Product manual

Chapter 7: Technology Background
289
RAID Controllers
LUN Affinity
VTrak subsystems with two controllers include a LUN Affinity feature. Normally,
either controller can access all logical drives. LUN Affinity enables you to specify
which controller can access each logical drive. Use this feature to balance the
load of your logical drives between the two controllers.
To use LUN Affinity you must:
• Have two controllers in the subsystem
• Enable LUN Affinity under controller settings.
On subsystems with two controllers, when Cache Mirroring is disabled, LUN
Affinity is enabled automatically.
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 when you create a
logical drive. you can also change them under logical drive settings.
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.