Exchange 2007 comes with a new feature for monitoring the resources on the transport servers known as Back Pressure. The feature runs only on hub and edge transport servers. Exchange Transport Service is responsible for the running it.
The following resources are closely monitored by the Back Pressure feature.
1. The available disk space on the drive that has the transport database (Mail.que).
2. The available disk space on the drive that has the transport database log files.
3. Memory used by all processes
4. Memory used by EdgeTransport.exe process.
5. Number of uncommitted transport database transactions in the memory, known as Version Buckets.
There are three levels for the status of these counters – Normal, Medium and High. Each of the levels have pre-defined threshold values for each counter that is being monitored.
Typical symptoms of Back Pressure feature being kicked in is when you have emails stuck in the Drafts folder or get a “4.3.1 Insufficient System Resources” ndr from exchange.
For example, the available disk space on the drive that has the transport database should be atleast 500mb. If it goes below that level, the transport servers stop sending and receiving emails. The available space threshold was 4gb for pre SP1 servers, which was bit of an overkill. Similarly, all resources that are monitored has a pre-defined threshold limits.
Can we change any of these settings? Of course you can! All settings related to the back pressure feature is stored in edgeTransport.exe.config file which is located in the bin directory. By default, it is in C:Program FilesMicrosoftExchange ServerBin. Open the config file in notepad and have a look at the settings & change any if needed. The Transport Service will have to be restarted for the changes to take effect.
Can we disable Back Pressure feature altogether? And the answer is yes! You can edit the config file with the entry “false” for , save the file and restart the transport service. Job done! It is not recommended to disable the feature as it will be useful for to know when resources are running out. If you want some time to sort out the resource issues like increasing the disk space available but keep the server operational at the same time, disable the feature, fix the resource issue and enable it again.
Can we change the resource monitoring interval? The default interval is 2 seconds. You can modify the value with anything between 1 and 30 seconds. Edi the config file with the value of your choice for the entry , save the config and restart the transport server.
An important point to note is that the back pressure feature is only available for hub and edge transport servers. Check this article for more information regarding changing the back pressure settings.