Dell PowerEdge RAID Controller H800 - Manual (Page 29)

Storage Controller Features 29
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 are 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 cards manage the data in cache independently of
the operating system or any applications. Use Dell 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 PERC H700 or H800 cards. 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.
PERC7.2_UG.book Page 29 Thursday, March 3, 2011 2:14 PM