Exchange 2010 Database Issues Due to Back Pressure

I just came across a problem with Public Folder connectivity failing intermittently during production hours; sometimes it was a total dismount of the PF Database. The problem was due to Back Pressure, which is a server resource being overloaded refusing connections.
All kinds of wacky things can happen from Back Pressure, from DB dismounts to messages becoming stuck in the queues.

There are three degrees of back pressure:

- Normal – everything is humming along smoothly

- Medium – Moderately over-utilized; the server begins limiting some connection types. Usually only external mail flow will be rejected.

- High – Severely over-utilized; the server rejects any new connections.

In our case disk utilization was the culprit, because we were above the  default 25% threshold for disk space, which caused the server to drop the Public Folder database. The problem was further exacerbated by the fact that we had no more space to extend the volume any more :(

The resolution was to adjust the Back Pressure threshold, which in my opinion is a little too stringent in the first place, but (disclaimer) you should fix the underlying issue so it doesn't keep happening.

To adjust the thresholds, open: C:\Program Files\Microsoft\Exchange Server\V14\Bin\EdgeTransport.exe.config

**Note** If you installed Exchange in a different location, look in the "bin" folder of your install directory.

In the "EdgeTransport.exe.config" file, modify these values to suit your environment:

PercentageDatabaseDiskSpaceUsedHighThreshold (Default 0)
PercentageDatabaseDiskSpaceUsedMediumThreshold (Default 0)
PercentageDatabaseDiskSpaceUsedNormalThreshold (Default 0)

The formula for disk utilization is:

Free space percentage = (disk size – 500MB) / disk size

The results are expressed as a percentage of the total disk space that is being used. 
By default, the medium level of hard disk drive utilization is 2 percent less than the high level, the normal level is 4 percent less than the high level.
Thus, you need to adjust the thresholds listed above to the amount of disk space that you need for the database(s) to stop dismounted and/or rejecting messages.