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

• Free space on the hard disk that stores the message queue database.
  
  
• Free space on the hard disk 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's used by all other 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 isn't 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 uses tarpitting to delay server response 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.
  
  

The following sections explain how Exchange handles the situation when a specific resource is under pressure.

By default, the message queue database is stored at C:\Program Files\Microsoft\Exchange Server\V14\TransportRoles\data\Queue. Exchange monitors the hard disk space utilization for this location. The high level of hard disk space utilization is calculated by using the following formula:

100 * (hard disk size - fixed constant) / hard disk size

The value of fixed constant is 500 megabytes (MB).

The results of this formula are expressed as a percentage of the total hard disk space that's being used. The results of the formula are always rounded down to the nearest integer. By default, the medium level of hard disk utilization is 2 percent less than the high level. By default, the normal level of hard disk utilization is 4 percent less than the high level.

For more information about the message queue database, see Understanding Transport Queues.

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. This parameter is used in the formula that calculates hard disk utilization.

Note:
The value of the DatabaseCheckPointDepthMax parameter applies to all transport-related Extensible Storage Engine (ESE) databases that exist on the Hub Transport server or Edge Transport server. This would include the message queue database and the IP filter database.

By default, the high level of hard disk utilization is calculated by using the following formula:

100 * (hard disk size - Max(5 GB, 3*DatabaseCheckPointDepthMax)) / hard disk size

The results of the formula are always rounded down to the nearest integer. By default, the medium level of hard disk utilization is 2 percent less than the high level. The normal level of hard disk utilization is 4 percent less than the high level.

For more information about the message queue database, see Understanding Transport Queues.

A list of changes that are made to the message queue database is kept in memory until those changes can be committed to a transaction log. Then the list is committed to the message queue database itself. These outstanding message queue database transactions that are kept in memory are known as version buckets. The number of version buckets may increase to unacceptably high levels because of an unexpectedly high volume of incoming messages, spam attacks, problems with the message queue database integrity, or hard disk performance.

When Exchange starts receiving messages, these messages are grouped together in batches and then prepared as version buckets. If an incoming message has a large attachment, it can be separated into multiple batches. These batches that are being processed are known as batch points. The number of outstanding batch points can exceed the set thresholds, especially when there's an unexpectedly high volume of incoming messages with large attachments.

When version buckets or batch points are under pressure, the Exchange 2010 transport server will start throttling incoming connections by delaying acknowledgement to incoming messages. Exchange will reduce the rate of inbound message flow by tarpitting, which introduces a delay to the MAIL FROM commands. If the resource pressure condition continues, Exchange will gradually increase the tarpitting delay. After the resource utilization returns to normal, Exchange will gradually start reducing the acknowledgement delay and ease into normal operation. By default, Exchange will start delaying message acknowledgements 10 seconds when under resource pressure. If the resources continue to be under pressure, the delay is increased in 5-second increments up to 55 seconds.

Exchange 2010 keeps a history of version bucket and batch point resource utilization. If the resource utilization doesn't go down to normal level for a specific number of polling intervals, known as the history depth, Exchange will stop the tarpitting delay and start rejecting incoming messages until the resource utilization goes back to normal. By default, the history depths for version buckets and batch points are in 10 and 300 polling intervals respectively.

By default, the high level of memory utilization by the EdgeTransport.exe process is calculated by using the following formula:

75 percent of the total physical memory or 1 terabyte, whichever is less

This calculation doesn't include virtual memory that's available on the hard disk in the paging file, or the memory that's used by other processes. The results of this formula are expressed as a percentage of the total memory that's used by the EdgeTransport.exe process. The results of the formula are always rounded down to the nearest integer.

By default, the medium level of memory utilization by the EdgeTransport.exe file is calculated as 73 percent of the total physical memory or 2 percent less than the value of the high level, whichever is less. By default, the normal level of memory utilization by the EdgeTransport.exe file is calculated as 71 percent of the total physical memory or 4 percent less than the value of the high level, whichever is less.

If the memory utilization of the EdgeTransport.exe process is higher than the specified normal level, garbage collection is forced. Garbage collection is a process that checks for unused objects that exist in memory, and reclaims the memory that's used by those unused objects.

Exchange 2010 keeps a history of the memory utilization of the EdgeTransport.exe process. If the utilization doesn't go down to normal level for a specific number of polling intervals, known as the history depth, Exchange will start rejecting incoming messages until the resource utilization goes back to normal. By default, the history depth for EdgeTransport.exe memory utilization is 30 polling intervals.

By default, the high level of memory utilization by all processes is 94 percent of total physical memory. This value doesn't include virtual memory that's available on the hard disk in the paging file.

When the specified memory utilization level is reached, message dehydration occurs. Message dehydration is the act of removing unnecessary elements of queued messages that are cached in memory. Complete messages are cached in memory for enhanced performance. Removal of the MIME content of queued messages from memory reduces the memory that's used at the expense of higher latency because the messages are read directly from the message queue database. By default, message dehydration is enabled.

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