Conditions under which write-back is employed, Conditions under which write-through is employed – Dell PowerEdge RAID Controller 5i User Manual
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PERC 5 Features
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 present and in good
condition (healthy), or if a learn cycle is in process. Dell recommends 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.
Learn Cycle Completion Time Frame
The time frame for completion of a learn cycle is a function of the battery charge capacity and the
discharge/charge currents used. For PERC 5, the expected time frame for completion of a learn cycle is
approximately seven hours and consists of the following parts:
•
Learn cycle discharge cycle: approximately three hours
•
Learn cycle charge cycle: approximately four hours
Learn cycles shorten as the battery capacity derates over time.
NOTE:
See the storage management application for additional information.
During the discharge phase of a learn cycle, the PERC 5 battery charger is disabled. In this phase, the
battery voltage is monitored through the smart battery bus (SMBus) using the battery gas-gauge. When
the battery reaches the discharge capacity threshold (DCT), the dummy load is disabled and the battery
charger is re-enabled. At this point, the battery charger detects that the battery is below the fast charge
trigger voltage (FCTV) and initiates a fast-charge of the battery. The learn cycle completes once the
battery fast charge is completed.
Dell_PERC5_UG.book Page 20 Tuesday, February 13, 2007 6:02 PM