If you use the RAID Web Console / MegaRAID Storage Manager software to perform a RAID Controller update, you may be given the option of an online firmware update. This is a new feature that has been made available in RAID Web Console 2 Version 15 and newer.
Normally when you flash the firmware through RAID Web Console, the firmware is uploaded to the controller, and then saved into the flash memory. However, the firmware upgrade does not actually take effect until the system is restarted. The online firmware update attempts to restart the controller with the updated firmware without needing to reboot the system.
In most situations, no. This should definitely not be attempted if your operating system is running from the RAID volumes hosted by the controller, or if you have any mission critical system functions hosted by that controller, including virtual machines, backup volumes or file shares. There will be a brief loss of controller access, and Windows Blue-screen crashes have been observed when using the online firmware update feature.