There are a million topics on this but I still struggle with it.
I am primarily in SPY and building a position in Vanguard Ex US. Even in looking at total stock market performance, Vanguard Total stockmarket still drew down 40%+ during the 08 recession. The Total Bond fund proved pretty resilient though and was down only about 4%.
I have been thinking maybe the best option is to allocate any excess performance to bonds. But I am unsure how others think about this