The I/O by File chart shows SQL Server I/O statistics for each SQL Server database file.
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The following counters can be displayed by clicking the arrow next to the chart name:

Wait per I/O

The wait time per I/O (in milliseconds) for the selected database file.

Current I/O rate

The current rate at which SQL Server is performing physical read and write operations to this file (Reads rate plus Writes rate).

Current Read Rate

The current rate at which SQL Server is performing physical read operations from this file.

Current Write Rate

The current rate at which SQL Server is performing physical write operations to this file.

Total I/O

The number of kilobytes read from or written to this file since SQL Server started (KB Read plus KB Written).

Total Wait Time

The total number of milliseconds that SQL Server has spent waiting for I/O operations on this file since SQL Server started.

Current Wait Time

The number of milliseconds that SQL Server has spent waiting for I/O operations on this file since the last time data for this page was collected.