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DELL PERC H700 and H800 Technical Guide 20
4.10 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 will be 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. You can use OpenManage or
the BIOS configuration utility to view and manage virtual disk cache settings.
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. Write-Back caching has a performance advantage over Write-Through caching. The default
cache setting for virtual disks is Write-Back caching. Certain data patterns and configurations
perform better with a Write-Through cache policy.
Write-Back caching is used under all conditions in which the battery is present and in good
condition.
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.
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.
It is
recommended that you use power backup system when forcing Write-Back to ensure there is no loss
of data if the system suddenly loses power.
4.11 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:
• Always Read Ahead—Read-Ahead capability 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.
• Adaptive Read Ahead—When selected, the controller begins using Read-Ahead if the two
most recent disk accesses occurred in sequential sectors. If the read requests are random, the
controller reverts to No Read Ahead mode. Note: The default read cache setting for virtual
disks is Adaptive Read Ahead.