Dell PowerEdge RAID Controller 6i - Manual (Page 32)

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About PERC 6 and CERC 6i Controllers
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 6, 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 deteriorates over time.
NOTE:
For additional information, see the OpenManage storage management
application.
During the discharge phase of a learn cycle, the PERC 6 battery charger is
disabled and remains disabled until the battery is discharged. After the
battery is discharged, the charger is re-enabled.
Virtual Disk Write Cache Policies
The write cache policy of a virtual disk determines how the controller handles
writes to that virtual disk. Write-Back and Write-Through are the two write
cache policies and can be set on a virtual disk basis.
Write-Back and Write-Through
In Write-Through caching, the controller sends a data transfer completion signal to
the host system when the disk subsystem has received all the data in a transaction.
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. The
controller then writes the cached data to the storage device in the background.
The risk of using Write-Back cache is that the cached data can be lost if there
is a power failure before it is written to the storage device. This risk is
mitigated by using a BBU on selected PERC 6 controllers. For information on
which controllers support a BBU, see Table 3-1.
Write-Back caching has a performance advantage over Write-Through
caching.
NOTE:
The default cache setting for virtual disks is Write-Back caching.
NOTE:
Certain data patterns and configurations perform better with a
Write-Through cache policy.
Dell_PERC6.1_UG.book Page 32 Wednesday, April 15, 2009 4:18 PM