Similarly, when the system reads from disk, it uses an input cache, and will usually read more data than requested. Buffer caches are monitored by SAR, reporting the number of cache reads and writes, the number of disk reads and writes, and the cache hit ratio. If these statistics show an imbalance in file system activity, the system administrator might wish to relocate some datasets in order to balance the load. Another monitored operation is tty device activity, traffic to and from modems and terminals. The number of input and output characters and interrupt rates is monitored.
System calls occur when user programs request system services.
System swapping and switching activity is one of the most important parameters that can be monitored on a UNIX system. It remains in memory until the system needs the space for use by another process, at which point the contents of the space allocated to the process is moved to swap space on disk. It also monitors the number of process context switches. The context of a process is generally defined as its state, including values of user variables and data structures and machine registers. Excessive swapping is an indication that the machine is memory-starved, and overall performance suffers.