databases on a single domain must sum to 30% or less of the pyhsical memory? What will happen if we exceed that figure? Will the effect on performanace be greater if we exceed the 30% rule or if we downsize the SGA etc. to stay within it?

I know this is not an exact science and I will have to re-tune everything afterwards but some initial pointers would be helpful please.