Exchange transport can detect when critical resources, such as available hard disk space and memory, are under pressure, and take action in an attempt to prevent service unavailability.
Back pressure prevents the system resources from being completely snowed under, and Exchange 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 Server 2007, when a Hub Transport or Edge Transport server is under resource pressure, it rejects 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 Transport or Edge Transport server that's 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's 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 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.
Each monitored system resource on a Hub Transport server or Edge Transport server; the three levels (Normal, Medium and High) of resource utilization are applied.
For example, by default, the message queue database is stored at
By default, the high level of hard disk drive space utilization is calculated by using the following formula: 100*(hard disk drive size - 4 GB) / hard disk drive size. As the available free hard disk drive space decreases, the hard disk drive utilization increases. So, we require at least 4GB free space on the hard disk drive containing message queue database. Otherwise, the hard disk drive space utilization will reach the high level and Exchange stops accepting any new connections and messages.
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.
Message queue database is kept in memory until change can be committed to a transaction log. Those message queue database transaction that kept in memory are known as version bucket The number of the version buckets may be increase to high volume of incoming message problem hard drive performance. If the message with large attachment, it can be separated in to multiple batches and called batch points. If the buckets or batch point are under pressure exchange server start throttling for incoming connection, exchange will introduce the delay to the MAIL FROm commands.
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.
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