ConstReadonlyLarge: 1use when driver uses disk or near ~10ms delays
ReadonlyNormal: 2use when driver uses network or near ~50ms delays
ReadonlySmall: 3use when driver uses API or near ~100ms delays
ReadonlyVeryLarge: 0use when driver is synchronous or near synchronous
ReadonlyVerySmall: 4use when driver uses API which have cost for operations
Speed of the Tachyon Storage driver.