504 DS8800 Performance Monitoring and Tuning
14.3 Multiple Allegiance
Normally, if a System z host image (server or logical partition (LPAR)) sends an I/O request to
a device address for which the storage subsystem is already processing an I/O originating
from another System z host image, the storage subsystem sends back a
device busy
indication and the I/O must be retried. This response delays the new request and adds to
processor and channel overhead (this delay is reported in the RMF Device Activity Report
PEND time column).
With older storage subsystems (before the DS8000 or ESS), a device has an implicit
allegiance, that is, a relationship created in the disk control unit between the device and a
channel path group, when an I/O operation ...