Unlike a normal system dump, which is a snapshot of the system taken when it is stopped, memory is constantly changing in system state 3. Thus, because of the volatility of the kernel structures, some of the information obtained with dumpmemory may not be valid. Nevertheless, dumpmemory can be very useful when you are analyzing a hung, or poorly performing application, or when you need to diagnose and correct poor system performance.
dumpmemory prompts you for a filename, in which it will save the memory dump. The file you name should be in a filesystem with enough space to hold all the memory on your system because that's what will be dumped.