diff --git a/storage/storage.go b/storage/storage.go
index f3e48ca8..84de7b57 100644
--- a/storage/storage.go
+++ b/storage/storage.go
@@ -24,7 +24,12 @@ type StorageDriver interface {
 	// numStats < 0, then return all stats stored in the storage.
 	RecentStats(containerName string, numStats int) ([]*info.ContainerStats, error)
 
+	// Read the specified percentiles of CPU and memory usage of the container.
+	// The implementation decides which time range to look at.
 	Percentiles(containerName string, cpuUsagePercentiles []int, memUsagePercentiles []int) (*info.ContainerStatsPercentiles, error)
 
+	// Returns samples of the container stats. If numSamples < 0, then
+	// the number of returned samples is implementation defined. Otherwise, the driver
+	// should return at most numSamples samples.
 	Samples(containername string, numSamples int) ([]*info.ContainerStatsSample, error)
 }