Chapter 9 High Availability for FortiOS 5.0 : Operating a cluster : Upgrading cluster firmware : Changing how the cluster processes firmware upgrades
  
Changing how the cluster processes firmware upgrades
By default cluster firmware upgrades proceed as uninterruptable upgrades that do not interrupt traffic flow. If required, you can use the following CLI command to change how the cluster handles firmware upgrades. You might want to change this setting if you are finding uninterruptable upgrades take too much time.
config system ha
set uninterruptible-upgrade disable
end
uninterruptible-upgrade is enabled by default. If you disable uninterruptible-upgrade the cluster still upgrades the firmware on all cluster units, but all cluster units are upgraded at once; which takes less time but interrupts communication through the cluster.