Using AWE Memory in SQL Server 2000

Using AWE Memory in SQL Server 2000
By : Brad McGehee
May 31, 2003  

If you are using SQL Server 2000 Standard Edition under Windows NT 4.0 or Windows 2000 (any version), or are running SQL Server 2000 Enterprise Edition under Windows NT 4.0 or Windows 2000 Server, or if your server has 4GB or less of RAM, the "awe enabled" option should always be left to the default value of 0, which means that AWE memory is not being used.

The AWE (Advanced Windowing Extensions) API allows applications (that are written to use the AWE API) to run under Windows 2000 Advanced Server or Windows 2000 Datacenter Server to access more than 4GB of RAM. SQL Server 2000 Enterprise Edition (not SQL Server 2000 Standard Edition) is AWE-enabled and can take advantage of RAM in a server over 4GB. If the operating system is Windows 2000 Advanced Server, SQL Server 2000 Enterprise Edition can us up to 8GB of RAM. If the operating system is Windows 2000 Datacenter Server, SQL Server 2000 Enterprise can use up to 64GB of RAM.

By default, if a physical server has more than 4GB of RAM, Windows 2000 (Advanced and Datacenter), along with SQL Server 2000 Enterprise Edition, cannot access any RAM greater than 4GB. In order for the operating system and SQL Server 2000 Enterprise Edition to take advantage of the additional RAM, two steps must be completed.

Exactly how you configure AWE memory support depends on how much RAM your server has. To configure Windows 2000 (Advanced or Datacenter), you must enter one of the following switches in the boot line of the boot.ini file, and reboot the server:

•4GB RAM:  /3GB (AWE support is not used)

•8GB RAM:  /3GB /PAE

•16GB RAM:  /3GB /PAE

•16GB + RAM:  /PAE
The /3GB switch is used to tell SQL Server to take advantage of 3GB out of the base 4GB of RAM that Windows 2000 supports natively. If you don't specify this option, then SQL Server will only take advantage of 2GB of the first 4GB of RAM in the server, essentially wasting 1GB of RAM.

AWE memory technology is used only for the RAM that exceeds the base 4GB of RAM, that's why the /3GB switch is needed to use as much of the RAM in your server as possible. If your server has 16GB or less of RAM, then using the /3GB switch is important. But if your server has more than 16GB of RAM, then you must not use the /3GB switch. The reason for this is because the 1GB of additional RAM provided by adding the /3GB switch is needed by the operating system in order to take advantage of all of the extra AWE memory. In other words, the operating system needs 2GB of RAM itself to mange the AWE memory if your server has more than 16GB of RAM. If 16GB or less of RAM is in a server, then the operating system only needs 1GB of RAM, allowing the other 1GB of RAM for use by SQL Server.

Once this step is done, the next step is to set the "awe enabled" option to 1 within SQL Server Enterprise Edition, and then restart the SQL Server service. Only at this point will SQL Server be able to use the additional RAM in the server.

One caution about using the "awe enabled" setting is that after turning it on, SQL Server no longer dynamically manages memory. Instead, it takes all of the available RAM (except about 128MB which is left for the operating system). If you want to prevent SQL Server from taking all of the RAM, you must set the "max server memory" option (described in more detail later in this article) to a figure that limits SQL Server to the amount or RAM you specify. (7.0, 2000) Updated 1-2-2004