Understanding Write Cache Policy

The cache controller writes a block of data to cache memory, which is much faster than writing to the physical disk. The cache controller sends an acknowledgement of data transfer completion to the host system. 


Write-Back Versus 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 when system activity is low or when the write buffer approaches capacity. The cached data is not written to the storage device immediately. 


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. 


Write-back caching has a performance advantage over write-through caching. 


NOTE: 
The default cache setting is write-back caching.  


NOTE: 
Certain data patterns and configurations perform better in 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. 


Conditions Under Which Write-Through is Employed
Write-through caching is used under all conditions in which the battery is missing or in a low-charge state. Low-charge state is when the battery is not capable of maintaining data for at least 24 hours in the case of a power loss. 


Conditions Under Which Forced Write-Back With No Battery is Employed
Write-Back mode is available when the user selects 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