NEC Express5800 series User Manual page 91

The Write Policy has the following modes depending on combination with WrtThru for BAD BBU.
Select a mode suitable for your environment.
Write WBack
Policy
WThru
IMPORTANT:
If constant write back mode is selected, the controller operates in
write back mode even if the battery has failed or insufficiently
charged. The data in cache memory may be lost at an occurrence of
power failure.
Be sure to use UPS when specifying the constant write back mode
for write policy.
WrtThru forBAD BBU
Checked
Normal write back mode
(recommended)
The controller uses cache memory
for writing. However, if the battery is
being charged or failed, the
controller operates in WThru (write
through) mode automatically. Thus,
this mode can provide higher data
security.
Write through mode
The controller does not use cache
memory for writing data.
This mode can provide the highest
data security, however, the writing
performance is lower than that in
write back mode.
Configuring Your Storage and I/O Blade
Unchecked
Constant write back mode
The controller always uses cache
memory for writing regardless of
battery charged status or existence
of battery.
Be sure to use UPS when specifying
this mode for write policy.
* This mode is unavailable.
If you do not check "WrtThru for BAD
BBU" at creation of VD, this item is
automatically checked after the VD
has been created.
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