Virtual Disk Write Cache Policies
The write cache policy of a virtual disk determines how the controller handles writes to the virtual disk. Write-Back and Write-Through are the two write cache policies that can be set on virtual disks individually.
All RAID volumes are presented as Write-Through to the operating system independent of the actual write cache policy of the virtual disk. The Shared PERC 8 cards manage data in the cache independently of the operating system or any applications.
NOTE: Use the Dell Chassis Management Controller (CMC) to view and manage virtual disk cache settings.
Write-Back
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 during or when the data is written to the storage device. This risk is mitigated by using a Non-Volatile Cache on the Shared PERC 8 card.
NOTE: The default cache setting for virtual disks in single and dual controller systems is Write-Back caching.
Conditions Under Which Write-Back Is Employed
Write-Back caching is the default caching mode for single and dual Shared PERC 8 card configurations. Write-Back caching can be configured under all conditions in which the battery is present and in good condition.
If a virtual disk is already created on the VRTX system with the firmware version that does not support write-back caching, then the default will remain as write-through. The virtual disk must be manually changed to write-back before it goes to write-back mode, even if the system has been updated to a write-back supported firmware version.
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 loses power.
Write-Back mode is available when you select Write-Back Force. When Write-Back Force mode is selected, the virtual disk is in Write-Back mode even if the battery is not present.
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.
NOTE: Most configurations and workloads experience some reduction in performance when configured with Write-Through cache, as compared to Write-Back cache.
