Apple Xserve RAID User's guide User's Guide download pdf (Page 59)

RAID Overview 59

Once you have deﬁned a group of drive modules as a RAID set, the controller groups those

drives into “logical disks.” On the Xserve RAID system, each logical disk appears to the host

system and the RAID Admin software as one disk, regardless of the number of actual drives in

that logical unit. See “Xserve RAID Schemes” on page 66 for examples of RAID sets you can

conﬁgure on the Xserve RAID system. Each controller in the Xserve RAID system can have a

maximum of three logical disks.

You can also use the Apple RAID software built into Mac OS X (part of Disk Utility, located in

Applications/Utilities) to augment RAID storage on the system. See the document “Using

RAID Admin and Disk Utility” on the CD that came with your system for more about using

this software with the RAID sets you establish.

Data Storage Methods

The controller stores and retrieves data on a RAID system using one of two methods,

“striping” or mirroring. You can also combine these methods in some RAID sets.

Data Striping

Data striping is the foundation of RAID. Multiple hard disk drives in a RAID group, referred to

as a set or array, are divided (partitioned) into stripes. The controller spreads stripes across

the disks in alternating sections on each drive.

In environments with intensive input-output (I/O) requirements, such as servers,

performance is optimized by writing data across stripes large enough that each data record

can be written entirely to one stripe. This technique ensures that each drive will be working

on a different I/O operation and the number of simultaneous I/O operations performed by

the array is maximized.

In data-intensive environments such as digital video editing, performance is optimized by

writing data across small stripes so that each record spans all drives. This method ensures

that access to large records is very fast because data is transferred in parallel across multiple

drives.

Data Mirroring

To mirror data, the RAID controller creates equal partitions on two different disks. One

partition is primary; the other is the mirrored partition. The primary and mirrored partitions

are synchronized; that is, anything written to one disk is also written to the other. Mirrored

data is very secure because if one disk fails, the data is available from the other disk. Because

mirroring involves duplicating all data, two mirrored drives store half as much data as drives

that are not mirrored.

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Apple Xserve RAID User's guide User's Guide Page 59