How to Set the /3GB Startup Switch in Windows

How to Set the /3GB Startup Switch in Windows

Topic Last Modified: 2006-04-19

Windows Server 2003 includes support for a startup switch that lets you tune the allocation of use of memory and memory address space. Regardless of the amount of physical memory in your system, Windows uses a virtual address space of 4 GB, with 2 GB allocated to user-mode processes (for example, applications) and 2 GB allocated to kernel-mode processes (for example, the operating system and kernel-mode drivers). On systems that have 1 GB or more of physical memory, these two startup switches can be used to allocate more memory to applications (3 GB) and less memory to the operating system (1 GB). This additional virtual address space helps reduce the amount of memory fragmentation in the virtual address space of the Exchange information store process.

The /3GB switch is used to effect this allocation change. The switch is entered in the system’s boot.ini file and takes effect after a restart.

Before You Begin

Before you perform the procedure in this topic, be aware of the following:

The /3GB switch is supported only on the following operating systems:

• Windows 2000 Advanced Server
  
• Windows 2000 Datacenter Server
  
• Windows Server 2003 Standard Edition
  
• Windows Server 2003 Enterprise Edition
  
• Windows Server 2003 Datacenter Edition
  

Important:
The /3GB switch should not be used on Windows 2000 Server because it is unsupported and can cause application or operating system crashes.

Procedure

To Set the /3GB Startup Switch in Windows Server 2003

1. Right-click My Computer and select Properties. The System Properties dialog box will appear.

2. Click the Advanced tab.

3. In the Startup and Recovery area, click Settings. The Startup and Recovery dialog box will appear.

4. In the System startup area, click Edit. This will open the Windows boot.ini file in Notepad.

5. In the [Operating Systems] section, add the following switches to the end of the startup line that includes the /fastdetect switch: /3GB

6. Save the changes and close Notepad.

7. Click OK two times to close the open dialog boxes, and then restart the computer for the change to take effect.

To Set the /3GB Startup Switch in Windows 2000 Advanced Server or Windows 2000 Datacenter Server

1. In Windows Explorer, navigate to the system partition. This is the partition that holds hardware specific Windows files such as Boot.ini and NTLDR.

2. If you cannot see the Boot.ini file, it could be because the folder options are set to hide protected operating system files. If this is the case, in the Explorer window, click Tools, Folder Options, and then click View. Clear the Hide protected operating system files (Recommended) check box. When prompted, click Yes.

3. After the Boot.ini file is visible in Explorer, right-click the file, click Open With, and then click Notepad to open the file.

4. Add the /3GB switch to the end of the line starting with "multi" under the [Operating Systems] section.

5. Save the changes and close Notepad.

6. Restart the computer for the change to take effect.

For More Information

• For more information about the /3GB startup switch, see Optimizing Memory Usage in Exchange Server 2003.
  
• For more information about optimizing Exchange Server memory usage and performance, see the following Microsoft Knowledge Base articles:
  
  • 266096, Exchange 2000 requires /3GB switch with more than 1 gigabyte of physical RAM
    
  • 325044, How to troubleshoot virtual memory fragmentation in Exchange Server 2003 and Exchange 2000 Server
    
  • 815372, How to optimize memory usage in Exchange Server 2003
    
  • 308356, Memory usage by the Lsass.exe process on domain controllers that are running Windows Server 2003 or Windows 2000 Server
    
  • 810371, Using the /Userva switch on Windows Server 2003-based computers that are running Exchange Server
    
  • 823440, Use of the /3GB switch in Exchange Server 2003 on a Windows Server 2003-based system
    
  • 822178, Overview of dependencies and requirements for Exchange Server 2003 features