Conditions under which write-back is employed, Conditions under which write-through is employed, Virtual disk read policies – Dell PowerEdge RAID Controller 6i User Manual

Page 33

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About PERC 6 and CERC 6i Controllers

33

Conditions Under Which Write-Back is Employed

Write-Back caching is used under all conditions in which the battery is
present and in good condition.

Conditions Under Which Write-Through is Employed

Write-Through caching is used under all conditions in which the battery is
missing or in a low-charge state. Low-charge state is when the battery is not
capable of maintaining data for at least 24 hours in the case of a power loss.

Conditions Under Which Forced Write-Back With No Battery is
Employed

Write-Back mode is available when the user selects Force WB with no
battery. When Forced Write-Back mode is selected, the virtual disk is in
Write-Back mode even if the battery is not present.

CAUTION:

It is recommended that you use a power backup system when forcing

Write-Back to ensure that there is no loss of data if the system suddenly loses
power.

Virtual Disk Read Policies

The read policy of a virtual disk determines how the controller handles reads
to that virtual disk. Some read policies are:

Always Read Ahead - Read-Ahead capability allows the controller to read
sequentially ahead of requested data and to store the additional data in
cache memory, anticipating that the data is required soon. This speeds up
reads for sequential data, but there is little improvement when accessing
random data.

No Read Ahead - Disables the Read-Ahead capability.

Adaptive Read Ahead - When selected, the controller begins using Read-
Ahead if the two most recent disk accesses occurred in sequential sectors.
If the read requests are random, the controller reverts to No read ahead.

Dell_PERC6.1_UG.book Page 33 Wednesday, April 15, 2009 4:18 PM

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