Product manual

VTrak E-Class Product Manual
362
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 on the RAID
controller. See “Making Controller Settings” on page 86 (WebPAM PROe) or
page 216 (CLU).
Read Cache Policy
• Read Cache – The read cache is enabled but no pre-fetch action.
• Read Ahead – The read cache and predictive pre-fetch feature are enabled.
Read-ahead anticipates the next read and performs it before the request is
made. Can increase read performance.
• Forced Read Ahead – The read cache and aggressive pre-fetch feature are
enabled. See “Forced Read-Ahead Cache” below.
• 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.
• Adaptive Writeback – See “Adaptive Writeback Cache” below.
• 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 “Viewing Battery Information” on page 89 or page 222.
Forced Read-Ahead Cache
On the VTrak subsystem, you can set the logical drive read cache policy to
Forced Read Ahead an enable the aggressive pre-fetch feature.