http://publib.boulder.ibm.com/infocenter/zos/v1r9/index.jsp?topic=/com.ibm.zos.r9.ieae200/mih.htm
Missing interrupt handler (MIH)
z/OS V1R9.0 MVS Initialization and Tuning Reference
SA22-7592-15
The MIH detects missing interrupt conditions. The following device
conditions
qualify as missing interrupts if the specified time interval has
elapsed:
- Primary status interrupt pending
- Secondary status interrupt pending
- Start pending condition
- Idle with work queued
- Mount pending
If the MIH detects a missing interrupt, processing will be done that
is
dependent on the detected condition. For any detected missing
interrupt, the
MIH performs one or more of the following actions:
- Invokes the device dependent MIH exit (if it exists)
- Records the condition in the SYS1.LOGREC data set
- Notifies the system operator
- Clears the condition
- Simulates an interrupt
- Redrives the device
- Requeues the I/O request
When specifying time intervals, consider the following:
- The MIH detects a missing interrupt condition within 1 second of
the time
interval that you specify.
- If the time interval is too short, a false missing interrupt can
occur
and cause early termination of the channel program. For example, if a
30-second
interval is specified for a tape drive, a rewind might not complete
before
the MIH detects a missing interrupt.
- If the time interval is too long, a job or system could hang
because the MIH has not yet detected a missing interrupt. For example,
if
a 15-minute time interval is specified for a tape drive used as an IMS™
log tape, the
MIH could delay IMS for 15 minutes because of MIH detection.
- During IPL (if the device is defined to be ONLINE) or during the
VARY
ONLINE process, some devices may present their own MIH timeout values,
via
the primary or secondary MIH timing enhancement contained in the
self-describing
data for the device. The primary MIH timeout value is used for most I/O
commands,
but the secondary MIH timeout value may be used for special operations
such
as long-busy conditions or long running I/O operations. Any time a user
specifically
sets a device or device class to have an MIH timeout value that is
different
from the IBM-supplied default for the device class, that value will
override
the device-established primary MIH time value. This implies that if an
MIH
time value that is equal to the MIH default for the device class is
explicitly
requested, IOS will not override the device-established
primary MIH time value. To override the device-established primary MIH
time
value, you must explicitly set a time value that is not equal to the
MIH default
for the device class.
Notes:
- To cancel MIH processing for specific devices, specify 00:00 for
the time
interval value (mm:ss) defined for the associated devices.
- During IOS recovery processing, the system will override your
time interval
specification and may issue MIH messages and MIH error records in
SYS1.LOGREC
at this IOS determined interval.
- Any dynamic change that causes a device's UCB to be deleted and
then
re-added will cause the device's MIH time interval to be reset to the
default MIH setting for its device class. Once the dynamic change is
completed,
users should reissue the SETIOS MIH MVS™ command to reestablish any
specific MIH
setting for the device. For more information about the SETIOS MIH
command, see z/OS MVS System Commands.
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