Why /3GB can negatively affect your ISA Server

1. Introduction

While many Microsoft Application Servers such as Exchange take full advantage of /3GB switch in boot.ini, ISA Server can be seriously affected if you use this switch on it. Before we discuss what this switch does and how can affect ISA Server I strongly recommend you to read this great post from Windows Performance Team. After read that, we can be in the same page to talk about the side effects on ISA Server.

2. Why this is bad for ISA?

When you add /3GB on boot.ini what you are telling to the Operating System is to use 1GB for Kernel and 3GB for User Mode space. The problem is that the main ISA Server service runs in Kernel Mode (fweng) and limiting this driver to use only 1GB can be very dangerous. Besides when you add this switch you also reduce the memory available for:

• Nonpaged Pool

• Paged Pool

• System Page Table Entries (PTEs)

Considering that ISA Server is the default gateway for many of your networks, during a period of high network activity, the usage of non-paged pool memory may cause the server to stop responding because there will be no more memory available. The reason why is because when you use /3GB the maximum size of the nonpaged pool reduces from 256 MB to 128 MB.

3. ISABPA is your friend

Many people still not aware of how powerful ISA BPA can be when the subject is: identify what is wrong and tell you about it. When you run a Health Check on your ISA Server using ISABPA and this machine has /3GB the following alert will show up in the report:

Figure 1 – ISABPA alerts about /3GB.

Don’t know how to download ISABPA, easy: now can just type www.isabpa.com and you will be redirected to the download page for ISABPA.

4. Conclusion

Many myths are behind /3GB switch and many system admin still think that this could be beneficial for all servers’ regardless; untrue statement. Each server/application has its own needs and you need to carefully analyze and understand these needs before add such switch in your boot.ini.