Windows 2008 R2 AD servers on fanless flash based systems

Yes. I have deployed several Server 2008 R2 boxes on the Supermicro X7SPE-HF-D525 board (64-bit dual-core Atom D525, 1.8 GHz). With 4GB of RAM and two 2.5" SATA drives setup in RAID1 using the onboard ICH9R controller, it prices out well under US$500 and consumes exactly 25 watts of power: i.e., no special cooling requirements. These environments generally have had about 10 users and have used these servers for both file shares and domain services without any performance issues. I have no doubt that this would scale to 20 users without issues, especially if it's being used only for domain services.

Put it in the CSE-503-200B case and it will have exactly the same form factor as a rackmount switch or firewall, so you can mount in a 2-post telco/switch rack:

If the site doesn't have a rack, this server form factor is perfect for mounting flush against a wall. There are some holes pre-drilled in the bottom of the case that make it easy to screw the server onto the wall with the top cover off, and then reinstall the top cover.

Be sure to order the MCP-220-00044-0N bracket if you plan to mount two 2.5" drives for a RAID1 array. Alternately, if you prefer, you can mount drives to the inside of the case using 3M Command adhesive strips. This is especially helpful if you want to install a PCIe card in the server without clearance issues.

Of course, it would perform even better with solid state storage instead of mechanical disks.

Valid idea to a common problem we all face at some point. But have you consider a solution like the HP ProLiant MicroServer, it seems to be aimed at the type of work environment you are describing and can be obtained for around $350.00 USD.

Since the ProLiant MicroServer uses a low power CPU heat shouldn't be that much of a problem, small form factor and low cost are equally good.

I guess there is no right or wrong answer.

Though I don't have experience with this configuration, I think the overall approach is valid.

It is true a laptop will power its drive down so power-wise it may be a wash. But keep in mind in this case dust is also an issue. A system designed to be fanless would hopefully have some kind of heat sink configuration to mitigate the lack of a fan to move air over components to cool them. And less moving parts means less likely to have things gum up with dust in them in general.

Another potential advantage with flash-based storage is that these types of locations generally also have poor power quality/availability. In theory having solid state storage will minimize the impact of power glitches vs. rotating cylinder storage. Though I would still invest in a UPS or at least strong surge protection of some kind if at all possible. Mostly because it will probably repay its cost in not losing systems regularly when power goes wonky and then having to ship them all over the place.

Space-wise, 32GB is probably sufficient.

You might also consider looking into caching the login credentials local to the specific office that needs them (vs. using an RODC). This can be accomplished on local machines and also on Windows 2008 R2 server, depending on configuration. This decreases security for one system/user/office, but then the only locally stored authentication information will be restricted to local users instead of potentially encompassing your entire organization. Probably not a major issue, but thought it was worth throwing out there.

Laptop HDs can be spun down when not in use wheras a lot of flash doesn't ever power down so heat/powerwise any advantage can entirely depend on the specific products and the usage pattern of the machine. Likewise with reliability. It doesn't sound like you are going to see the major benefit of shock resistance.

Also, even with a fanless system you will still build up heat and particularly if this is on all the time with high ambient temp you will need heatsinks/a heat dissipating chassis that can maintain the running system within operating limits (remember at high ambient temps you haven't got to just worry about the traditionally hot components but the rest will also have a shorter reliable lifetime)

Really you need to put something together and test it in as near to the expected conditions as possible.

Your ideas are sound, but you should consider those RODCs to be disposable. If one does experience a hardware failure (especially Storage), you will need to forcefully remove it from AD, but this is not terribly complicated.

Check your current DCs for their memory usage, but 800 users isn't going to be terribly huge. Off the top of my head, you'd probably be fine with 2GB of RAM, but 4 is obviously better.