I believe FWASYNC_MAXBUF is used by the fwm process on a SMS to buffer incoming operations that fwm needs to perform. In R77.30 management and earlier, fwm had far too many responsibilities and was mostly single-threaded. This was a bad combination and led to fwm daemon crashes under heavy load which would eject the administrator from the SmartDashboard and all other GUI tools.
fwm still exists in R80+ management, but mainly to service the older legacy GUIs like the Smartview Monitor and SmartDashboard and a few other operations. Given the wholesale changes in R80+ management, fwm has much less to do these days and has been pretty stable from what I've seen; the new cpm daemon has taken over most of fwm's prior responsibilities. As such I wouldn't worry too much about increasing this buffer for fwm in R80+ management, unless you are actively seeing warnings about running out in $FWDIR/log/fwm.elg.
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