Users Guide

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l Force Write Back. When using force write-back caching, 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.
l Write Back Enabled. When using write-back enabled caching, the controller firmware disables the write cache if it does not detect the presence of a
charged battery over a specified period of time. For example, on some controllers, the write cache is disabled if the firmware cannot detect a charged
battery within 72 hours.
l Write-Through. When using write-through caching, 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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l Write Cache Enabled Protected. When the write cache is enabled, the controller writes data to the write cache before writing data to the physical disk.
Because it takes less time to write data to the write cache than it does to a disk, enabling the write cache can improve system performance. After data
is written to the write cache, the system is free to continue with other operations. The controller, in the meantime, completes the write operation by
writing the data from the write cache to the physical disk. The Write Cache Enabled Protected option is only available if the controller has a functional
battery. The presence of a functional battery ensures that data can be written from the write cache to the physical disk even in the case of a power
outage.
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l Write Cache Disabled. This is the only available option if the controller does not have a functional battery.
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Cache Policy
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Does my controller support this feature? See "Supported Features"
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The Direct I/O and Cache I/O cache policies apply to reads on a specific virtual disk. These settings do not affect the read-ahead policy. The cache policies are
as follows:
l Cache I/O. Specifies that all reads are buffered in cache memory.
l Direct I/O. Specifies that reads are not buffered in cache memory. When using direct I/O, data is transferred to the controller cache and the host system
simultaneously during a read request. If a subsequent read request requires data from the same data block, it can be read directly from the controller
cache. The direct I/O setting does not override the cache policy settings. Direct I/O is also the default setting.
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Disk Cache Policy
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Does my controller support this feature? See "Supported Features"
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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 can
improve system performance.
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The cache policies are:
l Enabled — Specifies that the disk cache policy is enabled.
l Disabled — Specifies that the disk cache policy is disabled.
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Cluster-enabled RAID Controllers
NOTE: Storage Management does not allow you to select the Write-Back policy for controllers that do not have a battery. This restriction protects
a controller without a battery from the data loss 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.
NOTE: Write-through is the default write policy setting when cluster mode is enabled. In cluster mode, the PERC 3/DC, 4/DC, and 4e/DC
controllers only allow write-through caching.
NOTE: Storage Management does not allow you to select the Write Cache Enabled Protected policy for controllers that do not have a battery.
This restriction protects a controller without a battery from the data loss that may occur in the event of a power failure. When using the Create
Virtual Disk Advanced Wizard on a controller without a battery, the wizard will either display Write Cache Disabled as the only available option
or the wizard will display no option at all for write policy.
NOTE: Cache policy is not supported on any controller that does not have a battery.
NOTE: On PERC 5 and PERC 6 controllers, for virtual disks based on SATA drives, the default Disk Cache Policy is Enabled; for virtual disks based on
SAS drives, it is Disabled.