Users Guide

Controller — Supported Stripe Sizes
When creating a virtual disk, you must specify the stripe size for the virtual disk. Different controllers have different limitations
on the stripe sizes they can support. For information on the supported stripe sizes for a controller, see the virtual disk
specifications section for the controller in Supported Features.
RAID Controller Read, Write, and Disk Cache Policy
When creating a virtual disk, you can specify the read, write, and disk 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.
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 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.
NOTE: Write-through is the default write policy setting when cluster mode is enabled.
Disk Cache Policy
Does my controller support this feature? See Supported Features.
NOTE: The Disk Cache Policy feature can be used to manage only write cache of disk drive.
The Disk Cache Policy feature is supported on Non-RAID SAS/SATA physical disks (HDDs) connected to PERC hardware
controllers and on RAID SAS/SATA physical disks connected to Software RAID controller. This feature is not dependent on the
controller mode and can function as usual when the controller is running in RAID or HBA mode.
Set the physical disk caching policy of all members of a virtual disk by enabling the Disk Cache Policy. When this feature is
enabled, the physical disk writes data to the physical disk cache before writing it to the physical disk. Because it is faster to
write data to the cache than to a disk, enabling this feature improves system performance.
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Controllers