
uses channel command retry to disconnect from the channel and recover from the
error or wait for the required resource.
Channel Command Retry–Control Unit Initiated
The control unit initiates retries for delayed selection and device orientation.
Delayed Selection and Device Orientation
Internal Disk accepts initial selection without selecting the physical device. The
command chain proceeds until an operation requires the device. When Internal
Disk determines that it needs the physical device, it presents channel command
retry to the channel. At the same time, Internal Disk selects and orients the device
to the correct position. If the device is doing an internal operation, the channel
remains in retry status until the device becomes available and activity completes.
When the device activity is complete, Internal Disk presents device end. The
channel then sends the command again, and the operation proceeds normally.
System and Selective Reset
System reset affects all status and allegiances for that logical path on all attached
logical volumes. Selective reset affects the status and allegiance of only the
selected logical volume.
The effect of system and selective reset depends on the operation in progress. For
example, asynchronous (internal) operations complete normally. For read hits in
cache, the operation stops without affecting the cache image. For write operations,
Internal Disk terminates the writing to the cache and discards the record. The
modified record is not written to the logical volume.
Effect of System Reset
A system reset affects all logical volumes that have allegiance to the logical path
issuing the reset. Status is cleared for all affected logical volumes. Logical
volumes that were Reserved are Released.
A system reset causes the path group ID for the logical path to reset. The
parameters associated with Set System Characteristics - PSF Order Code x'1D' are
also cleared by the system reset. See "Set System Characteristics - PSF Order
Code x'1D'" on page 4-129 for a description of the parameters.
Effect of Selective Reset
Selective reset affects only the chain in process at the time of the selective reset.
The chain processing is terminated and ending status is cleared. It does not alter
the reserved state of a logical volume or the valid path group ID for an interface.
Reset Notification
The control unit notifies the host that a system reset has been received. Any
system resetting event on Internal Disk causes a later channel initiated selection to
be unit checked. The sense data contains format 0, message 8 with a program
action code of X
'
16
'
.
The 3990 Model 2 has a procedure where the first selection on the interface will be
unit checked with sense data that includes equipment check and environmental
data present. This causes the event to be logged, which is appropriate for a stand
2-10
Internal Disk Subsystem Reference Guide
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