Promise Technology Pegasus R4, Руководство по эксплуатации продукта

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Pegasus R6, R4 Product Manual
98
RAID Controllers
RAID controller technology includes;
• Cache Policy (page 98)
• Capacity Coercion (page 98)
Cache Policy
As it is used with Pegasus, 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 25.
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.
• 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. Pegasus 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.
Capacity Coercion
This feature is designed for fault-tolerant logical drives (RAID 1, 1E, 5, 6, and 10).
It is generally recommended to use physical drives of the same size in your disk
arrays. When this is not possible, the system adjusts for the size differences by
reducing or coercing the capacity of the larger drives to match the smaller ones.
With Pegasus, you can choose to enable capacity coercion and any one of four
methods.
Enable capacity coercion and choose the method in the Controller Settings
menu. See “Making Controller Settings” on page 25.