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Table Of Contents

Storage Concepts and Technologies

RAID 1+Spare (Mirror + Spare)

Select RAID1 + Spare to dedicate another disk as the automatic backup when one member of

the mirrored configuration fails. This ensures that if a disk fails a spare disk is available to

automatically replace the failed disk (Figure 10). The following design considerations apply:

• Minimum Number of Disks: 3

• Advantages: Excellent redundancy. Good performance.

• Disadvantages: Costly.

The spare disk is not used until a working disk fails. For example, in a two-disk mirror

where each disk is 80 GB, the total raw space is 160 GB, of which 80 GB is protected and 80

GB is available for use. There is also 80 GB of space available. This means that of the 240 GB

of total space, only 80 GB is actually usable at any given time.

Figure 10: RAID 1+Spare

RAID 5 (Striped + Parity)

RAID 5 is one of the more popular and versatile levels implemented due to its excellent

performance (that is, high Read transaction rate, medium Write transaction rate, and low

ratio of parity disks to data disks) and good fault tolerance. It uses block-level striping with

parity data distributed across the disks in the array. This means that every time a block is

written to a disk in the array, a parity block is generated within the same stripe.

RAID 5 stores parity data which can be used to rebuild data should data become lost or

corrupted, but not full redundant data. The calculated value is stored on each disk on blocks

allocated to parity. This means that the amount of usable space for the protected disk is

decreased due to the parity space requirements. The amount of available protected space is

decreased by the size of the single disk. For example, if there are three 80 GB disks in the

array with a total of 240 GB of space configured as RAID 5, the usable space equals 240

minus 80 which equals 160 GB of protected usable space. If one of the drives fails, the

missing information can be recreated using parity bits stored on the remaining members. Use

this RAID level for applications such as Intranet servers; database servers; Web, e-mail, and

News servers; and File and Application servers (Figure 11).

The following design considerations apply:

• Minimum Number of Disks: 3

• Advantages: Good balance between space usage and disk protection. Higher read

performance than RAID1; similar to RAID 0.

• Disadvantages: Because of the parity that must be calculated, write transactions are

somewhat slower than read transactions. It is resource-intensive to rebuild in the event

of a disk failure (as compared with RAID 1).

12 EDCS-593805 v1.0

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