Apple Xserve RAID User's guide Technical Information download pdf (Page 17)

Chapter 2 Working With RAID Arrays 17

• “Use drive cache” enables the built-in write cache on each drive in the array. This

option improves efficiency and speed of data transfer, but the drive cache contents

will be lost if power to the system fails, unless you are using an uninterruptible

power supply (UPS).

Important: It is recommended that you use a UPS when you have drive cache

enabled. Without UPS backup, you could lose data in the event of a power failure.

In the event of a power failure, the system detects UPS power, immediately flushes

the data in the drive cache on a drive, and then turns off the cache. When AC power

is restored, the system turns on the drive cache again.

6 Check the summary of the array you’ve chosen and click Create Array.

Note: The RAID controller automatically treats any drive not assigned to an array as a

global hot spare for all arrays on that controller.

About Spare Drives

A spare drive is any drive that has not been assigned to an array. On the Xserve RAID

system, an unassigned drive is “hot” (always spinning and ready for use) and global

(available to any array on its controller). A spare drive is used automatically by the RAID

controller for its group of drives (either drives 1 to 7 or drives 8 to 14) whenever a drive

within an array fails. (A controller can only use available drives on its side of the system;

it cannot treat a drive on the other side as a hot spare.)

When you assign a previously unused drive to a new array, it no longer serves as a hot

spare.

Modifying a RAID Array

Once a RAID array is established, you can expand it by adding one or more drives to it.

Expanding an array preserves the data on the original array. Be sure that your file

system supports expansion. See “Expanding an Array” on page 38 for details on

expanding an array.

You can also subdivide, or “slice,” an array into as many as six parts. Slicing an array

erases any data on it. See “Slicing an Array” on page 35 for details on slicing.

Note: You cannot change an array’s type or remove drives from it. You can turn the

drive cache on or off.

Should you want to make any change except adding available drives to an array or

slicing it, you must back up all data in the array, delete the array, and create a new array

with the parameters you want.

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Apple Xserve RAID User's guide Technical Information Page 17