User's Manual

86 Controllers
Write Policy
Does my controller support this feature? See Supported Features
The write policies specify whether the controller sends a write-request
completion signal as soon as the data is in the cache or after it has been
written to disk.
• Write-Back —When using write-back caching, 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 more
quickly retrieve data from the controller cache than they could from the
disk. Write-back caching also entails a data security risk, however, since a
system failure could prevent the data from being written to disk even
though the controller has sent a write-request completion signal. In this
case, data may be lost. Other applications may also experience problems
when taking actions that assume 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 exception are PERC
S100 and 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 —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.
•
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.