Users Guide
information.
l Disk migration or foreign configurations. Some controllers enable you to move physical disks that contain one or more virtual disks to another controller.
The receiving controller is able to recognize and import the foreign configuration (virtual disks). See "Foreign Configuration Operations" for more
information.
Controller-supported RAID Levels
RAID controllers may support different RAID levels. For information on which RAID levels a controller supports, see the supported RAID levels section for the
controller in "Supported Features."
Controller-supported Stripe Sizes
When creating a virtual disk, you may need to specify the stripe size for the virtual disk. Different controllers have different limitations on the stripe sizes they
can support. For information on the stripe sizes a controller supports, see the virtual disk specifications section for the controller in "Supported Features."
RAID Controller Read, Write, Cache, and Disk Cache Policy
When creating a virtual disk, you specify the read, write, and cache policies for the virtual disk. The following sub-section describes these policies.
Read Policy
Does my controller support this feature? See "Supported Features"
The read policies indicate whether or not the controller should read sequential sectors of the virtual disk when seeking data.
l Read-Ahead. When using read-ahead policy, the controller reads sequential sectors of the virtual disk when seeking data. Read-ahead policy may
improve system performance if the data is actually written to sequential sectors of the virtual disk.
l No-Read-Ahead. Selecting no-read-ahead policy indicates that the controller should not use read-ahead policy.
l Adaptive Read-Ahead. When using adaptive read-ahead policy, the controller initiates read-ahead only if the two most recent read requests accessed
sequential sectors of the disk. If subsequent read requests access random sectors of the disk, the controller reverts to no-read-ahead policy. The
controller continues to evaluate whether read requests are accessing sequential sectors of the disk, and can initiate read-ahead if necessary.
l Read Cache Enabled. When the read cache is enabled, the controller reads the cache information to see if the requested data is available in the cache
before retrieving the data from the disk. Reading the cache information first can provide faster read performance because the data (if available in the
cache) can more quickly be retrieved from the cache than from the disk.
l Read Cache Disabled. When the read cache is disabled, the controller retrieves data directly from the disk and not from the cache.
Write Policy
Does my controller support this feature? See "Supported Features"
The write policies specify whether the controller sends a write-request completion signal as soon as the data is in the cache or after it has been written to
disk.
l Write-Back. When using write-back caching, the controller sends a write-request completion signal as soon as the data is in the controller cache but
has not yet been written to disk. Write-back caching may provide improved performance since subsequent read requests can more quickly retrieve data
from the controller cache than they could from the disk. Write-back caching also entails a data security risk, however, since a system failure could
prevent the data from being written to disk even though the controller has sent a write-request completion signal. In this case, data may be lost. Other
applications may also experience problems when taking actions that assume the data is available on the disk.
NOTE: Read, write, and cache policies are not supported on the CERC SATA1.5/2s controller.