Digital Equipment HSZ40, Справочное руководство

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2–18 ADD UNIT
PREFERRED_PATH=THIS_CONTROLLER
PREFERRED_PATH=OTHER_CONTROLLER
NOPREFERRED_PATH (Default)
___________________________ Note ____________________________
All partitions on a container must be addressed through the same controller. If
you set
PREFERRED
_
PATH
for one partition, all partitions on that container will
inherit the same path.
____________________________________________________________
The
PREFERRED
_
PATH
switch allows you to balance I/O load by specifying the controller
through which the unit will be accessed. If you set
NOPREFERRED
_
PATH
for a unit, it can be
accessed through either controller.
The controllers only use the
PREFERRED
_
PATH
setting if they are in a dual-redundant
configuration. If one controller fails, all the devices are accessed through the remaining
controller, ignoring the
PREFERRED
_
PATH
setting.
___________________________ Note ____________________________
The
PREFERRED
_
PATH
and
NOPREFERRED
_
PATH
switches are valid only for HSZ
controllers in dual-redundant multiple bus-failover configurations. The preferred
path for units on dual-redundant HSZ controllers is determined by the first digit
in the unit number (the controller target ID) specified in the
ADD UNIT
command.
____________________________________________________________
When the failed controller is restarted, the drives automatically return to the controller
specified by the
PREFERRED
_
PATH
switch.
You can specify the
PREFERRED
_
PATH
switch on a single controller; however, the switch
will not take effect until you add a second controller and configure the two controllers for
dual-redundancy.
READ_CACHE (Default)
NOREAD_CACHE
The
READ
_
CACHE
switch allows you to enable or disable the use of read cache with a unit.
Read caching improves performance in almost all situations, so it is generally good to
leave it enabled. However, under certain workloads, like a backup, there may be a low
probability for a cache hit and it may be beneficial to turn read caching off to remove the
overhead of caching.
RUN (Default)
NORUN
The
RUN
switch allows you to enable and disable a unit's availability to the host.