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  • iNETZO 93 posts 412 karma points
    Feb 24, 2020 @ 19:08

    Is it a problem to set fcnMode to Disabled?


    This week we had performance issues and high CPU usage with a large Umbraco website. It quite heavily uses ImageProcessor for resizing a lot of images. We found out that moving the ImageProcessor cache outside the wwwroot boosted the performance a lot but it got even better when fcnMode in Web.config was set to "Disabled" instead of "Single", which is default for Umbraco.

    What are the consequences if fcnMode is set to Disabled except that view files are no longer being monitored? We use a self-hosted server, no cloud hosting.

    Best regards,


  • Martin Blackwell 14 posts 72 karma points c-trib
    Feb 27, 2020 @ 13:28
    Martin Blackwell

    Hi iNETZO,

    Have you managed to get any more information relating to this?

    I am seeing a very similar issue on another large client, that again heavily uses the ImageProcessor and we are experiancing 100% CPU useage around the time of save and publish events.

    We haven't set the fnMode to "Disabled" yet, as the client wants evidence that this is the root cause of the issue.

    Many thaks,


  • iNETZO 93 posts 412 karma points
    Feb 27, 2020 @ 17:58

    Hi Martin,

    Did you also moved the cache-directory outside the root folder of your site? We moved the cache folder on the same level as the root-folder of the website and changed in the file “config/imageprocessor/cache.config”

        <setting key="VirtualCachePath" value="~/app_data/cache" />


        <setting key="VirtualCachePath" value="~/../cache" />

    We did delete the cache-folder from AppData\Temp to reduce the number of files under the websiteroot. Don't forget to set the right permissions on the folder (Modify-permission for IIS_IUSRS). This helped us a lot in reducing CPU usage dramatically. But it even got much better changing the fcnMode to “Disabled”. We’re trying this option for now and wait until there are people complaining ;-)

    Best regards,


  • Dan Diplo 1490 posts 5809 karma points MVP 4x c-trib
    Feb 27, 2020 @ 14:59
    Dan Diplo

    Setting fcnMode to disabled will disable all file watchers within the application. This means that when watched .config files change (eg. web.config) then the app domain will no longer restart. So think whether this will be a problem for you?

    See what Shannon wrote about this:

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