Users Guide

Table 4. Write cache policies
Feature Description
Write-back 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.
NOTE: The default cache setting for virtual disks is Write-back caching. Write-back
caching is also supported for single drive RAID 0 virtual disks.
Write-through 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: Certain data patterns and configurations perform better with a write-through
cache policy.
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.
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 write-back 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 policy
The read policy of a virtual disk determines how the controller handles reads to that virtual disk.
Table 5. Read policies
Feature Description
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 slight improvement when accessing random data.
No read ahead Disables the read ahead capability.
NOTE: Adaptive read ahead is no longer supported. Selecting adaptive read ahead is equivalent to selecting the read ahead
option.
Virtual disk migration
The PERC 11 series supports migration of virtual disks from one controller to another without taking the target controller offline.
The controller can import RAID virtual disks in optimal, degraded, or partially degraded states. You cannot import a virtual disk
that is offline. When a controller detects a configured physical disk, it marks the physical disk as foreign, and generates an alert
indicating that a foreign disk was detected.
Disk migration pointers:
Features of PowerEdge RAID Controller 11
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