Dell PowerEdge RAID Controller 12 User’s Guide PERC H965i Adapter, PERC H965i Front, PERC H965i MX, and PERC H965e Adapter

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Virtual disk write cache and read-ahead policy

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

Table 1. Write cache policiesThe write cache policy of a virtual disk determines how the controller handles writes to the virtual disk.
Feature	Description
Write-back	The controller sends a data transfer completion response 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 of HDD-based VDs is write-back. Write-back caching is also supported for single drive RAID 0 virtual disks.
Write-through	The controller sends a data transfer completion response to the host system when the disk subsystem has received all the data in a transaction. NOTE:Certain data patterns and configurations perform better with a write-through cache policy.
Read-Ahead	Available in 8.8.0.0.18-26 and later versions of PERC12 controllers. The controller speeds up sequential reads on virtual disks. This is supported only on HDDs with write-back cache policy. NOTE:You can request for Read Ahead support on any VD including SSD but controller will change only the cache setting if it is supported.

NOTE: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. PERC cards manage the data in cache independently of the operating system or any applications.

NOTE:Use the Dell OpenManage Storage Management application or the HII Configuration Utility to view and manage virtual disk cache settings.