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Using AWE Memory on Windows 2000

SQL Server 2000

Microsoft® SQL Server™ 2000 Enterprise Edition uses the Microsoft Windows® 2000 Address Windowing Extensions (AWE) API to support very large amounts of physical memory. SQL Server 2000 Enterprise Edition can access amounts of memory approaching 8 GB on Windows 2000 Advanced Server and approaching 64 GB on Windows 2000 Data Center.

Standard 32-bit addresses can map a maximum of 4 GB of memory. The standard address spaces of 32-bit Microsoft Windows NT® 4.0 and Windows 2000 processes are therefore limited to 4-GB. By default, 2 GB is reserved for the operating system, and 2 GB is made available to the application. If you specify a /3GB switch in the Boot.ini file of Windows NT Enterprise Edition or Windows 2000 Advanced Server, the operating system reserves only 1 GB of the address space, and the application can access up to 3 GB. For more information about the /3GB switch, see Windows NT Enterprise Edition or Windows 2000 Advanced Server Help.

AWE is a set of extensions to the memory management functions of the Microsoft Win32® API that allow applications to address more memory than the 4 GB that is available through standard 32-bit addressing. AWE lets applications acquire physical memory as nonpaged memory, and then dynamically map views of the nonpaged memory to the 32-bit address space. Although the 32-bit address space is limited to 4 GB, the nonpaged memory can be much larger. This enables memory-intensive applications, such as large database systems, address more memory than can be supported in a 32-bit address space. For more information about AWE, see the MSDN® page at Microsoft Web site.

Enabling AWE Memory

You must specifically enable the use of AWE memory by an instance of SQL Server 2000 Enterprise Edition by using the sp_configure option awe enabled.

  • When awe enabled is set to 0, AWE memory is not used, and the instance defaults to using dynamic memory in standard 32-bit virtual address spaces.

  • When awe enabled is set to 1, AWE memory is used, and the instance can access up to 8 GB of physical memory on Windows 2000 Advanced Server and 64 GB on Windows 2000 Data Center.

When an instance of SQL Server 2000 Enterprise Edition is run with awe enabled set to 1:

  • The instance does not dynamically manage the size of the address space.

  • The instance holds all memory acquired at startup until it is shut down.

  • The memory pages for the instance come from the Windows nonpageable pool, meaning that none of the memory of the instance can be swapped out.

You must carefully manage the memory used by an instance of SQL Server when awe enabled is set to 1. If the instance acquires most of the available physical memory as nonpaged memory, other applications or system processes may not be able to get the memory they need to run. Use the max server memory configuration setting to control how much memory is used by each instance of SQL Server that uses AWE memory. For more information, see Managing AWE Memory on Windows 2000.

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