I've got a persistent live USB set up that I'm running Ubuntu 14.04 off of. When I boot up my computer, I'm given the option of running the persistent live USB or running the persistent live USB "from RAM". My understanding is that with the first option "everything" will be read off the USB (which will be very slow), whereas with the second option "everything" will be read off the USB and put into RAM (which will be very fast).
Is this correct? If so, can someone narrow down what "everything" means? Is it just the OS that gets read into memory, or are all the persistent files also read into RAM? Conversely, if I choose not to use the "from RAM" option, what is read from the USB and what is read into the PC's memory? Also, at what point are any new files I've added written to the USB if I'm using the "from RAM mode"? Is it just at shutdown? If that's the case, what happens if my computer shuts off unexpectedly? Would I lose any files I've created in the current session?