REF: Exchange 2010 Back Pressure

 

Back pressure is a system resource monitoring feature of the Microsoft Exchange Transport service that exists on Microsoft Exchange 2010 Hub Transport and Edge Transport servers. Exchange Transport can detect when vital resources, such as available hard disk drive space and available memory, are under pressure, and take action in an attempt to prevent service unavailability.

Back pressure prevents the system resources from being completely overwhelmed and Exchange server tries to deliver the existing messages. When utilization of the system resource returns to a normal level, the Exchange server gradually resumes normal operation.

In Exchange 2007, when a Hub or Edge server is under resource pressure, it rejected incoming connections. In Exchange 2010, incoming connections are accepted, but incoming messages over those connections are either accepted at a slower rate or are rejected. When an SMTP host attempts to make a connection to a Hub or Edge server that is in back pressure, the connection will succeed but when the host issues the MAIL FROM command to submit a message, depending on the resource that is under pressure, Exchange either delays the acknowledgement to the MAIL FROM command or rejects it.

The following system resources are monitored as part of the back pressure feature:

• Free space on the hard disk drive that stores the message queue database.
• Free space on the hard disk drive that stores the message queue database transaction logs.
• The number of uncommitted message queue database transactions that exist in memory.
• The memory that is used by the EdgeTransport.exe process.
• The memory that is used by all processes.

For each monitored system resource on a Hub Transport server or Edge Transport server, the following three levels of resource utilization are applied:

• Normal   The resource is not overused. The server accepts new connections and messages.
• Medium   The resource is slightly overused. Back pressure is applied to the server in a limited manner. Mail from senders in the authoritative domain can flow. However, depending on the specific resource under pressure, the server tar pits or rejects incoming MAIL FROM commands from other sources.
• High   The resource is severely overused. Full back pressure is applied. All message flow stops, and the server rejects all new incoming MAIL FROM commands.

By default, the message queue database transaction logs are stored at C:\Program Files\Microsoft\ExchangeServer\V14\TransportRoles\data\Queue. Exchange monitors the hard disk space utilization for this location. The EdgeTransport.exe.config file contains a DatabaseCheckPointDepthMax parameter that has a default value of 512 MB. This parameter controls the total allowed size of all uncommitted transaction logs that exist on the hard disk drive. This parameter is used in the formula that calculates hard disk drive utilization.

 

Blog Extended Reading
1. REF: Exchange 2007 Back Pressure

More Information & Reference

1. Exchange 2007 了解背壓
2. E14 Understanding Back Pressure
3. Understanding the EdgeTransport.exe.Config File