Dell UCSA-901, UCSE-900 manual Virtual disk write cache policies, Virtual disk read cache policies

NOTE: For more information on Foreign Configuration View, see Foriegn Configuration View.

5.Exit the BIOS Configuration Utility and reboot the system.

Virtual disk write cache policies

The write cache policy of a virtual disk determines how the controller handles writes to the virtual disk.

The write cache policies are:

Write-Back	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.
	NOTE: The default cache setting for virtual disks is Write-Back caching.
Write-Through	The controller sends a data transfer completion signal to the host system when the
	disk subsystem has received all the data in a transaction.
	All RAID volumes are presented as Write-Through 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.
	NOTE: Certain data patterns and configurations perform better with a Write-
	Through cache policy.

NOTE: Use the Dell OpenManage storage management application or the BIOS Configuration Utility to view and manage virtual disk cache settings.

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 forced write-back with no battery is employed

CAUTION: It is recommended that you use a power backup system when forcing Write-Back to ensure there is no loss of data if the system suddenly loses power.

Write-Back mode is available when you select 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.

Virtual disk read cache policies

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

Read Ahead	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.

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