Digital Equipment HSZ40, Справочное руководство

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2–80 SET unit-number
Select
WRITEBACK
_
CACHE
for all new RAIDsets and mirrorsets, and for all other units for
which you want this feature. The write-back cache improves performance for write
operations to the unit.
___________________________ Note ____________________________
It may take up to 5 minutes for the controller to flush unwritten data from the
cache once you disable write-back caching.
____________________________________________________________
Select
NOWRITEBACK
_
CACHE
for those units for which you want all writes to go directly to
devices without being cached.
Considerations When Using Write-back Caching
The following list summarizes considerations you must be aware of when using
write-back cache.
Two conditions will cause data contained within write-back cache to be lost: if
power from the main power supply and the external cache battery is
interrupted, or if the cache module is removed before the controller flushes
the data from the write-back cache.
•
When restarted, the controller attempts to flush any unflushed data within write-back
cache to the devices. However, by specifying the IGNORE_ERRORS or
IMMEDIATE_SHUTDOWN switch, you allow data to reside in write-back cache
when the controller is turned off, regardless of any errors detected.
•
RAIDsets and mirrorsets require data to be stored in write-back cache to
accomodate the write hole and to increase performance—without regard to the
WRITEBACK_CACHE switch setting.
•
If data is contained within the write-back cache while the subsystem is shut down,
do not perform any hardware changes until after the controller flushes the data to the
devices.
•
When restarted, the controller attempts to flush any unflushed data within the write-
back cache to the devices. However, by specifying the IGNORE_ERRORS or
IMMEDIATE_SHUTDOWN switch, you allow data to reside in write-back cache
when the controller is turned off, regardless of any errors detected.
Examples
To enable write protect and turn off the read cache on unit D102:
CLI> SET D102 WRITE_PROTECT NOREAD_CACHE
To set unit T47 to 1600 bpi: