Path Control Commands
Path control commands are primarily intended for environments where the disk data
sets are shared among application programs. The sharing may be across two
applications that are each running under different Operating Systems both of which
are controlled by VM, or data sets may be shared between LPARs.
Device Reserve
Description:
The Device Reserve command transfers 32 bytes of sense data to
the channel.
If the logical volume is busy or reserved to another LPAR, the Device Reserve
command is I/O queued. Internal Disk queues CCW chains using a FIFO queue
discipline. While the chain is queued, the channel emulator in Internal Disk
maintains the subchannel in a Start Pending state. If the logical device is not busy,
the Device Reserve command operates even if the device is not ready, or if the
physical drive is busy.
See “Effect of Subsystem and Device State on Status Presented to Command” on
page 2-5 for the operation of this command when other subsystem and device
status conditions occur.
The Device Reserve command includes all the Sense command functions, and also
reserves the logical volume to the channel path group of the channel sending the
command. If the command is on a channel path where the device paths are
grouped, the logical volume is reserved to the path group. If the command is
received on a channel path that is not grouped, the addressed logical volume is
reserved to that channel path. The device reservation lasts until that channel or
any channel in the channel path group successfully completes a Device Release
command to the logical volume, or all paths where the logical volume is reserved
are reset by a system reset or a subsystem IML.
When a Device Reserve command is sent to a logical volume in the contingent
allegiance state, much logical volume status testing normally performed during
initial selection is bypassed. This is so the control unit can present zero initial
status and report the error information from the last unit check. The Sense, Device
Reserve, Device Release, and Unconditional Reserve commands bypass the
normal logical volume status testing.
Notes:
1. Device reservation is reset for an individual channel path if a control unit
receives a system reset from that channel.
2. If a logical volume is reserved to a channel path or channel path group the
existing device reservation is reset for that channel path or channel path group
when any channel successfully completes the Unconditional Reserve
command. The logical volume becomes reserved to the channel path or path
group that sent the Unconditional Reserve command.
Hex Code
Data Address
Count
B4
The main storage location for the first sense byte.
32
4-118
Internal Disk Subsystem Reference Guide
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