1. A method comprising: receiving multiple write commands having corresponding write data; receiving multiple flush commands, the multiple flush commands defining corresponding flush epochs; issuing the write data to a persistent log on a physical storage device with consistency data; acknowledging an individual flush command that defines an individual flush epoch before confirming that at least some write data for the individual flush epoch has committed on the physical storage device; and after a crash, recovering the write data on the physical storage device to a consistent state using the consistency data.