Suppose I have an Oracle database whose data files are 256 GB in size. Is it a good idea to use a server with, say, 384 GB RAM in order to host the entire database in RAM?
Is there any difference if you only have, say, 128 GB RAM?
I'm talking about caching and Oracle inner workings, not memory based filesystem. Suppose OLTP, and a 100 GB working set.
Regards,
On
Assuming you are talking about Oracle using the memory for caching and other processes and not a memory based filesystem (which is an awful idea)... more memory is almost always better than less memory.
The real world answer is it depends. If your working set of data is a few GB or less then the extra memory wouldn't help as much.
How much memory you need and when extra memory stops helping depends on your application and what style of DB (OLTP,DSS) and there is no simple yes/no answer.
Use the views V$SGA_TARGET_ADVICE and V$PGA_TARGET_ADVICE to predict the performance improvement of additional memory.
Oracle records many statistics about physical (disk) and logical (total) I/O requests. People used to obsess over the buffer cache hit ratio. It can be helpful but that number doesn't tell the whole story. If the ratio is 99% then your cache is probably sufficient and adding more memory won't help. If it's low then you might benefit from more memory, or perhaps the processes that use disk aren't time critical.
Be careful before you request more memory. I've seen a lot of memory wasted because some people assume more memory will solve everything. Oracle has many I/O features to help reduce memory requirements. The "in-memory database" fad is mostly hype.