Software Support

RAID Controller Read, Write, Cache, And Disk Cache Policy
When creating a virtual disk, you can specify the read, write, and cache policies for the virtual disk. The following
subsection describes these policies.
Related Links
Changing The Virtual Disk Policy
Read Policy
Does my controller support this feature? See Supported Features.
The read policies indicate whether the controller should read sequential sectors of the virtual disk when seeking data.
• Read Ahead — The controller reads sequential sectors of the virtual disk when seeking data. Read ahead policy
may improve system performance if the data is written to the sequential sectors of the virtual disk.
• No Read Ahead — Selecting no read ahead policy indicates that the controller should not use read ahead
policy.
• Adaptive Read Ahead — 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 initiates read ahead if necessary.
• Read Cache Enabled — The controller reads the cache information to verify 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 be retrieved more quickly from the cache than
from the disk.
• Read Cache 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 if the controller sends a write-request completion signal as soon as the data is in the cache or
after it has been written to the disk.
• Write Back — 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 retrieve data quickly from the cache then from the disk. However, data loss may
occur in the event of a system failure which prevents that data from being written on a disk. Other applications
may also experience problems when actions assume that the data is available on the disk.
NOTE: Storage Management does not allow you to select the Write Back policy for controllers that do not
have a battery. The only exceptions are PERC S100 and PERC S300. This restriction protects a controller
without a battery from the loss of data that may occur in the event of a power failure. On some controllers,
the Write Back policy may be available in the controller BIOS even though it is not available in Storage
Management.
• Force Write Back — The write cache is enabled regardless of whether the controller has a battery. If the
controller does not have a battery and force write back caching is used, data loss may occur in the event of a
power failure.
• Write Back Enabled — The controller firmware disables the write cache if it does not detect the presence of a
charged battery over a specified period. For example, on some controllers, the write cache is disabled if the
firmware does not detect a charged battery within 72 hours.
• Write Through — The controller sends a write-request completion signal only after the data is written to the
disk. Write-through caching provides better data security than write-back caching, since the system assumes
the data is available only after it has been safely written to the disk.
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