While charging based on the amount of data stored is a reasonable metric, Wachs contended, the amount charged for I/O is flawed, given the work expended to read that data from disk or write that data to disk. The cost of handling those bits on disk may vary widely from one instance to another, Wachs pointed out.

"As a result, tenant bills for storage access may bear little to no relationship to the actual costs," the paper said.

Wachs mentioned a number of factors that can lead to this variance, the most prominent being the difference between random and sequential access on the disk.

In sequential access, data is written to or read from one portion of the disk in a continual stream of bits. In random access, the disk head must jump around to different parts of the disk to read or write data.

The difference between these two types of workloads can be immense, Wachs said.