Back pressure is a monitoring feature of the Microsoft Exchange Transport service that monitors system resources such as disk space and memory on Transport servers (the Hub Transport and the Edge Transport servers) and stops message/mail submission if it’s running low on these resources.
The following system resources are monitored as part of the back pressure feature:
- Free space on the hard drive that stores the message queue database.
- Free space on the hard drive that stores the message queue database transaction logs.
- The number of uncommitted message queue database transactions that exist in memory.
- The memory that’s used by the EdgeTransport.exe process.
- The memory that is used by all other processes.
- The number of messages in the Submission queue.
In this case, the error code 15004 is related to the “back pressure or Checkpoint depth is high” which might be the result of any of the above factors.
In order to prevent the reoccurrence of this aforesaid issue, we have to take the below preventive measures.
- Based on the calculation below, free hard drive space should be available for message queue databases and content conversion.
100 * (<hard drive size in MB> – 500 MB) / <hard drive size in MB>
This formula accounts for the fact that there’s unused space in the message queue database
1 GB = 1024 MB. The result is rounded down to the nearest integer.
- Make hard drive space available for the transaction logs of the Message Queue Database as per the below calculation.
100 * (<hard drive size in MB> – 1152 MB) / <hard drive size in MB>
1 GB = 1024 MB. The result is rounded down to the nearest integer.
- Monitor the memory used by EdgeTransport.exe and increase it as needed.
- When version buckets are under pressure, the Exchange server throttles incoming connections by delaying the acknowledgement of incoming messages. therefore, we need to check the number of uncommitted message queue database transactions in memory.
- We need to monitor the number of messages in the Submission queue as the Exchange throttles incoming connections by delaying the acknowledgement of incoming messages when the Submission queue is under pressure.
- It is necessary to monitor the memory utilization by other processes which may have an impact on Exchange performance.
Alternatively, we can disable the “Back Pressure” Feature, however, it is not recommended by Microsoft.
Please refer to the appended Micrsofot link.
The Hard disk IOPS performance also plays a key role in Exchange Server functionality. If any latency has been noticed on the Hard Disk, consider replacing it to avoid any Exchange server outage.