Users Guide

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.
The following are the disk cache policy options for Non-RAID disks connected to a PERC hardware controller:
• Enabled — Disk Cache Policy is enabled.
• Disabled — Disk Cache Policy is disabled.
• Unchanged — The disk uses the default write cache mode.
NOTE:
The Unchanged option is applicable only for PERC 9 and later of hardware controllers with the latest
firmware version.
NOTE: If you want to revert the Disk Cache Policy settings of the physical disks to the factory default settings, then
you must restart the server.
The following are the disk cache policy options for RAID disks connected to Software RAID controller:
• Default — The disk uses the default write cache mode.
• Enabled — Disk Cache Policy is enabled.
• Disabled — Disk Cache Policy is disabled.
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Controllers