This isn’t about ECC versus non-ECC RAM, as your servers probably only use ECC RAM to prevent RAM error crashes. Although more expensive than non ECC RAM it’s well worth it. In fact, if your server specs say ECC and you try the cheaper non-ECC type it probably won’t work. That being said, let’s get to the title, RAM jam and what it means.

Depending on your server brand, motherboard and firmware, the specs allow for specific maximum type and RAM chip density that can be used in each memory slot. While all servers that I can think of allow 4GB strips, usually installed in groups, the maximum RAM that your server can use is dictated by the number of available RAM slots and the firmware.

The two major server brands have mothboards with 4 to 8 slots in their lower-end popular models, still well suited to VDI, up to 24 slots for the top of the line models. Dell, 4-24 and HP, known in the business as Higher Price (HP), offers a wider range from 9 to 24! So why does this matter?

VDI is all about RAM. Each endpoint user has a separate instance of the Windows operating system running on the server, with a monitor, keyboard and mouse at each desk. So when you multiply 75- 100 users by 1GB of RAM each, plus other software running on the server you will be using a lot of RAM. And the RAM density is important.

In general, buy servers with the most RAM slots so your RAM chip density is low. Low means fast and cheap.

Realistically, once you get north of 96GB of RAM, the price of the RAM increases almost geometrically. So you may buy 24, 4GB ECC RAM strips for $100 each, or $2,400, but if you want twice as much, or have to use a higher density RAM chip as your server doesn’t have that many slots, the price jumps.

An 8GB, ECC RAM strip may be $600 each. So a 12 slot motherboard needs $7,200 of memory just to reach 96GB, three times the price! For that you could buy a new rack server or two for the project.

And if you’re thinking blade servers for their increased physical density, be aware that they may not have as many slots, although those may support really high density RAM for that magic 96GB. For example, the HP ProLiant BL2x220c G7 has six slots, supports 16GB chips in each slots or 96GB of ECC RAM, but will cost you $2,000 a piece, or $12,000 for the same 96GB!

So think and plan carefully. VDI is more than just throwing a bunch of stuff together. It takes planning, thought and skill to do it right.

February 2016
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