It’s been an interesting few weeks here.  We’ve been collecting data much faster than we anticipated, and we’ve had to upgrade hardware.  One thing we’ve had to do is bring several of our servers from 16 gigabytes of RAM to 32 gigabytes.

Memory is surprisingly inexpensive.  You can buy four gigabytes of RAM (2 DIMMs of 2 gigabytes each) for $55.  That’s fine for maxing out your 32-bit machine, or you can bring a typical 64-bit machine to eight gigabytes with those parts for only $110.

Most server RAM is more expensive–about double what desktop memory costs.  In addition, most servers only have eight memory slots, making it difficult or hideously expensive to go beyond 16 gigabytes.  The reason has to do with the way that memory controllers access the memory.  Controllers (and the BIOS, it seems) have to know the layout of chips on the DIMM, and most machines are set up to use single-rank or dual-rank RAM.  A 2-gigabyte DIMM that uses single-rank RAM will have eight (or nine, if ECC) 2-gigabit chips on it.  A dual-rank DIMM will have 16 (18) 1-gigabit chips.  Dual-rank will typically be less expensive because the lower density chips cost less than half of the higher density chips.

The inexpensive 2-gigabyte DIMMs are typically dual-rank, meaning that the components on it are 1-megabit chips.  If you want a 4-gigabyte DIMM, then you have to step up to 2-gigabit chips.  And those are very expensive.  The other option is to buy quad-rank memory, which uses the 1-gigabit chips.  Quad-rank 4-gigabyte DIMMs for servers are currently going for $70 or $80.  Figure $20 per gigabyte.

The only catch is that most older computers’ memory controllers don’t support the quad-rank DIMMs.  I do know that Dell’s PowerEdge 1950 server with the latest BIOS supports quad-rank.  The Dell 490 and 690 machines do not.

If you’re in the market for a new computer that you expect to load up with RAM, you should definitely make sure that it supports quad-rank memory.  If you’re adding memory to an older machine, you might save a lot of money by doing some research to see if you can upgrade the BIOS to support quad-rank.