How big should my OS drive be?

My name is Michael Champion and I've been working in support for more than 12 years here at Microsoft.  I have been asked by many customers "What is the recommended size for the OS partition for Windows Server?".  There are minimum recommendations in the technical documentation (and release notes), but those recommendations are more on the generic side.  There are times when that recommendation is fine, but other times they are not.

Take for example the Windows Server 2012 R2 disk recommendations.

System Requirements and Installation Information for Windows Server 2012 R2

Disk space requirements

The following are the estimated minimum disk space requirements for the system partition.

Minimum: 32 GB

Be aware that 32 GB should be considered an absolute minimum value for successful installation. This minimum should allow you to install Windows Server 2012 R2 in Server Core mode, with the Web Services (IIS) server role. A server in Server Core mode is about 4 GB smaller than the same server in Server with a GUI mode. For the smallest possible installation footprint, start with a Server Core installation and then completely remove any server roles or features you do not need by using Features on Demand. For more information about Server Core and Minimal Server Interface modes, see Windows Server Installation Options.

The system partition will need extra space for any of the following circumstances:

    • If you install the system over a network.
    • Computers with more than 16 GB of RAM will require more disk space for paging, hibernation, and dump files.

The trick here is that "minimum" is bolded meaning that you could need a larger space and does not take into account your actal memory, what applications may be installed, etc.  While it does state this, I can give you an idea based on the role and hardware configuration of the server and other factors what disk space you should have available.

Here are some good suggestions to follow when trying to calculate the size of an OS volume.

  • 3x RAM up to 32GB
  • 10-12GB for the base OS depending on roles and features installed
  • 10GB for OS Updates
  • 10GB extra space for miscellaneous files and logs
  • Any applications that are installed and their requirements. (Exchange, SQL, SharePoint,..)

Taking the full 32GB RAM, a simple OS build would require a drive about 127GM in size.  One may think this is too large for the OS when the minimum disk space requirement is 32GB but let's break this down a bit...

Why 3x RAM?

If you are using 32GB of RAM and you need to troubleshoot a bug check or hang issue, you will need a page file at least 100MB larger than the amount of RAM as well as space for the memory dump.  Wait, that is just over 2x RAM... There are other log files like the event logs that will grow over time and we may need to collect other logs that will take up GB of space depending on what we are troubleshooting and the verbosity of the data we need.

10GB-12GB for the base OS?

The base OS install size is about 10GB-12GB and that is just for the base files and depends on what roles and features are installed.

10GB for OS Updates?

If you are familiar with the WinSxS directory in the OS for 2008/R2 and up, this folder will grow as the server is updated over the life of the server.  We have made great strides in reducing the space taken up by the WinSxS folder but it still increases over time.

10GB extra space for miscellaneous files and logs?

This may seem to be covered in the 3x RAM but many times people will copy ISO, 3rd party install files or logs, and other things to the server.  It is better to have the space than not to have it.

How much for server applications then?

This part is variable and should be taken in consideration when purposing a server for a particular function.  In general server use, the 127GB can usually accommodate a single or even a dual purpose server.

Thank You,
Michael Champion
Support Escalation Engineer