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
FortiAnswers is the space dedicated to FortiSASE and FortiOS questions and suggestions.
1 Person is following this question.
Can FortiOS be configured to limit the number of IPSec VPN clients?
RADIUS authentication group matching not working
The FortiGuard category based filter on Web Filter is... gone?
Is there a CLI command that reports the version of OpenSSL used by FOS?