A Guide to Back Pressure in Microsoft Exchange Server
452 4.3.1 Insufficient system resources The resource pressure increased from Medium to High. The following resources are under pressure: Private bytes = 76% [High] [Normal=71% Medium=73% High=75%] The following components are disabled due to back pressure: Inbound mail submission from Hub Transport servers Inbound mail submission from the Internet Mail submission from Pickup directory Mail submission from Replay directory Mail submission from Mailbox server Loading of e-mail from the queuing database (if available) The following resources are in normal state: Queue database path (“C:Program FilesMicrosoftExchange ServerV14TransportRolesdataQueuemail.que”) = 15% [Normal] [Normal=95% Medium=97% High=99%] Queue database logging path (“C:Program FilesMicrosoftExchange ServerV14TransportRolesdataQueue”) = 15% [Normal] [Normal=94% Medium=96% High=98%] Version buckets = 0 [Normal] [Normal=80 Medium=120 High=200] Physical memory load = 60% [limit is 94% to start dehydrating messages.] Batch Point = 0 [Normal] [Normal=2000 Medium=4000 High=8000] Submission Queue = 0 [Normal] [Normal=1000 Medium=2000 High=4000]
This error is generally noticed because external servers are unable to deliver email and error messages begin appearing on the event log as the one shown above. You will notice the first error on a connection attempt via SMTP.
What is back pressure? The Microsoft Exchange Transport Service Resource Monitoring
- Free disk space on the drive(s) that store the message queue database and logs
- Uncommitted queue database transactions in memory
- Memory utilization by the EdgeTransport.exe process (the Microsoft Exchange Transport service)
- Overall memory utilization for the server
As you would expect, there are thresholds for each of these resources and depending on those thresholds you could be placed in any of the following states:
- Normal – all is well and the server is performing its role as intended.
- Medium – a resource is moderately over-utilized and the server begins limiting some connection types. Typically internal email flow remains functional while email from external or non-Exchange sources will be rejected.
- High – a resource is severely over-utilized. The server ceases to accept any new connections.
For disk space metrics the back pressure condition causes messages to be rejected. However for memory utilization metrics, before rejecting connections the server will first take actions to attempt to relieve the conditions. For example, the server will perform garbage collection (reclaiming memory from unused objects) or flush the server’s DNS cache. If after a certain number of polling intervals (which vary depending on the metric involved) the utilization is still above threshold, then the server will begin rejecting new connections as it does with disk space utilization. Ultimately the problems you will actually notice are delays or a total lack of message delivery.
Monitoring Event Logs
You can monitor event logs using the Event Log in Windows Server, PowerShell and the Get-EventLog cmdlet. Back pressure conditions are logged to the Application event log under a series of event IDs:
- Event ID 15004: Increase in the utilization level for any resource (eg from Normal to Medium)
- Event ID 15005: Decrease in the utilization level for any resource (eg from High to Medium)
- Event ID 15006: High utilization for disk space (ie critically low free disk space)
- Event ID 15007: High utilization for memory (ie critically low available memory)
Fixing Back pressure problems & Thresholds
- Queue database path (“C:Program FilesMicrosoftExchange ServerV14TransportRolesdataQueuemail.que”) = 15% [Normal] [Normal=95% Medium=97% High=99%]
- Increase Hard Drive space if this is an issue
- Queue database logging path (“C:Program FilesMicrosoftExchange ServerV14TransportRolesdataQueue”) = 15% [Normal] [Normal=94% Medium=96% High=98%]
- Increase Hard Drive space is this is an issue
- Version buckets = 0 [Normal] [Normal=80 Medium=120 High=200]
- Physical memory load = 60% [limit is 94% to start dehydrating messages.]
- Increase physical memory if this is an issue
- Private bytes = 76% [High] [Normal=71% Medium=73% High=75%]
- Increase physical memory if this is an issue. This can appear an issue if you are using dynamic memory.
- Batch Point = 0 [Normal] [Normal=2000 Medium=4000 High=8000]
- Submission Queue = 0 [Normal] [Normal=1000 Medium=2000 High=4000]
- Perform monitoring to determine issue. If high volume of emails you might need to upgrade your hardware or balance the load with another server.
Customizing Back Pressure Thresholds
The metrics used by Transport server resource monitoring to trigger back pressure are based on a combination of configurable settings as well as fixed algorithms. The configurable settings are stored in the EdgeTransport.exe.config file, located in the following directories by default:
- Exchange 2007: C:Program FilesMicrosoftExchange ServerBin
- Exchange 2010: C:Program FilesMicrosoftExchange ServerV14Bin
- Exchange 2013: C:Program FilesMicrosoftExchange ServerV15Bin
If you installed Exchange Server to a different location then you will find the config file in the bin folder of your installation directory. In Exchange 2010 and 2013 this is referenced by the environment variable $env:exchangeinstallpath.
The first two values control whether resource monitoring is enabled, and at what interval resource monitoring is performed. Though the option exists it is not recommended to actually disable resource monitoring. Instead the recommended approach is to identify the cause of resource over-utilization and correct it. The same goes for basically all of the configurable settings for resource monitoring. Rather than spend time tuning the settings you should resolve the underlying issue, for example by adding disk or memory capacity to the server, or by adding additional servers to assist with overall email traffic load. If you do happen to want to dive into the specific metrics and thresholds you can read more about them here:
Back pressure can cause serious problems in an Exchange Server environment due to the interruptions it causes to message delivery. Be sure to check your Transport servers for signs of back pressure, and take steps to resolve the underlying issues.