what is the effect of making my kvm's HD cache none vs default?

I am playing with KVM migration, and I got an error about unsafe migration due to hdd cache settings. I found the fix, which is to set the cache to none and it works after I made the change.

But that leads me to wonder the following:

1. what effect, performance wise, will this have on my kvm?
2. why can't i migrate with hdd cache?

Thanks