Users Guide
Controller — Supported Stripe Sizes
When creating a virtual disk, you must specify the stripe size for the virtual disk. Dierent controllers have dierent limitations on the stripe
sizes they can support. For information on the supported stripe sizes for a controller, see the virtual disk specications section for the
controller in Supported Features.
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.
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.
• 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 rst 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.
NOTE
: For PERC 10 Controllers ‘Force Write Back’ policy is same as the ‘Write Back’ policy.
• Write Back Enabled — The controller rmware disables the write cache if it does not detect the presence of a charged battery over a
specied period. For example, on some controllers, the write cache is disabled if the rmware 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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Controllers