Promise Technology EX4650A, EX8650 Capacity Coercion, Read Cache Policy, Write Cache Policy

SuperTrak EX Series User Manual

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 logical drive. Better performance. VTrak has a cache backup battery to protect data in the cache from a sudden power failure.

•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, 10, and 50). It is generally recommended to use physical drives of the same size in your disk arrays. When this is not possible, physical drives of different sizes will work but the system must adjust for the size differences by reducing or coercing the capacity of the larger drives to match the smaller ones. With SuperTrak, 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 117. The choices are:

•GBTruncate – (Default) Reduces the useful capacity to the nearest 1,000,000,000-byte boundary.

•10GBTruncate – Reduces the useful capacity to the nearest 10,000,000,000-byte boundary.

•GRPRounding – Uses an algorithm to determine how much to truncate. Results in the maximum amount of usable drive capacity.

•TableRounding – Applies a predefined table to determine how much to truncate.

Capacity Coercion also affects a replacement drive used in a disk array. Normally, when an physical drive fails, the replacement drive must be the same capacity or larger. However, the Capacity Coercion feature permits the installation of a replacement drive that is slightly smaller (within 1 gigabyte) than the remaining working drive. For example, the remaining working drives can be

80.5GB and the replacement drive can be 80.3, since all are rounded down to 80 GB. This permits the smaller drive to be used.

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