If you do performance tuning or troubleshooting with glance probably you have observed the terms logical I/O and physical I/O. This post is to clarify these shortly:
Physical I/O: the traffic toward the storage subsystem.
Logical I/O: the requests which come from running processes and threads trough system calls. If the requested data is not to be found in memory, it generates a physical I/O and the system fetches it from the disks.
Note that some tools are reporting either logical or physical I/O, and they simply flag it as “I/O” so you might not have the idea about what kind of I/O it reports. Built-in application performance monitoring tools are reporting the logical I/O (e.g. statspack, awr) while system tools like sar or iostat are reporting the physical I/O. To be sure use glance, it is capable of showing both physical and logical I/O if you select a process or thread.