Why are only 16 GB of memory usable on our server?

We've recently upgraded the RAM on a Windows Storage Server 2008 R2 Standard to 32 GB.

When I open the System panel, it indicates Installed memory: 32.0 GB (16.0 GB usable). Why is that?

I've been trying to dig up documentation that would indicate that this is a limit of the operating system version, but I've come up short. Looking at the version comparison on Wikipedia shows no version with 16 GB, but Storage Server isn't listed and Memory Limits for Windows and Windows Server Releases doesn't list Storage Serer 2008 R2.

I've since found an infographic in the blog post titled Windows Storage Server 2008 R2 is now available!, which indicates that the limit would be 32 GB.

As suggested by @Nico, opening perfmon shows that 16 GB of memory are "reserved for hardware".

The board of the server is an Intel S3420GPLX set up with an Intel Xeon SP X3430, both of which are supposed to support 32 GB of memory.

The installed RAM modules are 4 x 8 GB Hynix HMT31GR7CFR8A-H9 RDIMM.

Support advised us to perform a firmware upgrade of the mainboard. We performed the firmware upgrade, but it had no effect on the issue

Solution 1:

High end servers have options for pairing memory modules, like raid arrays for hard disks. You must view the BIOS option for that.

Solution 2:

Have you see this link ?

Running a Perfom /res would show you this "bug". Cause the missing RAM might be hardware reserved.

Also, have a look at this Microsoft's KB : http://support.microsoft.com/kb/978610

(I apologize, I cannot add any comment due to reputation)