This metric is not supported with Multi-Attach enabled volumes.
The total number of seconds spent by all read operations that completed in a specified period of time.
If multiple requests are submitted at the same time, this total could be greater than the length of the period. 
For example, for a period of 1 minutes (60 seconds): if 150 operations completed during that period, and each operation took 1 second, the value would be 150 seconds. 
For Xen instances, data is reported only when there is read activity on the volume.