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Audience: Systems Admins
Last Updated: 6/6/2011 11:36:07 PM
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Microsoft Exchange 2007: Dealing with Back pressure

By Erik Rodriguez

Tags: Exchange 2007 backpressure, exchange back pressure, Exchange 2007 disk full, Exchange 2007 not accepting mail, Exchange mailstore backup

This article contains information on Microsoft Exchange back pressure. Back pressure is feature which tries to combat resource saturation from message-related activity.

Exchange Message Store

If you are not familiar with how Exchange works, here is quick run down. There are two critical components of e-mail messages. First, there is the mailstore itself. The mailstore is one or more files ended in .edb which can be very large depending on how much mail the server has. The second part corresponding to the mailstore file, are the log files. These end in .log and stored in multiple files. Both parts must be fully intact for the mailstore to operate correctly. A corrupt database and/or logfile will result in "dirty" shutdown and force the mailstore offline.

Once the mailstore is offline, it must be re-mounted. This can only be done if the two components mentioned above are in a clean state. There are two common reasons why the mailstore goes offline:
  • No available disk space
  • Sudden disconnect of drive which houses the mailstore or log files
  • Mailstore file itself is currupt and must be repaired or restored from backup

What is back pressure?

Introduced in Exchange 2007, a feature called back pressure ensures that a servers hard drive does not fill up due to growing log or exchange database files. It will also monitor the number of messages in queue and a few other critical resources. The idea behind back pressure, is to avoid a complete overrun of functions caused by too much activity. If back pressure is activated, it is a sign that something bad is happening to your Exchange server.

In reference to free disk space, back pressure will activate at 2 different levels. First, the "medium" level will allow users to send messages among themselves inside your Exchange environment. No new messages will be processed from outside the Exchange organization. They will be accepted into a queue, but not processed or delivered to mailboxes. The default value for a medium back pressure event to trigger is less than 3% free disk space.

Back pressure will again activate at a "high" level. Once the amount of available hard drive space is less than 1% of the total, Exchange comes to a screeching halt. Mail flow will stop both inside and outside the organization. No mail will be submitted to any queues and incoming mail from outside the organization will be rejected. Here is the event received once medium back pressure occurs:

Event ID: 15005

Description: The Microsoft Exchange Transport service is rejecting message submissions because the available disk space has dropped below the configured threshold.

Resource utilization of the following resources exceed the normal level:
Queue database logging disk space ("C:\Program Files\Microsoft\Exchange Server\TransportRoles\data\Queue\") = 97% [Medium] [Normal=95% Medium=97% High=99%]
Physical memory load = 91% [limit is 94% before message dehydration occurs.]

Back pressure caused the following components to be disabled:
Inbound mail submission from the Internet
Mail submission from the Pickup directory
Mail submission from the Replay directory

The following resources are in the normal state:
Queue database and disk space ("C:\Program Files\Microsoft\Exchange Server\TransportRoles\data\Queue\mail.que") = 96% [Normal] [Normal=95% Medium=97% High=99%]
Version buckets = 1 [Normal] [Normal=40 Medium=60 High=100]
Private bytes = 7% [Normal] [Normal=71% Medium=73% High=75%]

Once disk space returns to a normal level, you will see the following event, and Exchange will resume normal operation.

Event ID: 15005

Description: Resource pressure decreased from Medium to Normal.

No components were disabled because of back pressure.
The following resources are in the normal state:
Queue database and disk space ("C:\Program Files\Microsoft\Exchange Server\TransportRoles\data\Queue\mail.que") = 95% [Normal] [Normal=95% Medium=97% High=99%]
Queue database logging disk space ("C:\Program Files\Microsoft\Exchange Server\TransportRoles\data\Queue\") = 95% [Normal] [Normal=95% Medium=97% High=99%]
Version buckets = 0 [Normal] [Normal=40 Medium=60 High=100]
Private bytes = 6% [Normal] [Normal=71% Medium=73% High=75%]
Physical memory load = 60% [limit is 94% before message dehydration occurs.]

Preventing back pressure

There are several ways to prevent this. First, make sure the exchange mailstore is backed up on a nightly basis. This will ensure the log files are committed to the Exchange database and will drastically reduce the amount of disk space used. I was shocked how quickly disk space grows on a busy Exchange server that isn't backed up nightly. Also, back pressure can be activated by other factors such as available memory and/or buckets. That is not discussed in this article, but having an adequate amount of physical memory should decrease the chances for such events.

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