A customer told me he did an upgrade from a VSS (2 6500 equipments). He loaded the new IOS on appliances and reboot the Standby-HOT unit. At this moment he noticed the service was down for 30-40 seconds and I think it's impossible cause all connections were redundants and should be managed by active unit.
I'm gonna to upgrade a VSS and I'd like to know if the customer afirmation is correct.
If you just upgrade the stand-by and reboot, due to different versions of code the VSS goes in to dual-active mode
Take a look at this document (all the way at the bottom). You have to follow the instruction on how to use eFSU without loss of any connectivity.