This happens during recovery. It can also happen during buffer prefetching, as an optimization (rather than performing multiple single-block reads). Database blocks that must be changed as part of recovery are read in parallel from the database.
Wait Time: Wait until all of the I/Os are completed
| Parameter | Description |
|---|---|
| files | This indicates the number of files to which the session is reading |
| blocks | This indicates the total number of blocks to be read |
| requests | This indicates the total number of I/O requests, which will be the same as blocks |
Oracle Database Internal Mechanism 