Promise Technology VTRAK M210i, Product Manual

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VTrak M610i, M310i, M210i Product Manual
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Cache Policy
As it is used with VTrak, the term cache refers to any of several kinds of high-
speed, volatile memory that hold data moving from your computer to the physical
drives or vice-versa. Cache is important because it can read and write data much
faster than a physical drive. There are read caches, which hold data as it is read
from a physical drive; and write caches, which hold data as it is written to a
physical drive.
In order to tune the cache for best performance in different applications, user-
adjustable settings are provided. Cache settings are made in conjunction with
logical drives:
•
When you create a logical drive. See “Creating a Logical Drive” on page 111
(WebPAM PROe) or page 156 (CLU).
• On an existing logical drive. See “Making Logical Drive Settings” on
page 120 (WebPAM PROe) or page 159 (CLU).
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 physical 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
physical drive at the same time. Safer.
If Write Policy is set to Write Back, and 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.
If Write Policy is set to Write Back, and the Adaptive Writeback Cache feature is
enabled, the Write Policy will remain at Write Back even when the battery is
offline. Any data in the controller cache will be lost in the event of a power failure.
Adaptive Writeback Cache
On the VTrak subsystem, you can set the logical drive write cache policy to Write
Through or Write Back.