APPX_IO_LOG=filename
APPX_FMT_IO_LOG=1

The first will tell Appx to log all AppxIO actions to filename. The second will tell Appx to write the log in an easier to read format.

The log file will show all reads, readnexts, writes, deletes, and rewrites. Each entry will show a complete dump of the data being read, written, or deleted. For read operations, the log will show the key number and key value being retrieved.

You'll also see log entries for each process that runs so you can tell which process caused each I/O.