Apple Xserve RAID User's guide User Manual download pdf (Page 15)

About RAID Levels

RAID Utility lets you set up RAID sets based on the following RAID levels:

Â Enhanced JBOD

Â RAID 0 (striping)

Â RAID 1 (mirroring)

Â RAID 0+1 (mirroring a striped pair)

Â RAID 5 (striping with distributed parity)

Enhanced JBOD

Using Enhanced JBOD, you can create a RAID set using a single drive. The resulting

RAID set doesn’t gain the performance or data protection of the other RAID levels, but

it does benefit from the data caching and battery backup provided by the RAID card.

An enhanced JBOD set can also be migrated to other RAID sets or moved to another

computer that has a Mac Pro or Xserve RAID Card installed.

RAID 0

RAID 0 offers improved performance but no data protection. Blocks of data are spread

across all of the drives in the RAID set in a process called striping. This allows better

performance because file contents move in parallel to and from the individual drives in

the set. RAID 0 also provides the most usable disk space; nearly all space on all drives is

available for user data. You can create a RAID 0 set using two, three, or four disks.

RAID 1

RAID 1 protects data against a drive failure and allows some increase in read

performance. Data is protected by duplicating the contents of each drive on a second

drive in the set, a process called mirroring. Because of the duplication, a volume based

on a RAID 1 set can’t be larger than half of the total space available on the drives in the

set. You can create a RAID 1 set using either two or four disks.

RAID 0+1

RAID 0+1 combines the performance of RAID 0 with the data protection of RAID 1 by

mirroring a striped set on a second pair of drives. Because mirroring duplicates all data,

this level offers less usable disk space than RAID 5. Usable space is half of the total

space available on the drives in the set. You need four disks to create a RAID 0+1 set.

RAID 5

RAID 5 is a compromise between the performance of RAID 0 and the data protection of

RAID 1. Performance is improved by striping data across the drives in the set. Data

protection is provided by parity information that is distributed across the drives. Data

can be recovered if any single drive fails. RAID 5 leaves you with more usable space

than RAID 1. RAID 5 needs only the equivalent of one drive’s worth of disk space to

store the parity information. You can create a RAID 5 set using either three or four disks.

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Apple Xserve RAID User's guide User Manual Page 15