Promise Technology Pegasus3 R4, Product Manual, Page: 200 / 232

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Technology Background Pegasus3 Product Manual
RAID Controllers
RAID controller technology includes:
• Cache Policy (“Read Cache Policy” and “Write Cache Policy”)
• “Capacity Coercion”
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.
This provides better performance.
Write Thru – Also “Write Through.” Data is written to the cache and the logical drive at the same
time. This is safer.