Topic Last Modified: 2009-04-07

On Microsoft Exchange Server 2007 computers that are running the Hub Transport role or the Edge Transport role, the Microsoft Exchange Analyzer Tool examines the amount of installed RAM together with the DatabaseMaxCacheSize value.

If the following conditions are both true, Exchange Analyzer generates a Best Practice message that contains the following information:

• The installed RAM is 4 gigabytes (GB) or more.
  
  
• The DatabaseMaxCacheSize parameter is not set to 512 megabytes (MB).
  
  

This message represents a change in the recommendation for the database maximum cache size.

By default, Exchange 2007 has a transport database cache size of 128 MB. Although this size may be sufficient for typical usage, a maximum database cache size of 128 MB may be insufficient on a Hub Transport or Edge Transport server that experiences higher than typical message rates or that experiences unexpected messaging loads. To better allow for cache growth, we recommend that the DatabaseMaxCacheSize value be increased from 128 MB to 512 MB on transport servers that have 4 GB or more of RAM.

The Microsoft Exchange Transport service uses the Extensible Storage Engine (ESE) for mail transport functionality. This feature provides the following benefits over earlier versions of Exchange, which use the NTFS file system for mail transport functionality:

• It improves performance. The ESE first writes transactions to log files and to RAM. Then, the ESE writes the transactions to the database file.
  
  
• It increases transactional integrity of data that is stored in the queue.
  
  
• All messaging queues are held in a single location, the transport mail queue database. In Exchange 2003, mail could be stored in two locations during processing: the file folder structure and the local information store.
  
  

Although the Mailbox server role dynamically increases the ESE cache size, the Hub Transport or Edge Transport roles do not dynamically increase the ESE cache size. Instead, by default, the ESE cache has a maximum size of 128 MB. This value is set in the DatabaseMaxCacheSize parameter in the EdgeTransport.exe.config file.

The Exchange 2007 resource monitor tracks the number of used version buckets that are currently stored in memory. When the number of used version buckets exceeds the thresholds that are specified in the EdgeTransport.exe.config file, the resource monitor logs event ID 15004. This event indicates that the server is experiencing resource back pressure. A back pressure event is a staged process in which the following behavior occurs:

1. When the first threshold is reached, the transport server tries to block new incoming SMTP messages.
   
   
2. When the second threshold is reached, the transport server prevents new Mailbox server connections. This is to allow the transport server time to try to clean uncommitted transactions from memory and to put the transactions into the queue database file.
   
   

The version bucket thresholds have the following default values:

Exchange version	Medium threshold	Maximum threshold
Exchange 2007 RTM	60	100
Exchange 2007 SP1	120	200

We recommend that you do not increase these values to try to work around a back pressure situation. Increasing these values may affect server availability.

To improve performance by using version buckets and to allow for increased database cache growth, increase the DatabaseMaxCacheSize value from 128 MB to 512 MB on Hub Transport or Edge Transport servers that have 4 GB or more of RAM.

To increase the DatabaseMaxCacheSize value in Exchange 2007

1. Log on to the Hub Transport or Edge Transport server.

2. Start Windows Explorer, and then locate the following folder:

   %ProgramFiles%\Microsoft\Exchange Server\Bin

3. In the details pane, open the EdgeTransport.exe.config file by using a text editor such as Notepad.

4. Locate the DatabaseMaxCacheSize parameter. To do this, press CTRL+F, type databasemaxcachesize in the Find what box, and then click Find Next. The parameter appears as follows:

   <add key = "DatabaseMaxCacheSize" value="134217728" />

5. Replace the value with 536870912. The parameter should appear as follows:

   <add key = "DatabaseMaxCacheSize" value="536870912" />

6. Save the changes to the EdgeTransport.exe.config file, and then restart the Microsoft Exchange Transport service. To restart the service, follow these steps:

   1. Click Start, click Run, type cmd, and then click OK.
      
      
   2. At the command prompt, type net stop msexchangetransport && net start msexchangetransport, and then press ENTER.
      
      

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