Product manual

Chapter 7: Technology Background
289
Read Cache Policy
• Read Cache – The read cache is enabled.
• Read Ahead – The read cache and the read-ahead feature are enabled.
Read-ahead anticipates the next read and performs it before the request is
made. Can increase read performance.
• No Cache – The read cache is disabled.
Write Cache Policy
• Write Back – Data is written first to the cache, then to the logical drive.
Better performance. VTrak has a cache backup battery to protect data in the
cache from a sudden power failure.
• Write Thru – Also “Write Through”. Data is written to the cache and the
logical drive at the same time. Safer.
If your write cache policy is set to Write Back, the write policy automatically
changes to Write Thru 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
When the battery comes back online, the write policy automatically changes back
to Write Back.
Also see “Cache Mirroring” on page 290.
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 will power the cache, see “Checking the Batteries” on page 128 or
page 182.
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