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I'm running an X-99 deluxe 3.1 system, and I currently have 16GB of ram in the form of a 4GBx4 DDR4 2666 Mhz Corsair Dominator RAM kit. The system itself is a custom watercooled machine that my girlfriend built me* last spring, and since it is time to perform a coolant flush anyway, now is an ideal time to upgrade the RAM.

Because the MB has 8 RAM slots, I have space to simply add another kit, which is without question the most economical option, at least in principle. That said, I'm pretty sure the last time I had occasion to add simply add more RAM to an existing system was some time around 1994 (It was an AMD 386/40 clone, complete to cutting edge XGA graphics (I think. It may have been SVGA; I bet my parents still have the system in their attic. I wonder if it would still boot?). I'm pretty sure things have gotten more complicated since I the day I went from 2MB of RAM to the limiless capacity represented by 4 whole megabytes!

So I'm hoping I can get some quick pointers on what I can and cannot do, and whether it is a good idea to add a second kit at all. Beyond "Is this an inherently bad idea", my specific questions would be the following:
--As I understand it, a second kit will have to be of an identical clock speed (as in "also 2666 MHz", rather than "at least 2666 MHz).
--I also assume that I am best off getting my RAM from the same OEM, and of the same model; that isn't a big deal. Since this is a custom loop, aesthetics were obviously a meaningful consideration in the first place and I'm pretty sure if I tried to mess up the (really amazing) looks of the system she painstakingly designed at assembled for me, my girlfriend would hide all the tools and refuse to let my touch it until I came to my senses.
--I am unclear on the issue of the kits needing to be the same size. I'm probably going to end up with a second 16GB kit simply to keep cost down, but it would be nice to know if it is possible to mix a 16 gig kit with a 32 gig kit, in the event that i come across a random sale of some kind.

I'm perfectly content being pointed at an existing guide, but if I'm going to follow a guide that is more than a couple of months old i want to have made sure that I'm being pointed in the direction by someone who is able to say "Yep, that is two years old but the information is still current" rather than just using Google and hope.

While it is generally best to match the existing DIMMs, it isn't an absolute requirement. Different DIMMs but still with the same specs (speed and timings) would be second best, followed by modules with different specs. If you mix speeds then all modules will likely need to be run at the speed of the slower modules, but it should still work.

Even with matched modules, you may need to downclock or relax the timings when you add a 2nd DIMM per channel.

You should be able to mix DIMM sizes without causing issues, though you'll still want to make sure each channel has the same mix of sizes. E.g., you should be able to install a 32GB kit, for a total of 48GB.

For a little extra insurance, stick to modules that the motherboard manufacturer has qualified for your motherboard.

And it is also best to avoid cheap no name memory as well. I've been using Crucial brand memory in all my systems since I built my first rig in 2000, and not once have I had any problems.

Honestly the best reference would be your motherboard manual (one of the few times it's worth looking at ) as it will generally outline any configurations that will or won't work

Keep in mind that populating all slots usually requires a drop in the top clock speed due to the added stress for the Integrated Memory Controller on your CPU. You might have to bump the voltage a small increment to the DIIMM slots or the VSSA for stability at 2666 MHz with all slots filled.

I'd like to revive this thread with a related question, if that's ok. I am current running with 16 GB in 2 DIMMs of DDR3-2133; cleaning the office, I have found another 16 GB in 2 DIMMs of DDR3-1600. I wouldn't mind having 32 GB of memory in this box, as it would allow me to allocate a lot more to the VMs I need to use for my work, while still keeping a healthy amount for the operating system and regular applications.

I'd need to run everything at 1600, though - would that be a significant hit to performance?

For reference, the other hardware is:

Intel 4790k @ 4.0 GHz (i.e. stock)
Asrock Z97 Extreme 4
Geforce GTX 1060 6 GB
Crucial MX300 525 GB
various and sundry other hard drives, your regular peripherals etc.
with Ubuntu 16.04.3 as the main operating system, and VMs of Windows 10, 7 and 2000.

https://www.anandtech.com/show/7364/mem ... -haswell/3

(mind you, these tests are likely run on Windows)

DancinJack wrote:

https://www.anandtech.com/show/7364/memory-scaling-on-haswell/3

(mind you, these tests are likely run on Windows)

Even with the Windows caveat, that was fantastically helpful. Thank you. It looks like the performance hit dropping from 2133 to 1600 will not be enormous. It's certainly worth trying, I think, and I can always pull the slower DIMMs again if I'm not happy with the result.