Use of the /3GB switch in Exchange Server 2003 on a Windows Server 2003-based system:

When you install Exchange Server 2003 on a Microsoft Windows Server 2003-based computer that has more than 1 gigabyte (GB) of physical random access memory (RAM) installed, and that is home to mailboxes or public folders, you must edit the Boot.ini file to optimize the virtual memory usage of the Information Store service.

Under typical circumstances and for each process, 2 GB of virtual address space is allotted for the user-mode process, and another 2 GB of virtual address space is allotted to the operating system. When you use the /3GB switch in Windows Server 2003, 3 GB of virtual address space is allotted for the user-mode process, and only 1 GB of virtual address space is allotted to the operating system. This reallocation of the extra 1 GB of address space helps to resolve the problem of memory fragmentation in the Store.exe virtual address space. With the larger address space allocated to Store.exe, memory can be more easily joined before all large memory blocks are used.

After you have installed Windows Server 2003, modify the Boot.ini file, and then add the /3GB and the /USERVA=3030 parameters to the startup line, as in the following example:

[Boot Loader]
Timeout=30
Default=multi(0)disk(0)rdisk(0)partition(2)\WINNT [Operating Systems] multi(0)disk(0)rdisk(0)partition(2)\WINNT="Microsoft Windows Server 2003" /fastdetect /3GB /USERVA=3030

Thanks to Microsoft

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