That's a good question, billhilly. In my truck I always have a real sharp Buck knife with a 4 inch blade. And I keep a crowbar in the bed. Old beat up rusty crowbar, nobody bothers to steal it.
If I came across a dog, or several dogs attacking someone, I would put the knife in my pocket and I would attack the dogs with the crowbar. I know these attack dogs are bad asses but I don't think they could take but one whack across the spine, or the head, I think the spine would get fractured real quick.
If I had to fight the dog with just the knife, I would start shoving it in to his chest or abdomen, two or three strokes like that would take the fight out of him.
If you get into a brawl with several dogs, take it for granted you are going to the hospital. If the dog manages to cut your femoral artery, or the popliteal artery [behind the knee] you are going to the morgue.

In a case like that teenage girl in Georgia I would have to attack those dogs and we would just have to see what happens. It was a life and death matter for that girl and in about one more minute of that attack, she would have been dead.