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Management Applications for PERC Cards 57
Write Policy Write Policy specifies the controller write policy. You can set the write
policy to Write-Back or Write-Through.
In Write-Back caching, the controller sends a data transfer
completion signal to the host when the controller cache has received
all the data in a transaction.
NOTE: If a Battery Backup Unit (BBU) is present, the default cache
setting is Write-Back. If no BBU is present, the default cache policy
default setting is Write-Through.
NOTE: If Write-Back is enabled and the system is quickly turned off and
then on, the controller may pause as the system flushes cache memory.
Controllers that contain a battery backup default to Write-Back caching.
In Write-Through caching, the controller sends a data transfer
completion signal to the host when the disk subsystem has received
all the data in a transaction.
Read Policy Read-Ahead enables the read ahead feature for the virtual disk.
You can set the parameter to Read-Ahead, No-Read-Ahead,
or Adaptive. The default is Adaptive-Read-Ahead.
Read-Ahead specifies that the controller uses Read-Ahead for the
current virtual disk. Read-Ahead capability allows the controller to
read sequentially ahead of requested data and store the additional
data in cache memory, anticipating that the data is required soon.
No-Read-Ahead specifies that the controller does not use read ahead
for the current virtual disk.
Adaptive specifies that the controller begins using Read-Ahead if the
two most recent disk accesses occurred in sequential sectors.
If all read requests are random, the algorithm reverts to
No-Read-Ahead; however, all requests are still evaluated for
possible sequential operation.
Table 5-2. Virtual Disk Parameters and Descriptions
(continued)
Parameter Description