Dell PowerEdge Expandable RAID Controller 3 Guide de l'utilisateur télécharger pdf (Page 32)

32 About PERC 6 and CERC 6/i Controllers

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 virtual disks individually.

All RAID volumes will be presented as Write-Through (WT) to the operating

system (Windows and linux) independent of the actual write cache policy of

the virtual disk. The PERC/CERC controllers manage the data in cache

independently of the OS or any applications. Use OpenManage or the BIOS

configuration utility to view and manage virtual disk cache settings.

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.

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.

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Dell PowerEdge Expandable RAID Controller 3 Guide de l'utilisateur Page 32