To view I/O statistics for database and log files, use fn_virtualfilestats.
The arguments for this function is ([Database ID], [File ID])
For example:
To obtain the I/O stats for the TempDB database file the syntax would be
SELECT * FROM ::fn_virtualfilestats (2, 1)
To obtain the I/O stats for the TempDB log file the syntax would be
SELECT * FROM ::fn_virtualfilestats (2, 2)