User's Manual

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.
NOTE: Write-through is the default write policy setting when cluster mode is enabled.
• Write Cache Enabled Protected — 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 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 policy is available only 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
when there is power failure.
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 either displays Write
Cache Disabled as the only available option or the wizard does not display any option for write
policy.
• Write Cache Disabled — This is the only option available when the controller does not have a
functional battery.
Cache Policy
Does my controller support this feature? See Supported Features.
The Direct I/O and Cache I/O policy apply to readings on a specific virtual disk. These settings do not
affect the read-ahead policy. The cache policies are:
• Cache I/O — Specifies that all reads are buffered in cache memory.
• 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 the
default setting.
NOTE: Cache policy is not supported on controllers that do not have a battery.
Disk Cache Policy
Does my controller support this feature? See Supported Features.
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.
64