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phantomscribe asked

What is 'conserve' mode on FortiGate?

When configuring FGT using GUI, sometimes I hit an error saying 'Cannot change configuration in conserve mode.' or some other CMDB modification error, ie. 'The CMDB add entry failed.' At this stage, I cannot modify/save any configuration changes to the FGT because the FGT has entered 'Conserve' mode. From my understanding, this 'Conserve' mode happens when the FGT is using up too much memory and the kernel will enter this mode to preserve memory, blocking user space processes from doing certain operations. Can someone elaborate more on this mode as how does it work, how to troubleshoot when it happens, how to prevent it, etc.
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stylus answered

Conserve mode was implemented to prevent a complete overrun of memory in the FortiGate unit to prevent it from freezing up. The proxies tended to use a lot of memory when buffering web pages, emails etc and holding them in memory during the scanning process. This occasionally lead to high memory usage when there were either many large files being processed simultaneously or many users downloading data.


There was originally a high and low watermark for the conserve mode. The low would enter conserve mode and prevent further new connections. The high watermark would become more aggressive and kill some of the largest memory hogging sessions.

There is a more recent KB article that you may find helpful : http://kb.fortinet.com/kb/microsites/search.do?md=displayKC&docType=kc&externalId=FD33103

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