User's Manual

Controllers 87
• 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.
NOTE: Write-through is the default write policy setting when cluster mode is
enabled. In cluster mode, the PERC 4/DC and 4e/DC controllers only allow
write-through caching.
• 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.
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 available option if 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 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:
• Cache I/O —Specifies that all reads are buffered in cache memory.