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This is best demonstrated when we just try to request a single CSS file from our IIS server. Sometimes we get the whole uncompressed file back. We'll wait 5 seconds and refresh, then the CSS file is gzipped, 1/5 the original size (which is what we want)... but if we wait another minute, it'll be back to it's original uncompressed version again.

Any ideas what could cause this or how to troubleshoot it?

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2 Answers 2

Look into frequentHitThreshold and frequentHitTimePeriod. These are used by the caching module when determining the compressibility of content.

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Static compression will only happen if the files being compressed are larger than the server defined minimum file size and if you have enough available disk space for the app pool (also a server defined setting). This TechNet article explains further:

http://technet.microsoft.com/en-us/library/cc730629%28WS.10%29.aspx

In your case, your app pool has likely run out of available disk space for compressed files in the cache directory.

It is also possible that if CPU usage on your server is high, compression may have been temporarily disabled (set with staticCompressionEnableCpuUsage and staticCompressionDisableCpuUsage). Here is an additional reference:

http://www.iis.net/ConfigReference/system.webServer/httpCompression

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