I'm not sure there is a "downside" to be had - it is an important requirement to every billing solution of any scale at all. The term is borrowed from accounti

I'm not sure there is a "downside" to be had - it is an important requirement to every billing solution of any scale at all. The term is borrowed from accounting (http://en.wikipedia.org/wiki/Double-entry_bookkeeping_system http://en.wikipedia.org/wiki/Double-entry_bookkeeping_system) and states that each transaction gets recorded in a different ledger. Our implementation at Twilio divides the two in stages of the transaction lifecycle that we allude to in the OP - in-memory for in-flight (when a call or SMS message is initiated) and disk for post-flight (when a call or SMS message is complete).