This check makes sure there is enough free memory on the current Confluence node for the external process pool (also known as sandboxes), which are used for specific resource intensive tasks, such as document conversion and PDF export.  This check applies to Confluence 6.10 and later. 

Each Confluence node in the cluster must have at least 2GB free, on top of Confluence's memory requirements, and that of your operating system and other essential utilities.  This memory requirement may be greater if you choose to configure more than the default number of sandboxes.  

この問題の解決方法

To resolve this issue you'll need to either provision more memory, or free up memory by moving non-essential applications or utilities to another server.

If you're not able to provision more memory at this time, you can choose to disable the external process pool using a system property:

• For Confluence 6.10 and 6.11, set document.conversion.sandbox.disable  system property to true and document conversions will be handled by Confluence. 
• For Confluence 6.12 and later, set pdf.export.sandbox.disable and document.conversion.sandbox.disable  system properties to true and both document conversions and PDF export will be handled by Confluence.
   

この警告を無視するとどうなりますか?

If the external process pool does not have enough memory, and the sandbox process hits an out of memory error, the sandbox will be terminated. A new sandbox will be spawned for the next request. 

• For document conversion, this means thumbnail generation will fail for the uploaded file. A placeholder thumbnail will be used on the page, and a download option will be provided in the file preview. 
• For PDF export, this means the export will fail, and the user requesting the export will see an error.