ConstReadonlyLarge: "Large"use when driver uses disk or near ~10ms delays
ReadonlyNormal: "Normal"use when driver uses network or near ~50ms delays
ReadonlySmall: "Small"use when driver uses API or near ~100ms delays
ReadonlyVeryLarge: "VeryLarge"use when driver is synchronous or near synchronous
ReadonlyVerySmall: "VerySmall"use when driver uses API which have cost for operations
Speed of the Tachyon Storage driver.