Is there an advantage for me to run 64-bit Oracle on a Solaris box with 16 GB of RAM? What are the disadvantages? (Patching I am aware of.)
The main advantage of 64-bit Oracle is that you can have a very large SGA (max with 32 bit is somewhere between 1.7 and 4 GB). If you were running 32 bit, the amount of RAM over 4 GB would not be available for the SGA sizing.
The following text is from Metalink Note:107201.1:
Oracle 64-bit advantages
The key market for 64-bit databases are high-performance systems for applications that have a very large working data set, and thus can make good use of the improved memory addressing capabilities of a 64-bit architecture.
Using the 64-bit architecture will also improve scalability and the potential for faster performance offered by the 64-bit machine. The current 64-bit Oracle release takes full advantage of the latest HP 64 bit PA-RISC processor technology.
A true 64-bit computing environment has the capability to process 64-bit data, instructions and addressing. The HP system is a true 64-bit environment, with 64-bit processor, 64-bit memory addressing capabilities, 64-bit Direct Memory Access (DMA) and a 64-bit kernel. In a 32-bit system, addressing is limited to 2(32) 32-bit words or 4 GB of memory. With 64 bits we can address 2(64) 64-bit words or 18 billion GB (Exabytes) of memory, representing a huge increase in the amount of memory that can be addressed. 64-bit processors achieve better performance by carrying out 64-bit integer and floating point integer arithmetic operations.
One important advantage of 64-bit memory addressing is the improved scalability of the machine. Applications can store more data in the larger amount of memory available and reduce considerably calls to the I/O subsystem. A large SGA is especially useful for OLTP and applications with a large working data set. More data can be held in memory, reducing I/O to disks and thereby increasing throughput. In the case of the 32-bit Oracle database, the amount of System Global Area (SGA), was limited to 1.75 GB on 32 bit HP machines. The SGA for 64-bit Oracle can be grown to occupy all possible physical memory on a 64-bit system. On HP's largest 64-bit system the physical memory could be as large as 32 GB. A very large memory also allows a greater number of in-memory processes.
The in-memory nature alone is extremely fast. Memory is accessed about 10,000 times faster than disk drives. For large applications that swap to disk frequently, simply moving to a 64-bit operating environment with generous amount of physical memory would increase the performance drastically. Such performance improvement is critical in an e-commerce environment, where there is a large number of connections to huge databases. With 32-bit HPUX, the limit on the size of the file was 4GB. This restriction is removed in the 64-bit environment.
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