Adaptec AHA-3980 User's Guide download pdf (Page 10)

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Creating and Managing Arrays

Stock Number: 510685-00, Rev. A Page: 1-2

Print Spec Number: 493069-00

Current Date: 3/3/99 ECN Date: 3/28/95

RAID 0 (Striped Disks)

In a RAID 0 array, data is distributed, or striped, across the disks in

the array. The array appears to the server as one large disk with a cap-

acity approximately equal to the combined capacity of the physical

disks. Because multiple reads and writes can be handled in parallel,

the input/output performance of the array is much better than that

of a single physical disk.

No redundant data is stored in RAID 0 arrays, so they are not true

RAID applications. If one disk fails, the entire array fails and all array

data is lost. The reliability of a RAID 0 array, therefore, is less than that

of any single disk in the array. The term RAID 0 is widely used for

these arrays, however, because they are conceptually similar to true

RAID applications.

RAID 1, RAID 1/0 (Mirrored Disks)

In RAID 1 and RAID 1/0 arrays, commonly called mirrored arrays,

disks are paired, with both disks in a pair containing the same data.

When data is written to a mirrored array, it is written twice—once to

each disk in the pair. A RAID 1 array has only one set of paired disks.

A RAID 1/0 array has multiple pairs, across which data is striped.

A mirrored array has good performance. In RAID 1 arrays, the read

performance can be much better than that of a single disk, while the

write performance is slightly worse. In RAID 1/0 arrays, both read

performance and write performance are better than those of a single

disk.

A mirrored array is also highly reliable, because both disks in a pair

must fail for the array to fail. In an array with five pairs of mirrored

disks, for example, the array can maintain its integrity if even five

disks fail—as long as each pair has one good disk.

The main disadvantage of a mirrored array is its cost. Because all

disks must have a twin, you must use twice the number of disks that

actually contribute to the array capacity. In an eight-disk array, for

example, you have only four disks of usable capacity.

RAID 5

A RAID 5 array is similar to a RAID 0 array; however, it contains

redundant information in the form of parity data, which is calcu-

lated block by block for all user data. The parity data is interspersed

across all disks in the array, occupying the equivalent capacity of

about one disk. If one disk in the array fails, the data on the failed

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Adaptec AHA-3980 User's Guide Page 10

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