Knowledge Base

Exchange memory use and the /3GB switch

Article ID: 328882

Article Last Modified on 10/25/2007

APPLIES TO

Microsoft Exchange Server 2003 Enterprise Edition
Microsoft Exchange Server 2003 Standard Edition
Microsoft Exchange 2000 Server Standard Edition

This article was previously published under Q328882

SUMMARY

This article describes issues that are related to virtual memory use in Exchange and the /3GB switch.

MORE INFORMATION

The /3GB Switch

By default, Windows can address a total of 4 gigabytes (GB) of virtual address space. By default, 2 GB of this is reserved for the kernel (operating system), and 2 GB is reserved for User mode programs such as Exchange.

When you put a /3GB switch into the Boot.ini file of the operating system, you reallocate the virtual address space distribution to give User mode programs 3 GB of space and limit the kernel to 1 GB. The /3GB switch is only required on computers that contain mailbox stores or public folder stores. This switch is not recommended on computers that do not have mailbox stores or public folder stores.

Important Only use the /3GB switch with the following operating systems:

Microsoft Windows Server 2003 Datacenter Edition
Microsoft Windows Server 2003 Enterprise Edition
Microsoft Windows 2000 Advanced Server
Microsoft Windows 2000 Datacenter Server
Microsoft Windows NT 4.0 Enterprise Server

Caution We support the use of the /3GB switch in Microsoft Windows Server 2003 Standard Edition in a production environment for use by Active Directory. Microsoft Exchange Server 2003 is supported in a production environment when you use this functionality. Otherwise, the /3GB switch in Windows Server 2003 Standard Edition is only for development and testing purposes.

Do not use the /3GB switch with the following operating systems:

Microsoft Windows 2000 Server
Microsoft Windows NT 4.0 Server

Windows 2000 Server and Windows NT 4.0 Server can only allocate 2 GB to User mode programs. If you use the /3GB switch with Windows 2000 Server or Windows NT 4.0 Server, you have 1 GB for kernel and 2 GB for User mode programs, so you lose 1 GB of address space.

Exchange on a Windows 2000 Server-Based Computer

The 2GB virtual memory address limit for User mode programs can contribute to several issues in Exchange when you have more than 1 GB of physical memory installed in your computer.

Exchange calculates the memory that is allocated to database buffers based on the amount of physical memory. Physical memory above 1 GB can cause a disproportionate amount of the 2 GB of virtual memory to be allocated to database buffers. This may cause virtual memory fragmentation problems, performance issues, or "out of memory" issues.

To work around this issue, use one or more of the following methods:

Reduce the Jet DBA Size

You might be able to reclaim some memory by reducing the JET DBA size. For more information about how to reduce the JET DBA size, click the following article number to view the article in the Microsoft Knowledge Base:

266768 How to modify the Store Database maximum cache size in Exchange 2000 Server

Reduce the Number of maxOpenTables in JET

You might be able to reclaim some memory by reducing the number of maxOpenTables in JET.

Set msExchESEParamMaxOpenTables on each storage group object to a lower value. The default for an 8 proc in Service Pack 2 (SP2) is 85,000. This is reduced to 27,600 in Service Pack 3 (SP3) for 8-procs. The 4-procs number is half of this value.

Note If you continue to experience memory problems, remove some RAM from your server to reduce it to 1GB or less. This helps you to confirm that the situation that is described in this article is causing the issue.

Exchange on a Windows 2000 Advanced Server-Based Computer

When a Windows 2000 Advanced Server-based computer has more than 1 GB of physical RAM, use the /3GB switch in the Boot.ini file to give User mode programs such as Exchange to have access to 3 GB of virtual address space. For more information this issue, click the following article number to view the article in the Microsoft Knowledge Base:

266096 Exchange 2000 requires /3GB switch with more than 1 gigabyte of physical RAM

Exchange on a Windows 2000 Datacenter Server-Based Computer

Use the /3GB switch with more than 1 GB of physical RAM on a Windows 2000 Datacenter Server-based computer. Exchange requires Service Pack 1 (SP1) to be supported on Windows 2000 Datacenter Server.

The /PAE Switch

Physical Address Extension (PAE) is a feature of Windows 2000 Datacenter Server and Windows 2000 Advanced Server that makes it possible for programs that were written for this API to use memory above 4 GB. Although there are programs such as Microsoft SQL Server 2000 that were written to take advantage of this API, Exchange 2000 Server is not compatible with it. Because of this, Exchange only addresses 3 GB if you use the /3GB switch.

Troubleshooting

Problems that are related to the issues that are described in this article typically become evident as performance problems. To troubleshoot these problems, use the Performance Monitor tool.

You can also use the Virtual Address Dump utility (VADUMP) to troubleshoot problems like this. For more information about the VADUMP tool, click the following article number to view the article in the Microsoft Knowledge Base:

317411 How to gather data to troubleshoot Exchange virtual memory issues

Additional Information

For more information about the issues that are described in this article, click the following article numbers to view the articles in the Microsoft Knowledge Base:

291988 A description of the 4 GB RAM Tuning feature and the Physical Address Extension switch

300573 Exchange 2000 Enterprise Server support on Windows 2000 Datacenter Server

266096 Exchange 2000 requires /3GB switch with more than 1 gigabyte of physical RAM

298064 Scalability planning for Exchange 2000 Server

266650 Information about BackOffice program support on Windows 2000 Datacenter Server-based computers