Would you mind to try Nginx caching in Plesk Onyx 17.8 Preview?
This would be very helpful to get your experienced feedback here http://www.surveygizmo.com/s3/4165754/Plesk-17-8-Preview
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This guide is for the older version of Magento platform. We recommend you to use the latest version of Magento. https://www.cloudways.com/blog/how-to-configure-varnish-cache-magento/