6. DATA SOURCES › 6.6 I/O Measurements and Device Activity › 6.6.3 I/O Measurements

6.6.3 I/O Measurements

How is I/O activity quantified? The answer is partially
determined for us by the measurement facilities provided by
the operating system.

The IOS has the responsibility of assigning a path to each
I/O request, requeueing the request if all paths are busy,
handling error conditions that could not be corrected by the
device controllers, and measuring the subsystem activity
level.  There is no direct measure of device busy time or
quantity of data transferred.  Measurement of device activity
must be done by a sampling technique because the CPU cannot
afford to spend time closely monitoring each device.

The IOS's response to an EXCP is limited to passing the I/O
request to the channel subsystem and notifying the AM when
the request is complete.  The channel subsystem carries out
most of the function that was assigned to IOS.  Because it is
dedicated to I/O management, the subsystem can keep track,
for each locally attached device, of the time spent in each
phase of a request's life (waiting for path, assigned but
disconnected, and connected for control or data transfer).

The SMF records generated for a batch job step, started task,
or TSO session contain EXCP sections that show I/O activity
by individual device address.  The SMF type 30 record
consolidates all I/O activity in the EXCP sections and
provides device connect time for each device accessed as
well.

The following subsections outline the I/O measurement data
elements in the CA MICS files of the Batch Information Area:

    1 - CA MICS EXCP Data Elements
    2 - CA MICS Device Connect Time Data Elements
    3 - APPC/MVS Data Transfer Elements