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.
• 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 first 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.
• 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,
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Controllers