User Guide

NOTE: Use the Dell OpenManage storage management application or the BIOS Configuration
Utility to view and manage virtual disk cache settings.
Write-Back And 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 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 Non-Volatile Cache on the PERC H710,
H710P or H810 card.
NOTE: The default cache setting for virtual disks is Write-Back caching.
NOTE: Certain data patterns and configurations perform better with 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 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:
• Always 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.
• Adaptive Read Ahead—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.
Reconfiguration Of Virtual Disks
An online virtual disk can be reconfigured in ways that expands its capacity and/or change its RAID level.
NOTE: Spanned virtual disks such as RAID 10, 50, and 60 cannot be reconfigured.
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