2. PERFORMANCE REPORTING ANALYSIS › 2.4 I/O Configuration Analysis › 2.4.4 DASD I/O Component Analysis Inquiry › 2.4.4.2 Technique Tutorial › 2.4.4.2.1 I/O Component Analysis for MVS/XA and MVS/ESA
2.4.4.2.1 I/O Component Analysis for MVS/XA and MVS/ESA
I/O component analysis for MVS/XA is greatly simplified,
since four components of I/O service time are directly
measured and need not be estimated with queuing
relationships, as was the case with pre-MVS/XA systems. The
four components of I/O service time measured by MVS/XA
systems are:
o Queue - The average amount of time that I/O requests
to this device were held enqueued within IOS before a
SSCH instruction could be issued. Such an enqueue
occurs when this particular system already has an I/O
operation in progress on a device at the time another
I/O operation is requested by this same system.
o Pending - The time that the I/O was blocked by other
I/Os that used some part of the physical path to the
device. This can be caused by head of string or control
unit contention or reserve delays issued from other
systems.
o Disconnect - The time that the device spent in seek,
device latency, and extra rotations due to rotational
positioning sensing (RPS) misses.
o Connect - The time that the device was connected to the
path. This includes the time required for both channel
control word (CCW) and data transfer.
o ESCON - The time that the device was delayed due to an
ES Connection Director (ESCD) port being busy.
The first three of these components can be addressed by
tuning. The fourth component is a function of data set block
sizes and can only be addressed by changing the
characteristics of the data sets on the device. The last of
these components can be managed by adjusting the I/O load on
the ES Connection Director ports.
The queue component of I/O response time is the sum of the
time that the I/Os wait on the device's UCB (that is, device
busy condition) and the time that the I/O queues behind the
LCU (logical control unit) due to the utilization of the path
by other I/Os. If the utilization of the device is in excess
of 30 percent, the I/O queue time can be reduced by spreading
the data sets on the volume across two or more devices. If
the device utilization is low, the only way to address I/O
queuing is to move the volume to a lower activity string.
The pending component of I/O response time represents the
average time between successfully issuing the start
subcommand (SSCH) is issued by the logical control unit (LCU)
and acceptance of the first command by the the device. Two
factors contribute to this delay. They are the utilization
of the path (for example, channels or control units) by other
I/Os from the system being measured, or device contention
conditions that result from I/Os issued by other systems.
These effects may be evaluated using the DASD Skew and Shared
DASD Analysis Reports.
The disconnect component of I/O response time represents the
portion of the device service time during which it is
disconnected from the channel. This time includes seek,
latency, and extra revolutions that result from RPS missed
reconnects. The value shown in the report should be compared
to the ideal disconnect time for the device type.
The ideal disconnect time may be calculated by dividing the
sum of the average seek time and rotational delay for the
device type by two. For example, the average seek time of a
3350 is 25 milliseconds and the rotational delay is 16.7
milliseconds. Hence, the ideal disconnect time for 3350 type
devices is approximately 21 milliseconds.
By comparing the ideal disconnect time to the actual for the
device, you can determine the impact of RPS missed
reconnects. If this time is significant, you can move the
volume to a less active device string where the RPS miss
contribution is lower. You can use the DASD Skew Report to
evaluate strings where RPS misses are causing performance
problems.
Delays due to ES Connection Director port busy occur when I/O
actions are delayed by the temporary unavailability of an
ESCON director port. Unless a dedicated port-to-port
connection from a channel to a controller has been defined
within the ESCON director, a port-to-port connection will be
established dynamically only for the duration of an I/O
operation, with the I/O processor requesting termination of
the connection upon I/O completion. A channel along which
the I/O process is initiated must pass a request that the
ESCON director establish a dynamic connection to any of the
defined routes to the device for which the I/O operation is
intended. Each controller attached to the device is attached
to a port on the ESCON director. If each of these ports is
busy, an ESCON port busy condition will occur.
Of course, if there is direct channel connection to a
controller, ESCON port delays are not possible.
It is also possible to link two ESCON directors together, for
instance, in order to increase the distance permitted between
processors and devices. If this has been done, the link
between the directors may also be a source of contention for
which the port busy condition will occur.
