Product manual

Chapter 7: Technology Background
363
The Forced Read-Ahead cache policy setting provides predictive pre-fetching of
data requests, allowing the controller to aggressively buffer large chunks of data
in cache memory to prevent frame drops on high-bandwidth video playback. Not
normally enabled for non-video applications.
Adaptive Writeback Cache
On the VTrak subsystem, you can set the logical drive write cache policy to Write
Thru or Write Back.
If you set the write cache policy to Write Back, your data is first written to the
controller cache, and later to the logical drive. This action improves performance.
To preserve the data in the cache in the event of a power failure, the subsystem
has a backup battery that powers the cache. To see an estimate of how long the
battery can power the cache, see “Viewing Battery Information” on page 89 or
“Viewing Battery Information” on page 222.
The Adaptive Writeback Cache feature protects your data by changing the write
cache settings while the cache backup battery is offline. When all of the following
conditions occur:
• The logical drive write policy is set to Write Back.
• The Adaptive Writeback Cache feature is enabled.
• The cache backup battery goes offline.
The write policy automatically changes to Write Thru. When the battery comes
back online, the write policy automatically changes back to Write Back.
To enable the Adaptive Writeback Cache option, see “Making Controller Settings”
on page 86 or page 216.
Host Cache Flushing
On the VTrak subsystem, you can enable or disable host cache flushing.
When enabled, host cache flushing guards against data loss in the event of a
power failure. However RAID performance is slightly reduced.
When disabled, the VTrak subsystem has greater sustained bandwidth and lower
latency, which are helpful for real-time video capture.
When you operate the VTrak with host cache flushing disabled, use a UPS to
protect against data loss.