Solution by Anonymous

This message will indicate which diagnostic log is reaching the maximum size limit. Example:
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The size of this folder C:\Program Files\Microsoft\Exchange Server\V15\Logging\Diagnostics\DailyPerformanceLogs has reached 95% of max size allowed - 5120 MB. Some data will be purged once it reaches the max limit.
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You can edit the maximum size limit for each diagnostic log in the Microsoft.Exchange.Diagnostics.Service.exe.config file. This file is usually located in C:\Program Files\Microsoft\Exchange Server\V##\bin\ (where ## is your exchange version, such as 15) and to edit the size limit for the file listed in the example, you would change the MaxSize value in this line: