RAID Classifications

RAID Classifications

With RAID technology, data is striped across an array of hard disk drives. This data-distribution technique complements the way the operating system requests data.

The six basic RAID classifications are RAID level 0 through RAID level 5. The ServeRAID controller on the system board supports three, industry-recognized RAID levels: 0, 1, and 5.

RAID Level 0: RAID level 0 stripes the data across all the drives in the array. This offers substantial speed enhancement, but provides for no data redundancy. The ServeRAID controller automatically assigns RAID level 0 to all logical drives in an array containing only one hard disk drive.

A hard disk failure within the array results in loss of data in the logical drive assigned level 0, but only in that logical drive. If you have logical drives assigned RAID level 1 or 5 in the same array, they will not lose data.

When you replace the failed drive, the ServeRAID controller automatically rebuilds all the logical drives assigned RAID levels 5 and 1 onto the replacement hard disk drive and defines the level 0 logical drive. However, the data that was in the failed level 0 logical drive is lost.

Though the risk of data loss is present, you might want to assign RAID level 0 to one of the logical drives, to take advantage of the speed offered with this RAID level. You could use this logical drive to enter data that you back up each day and for which safety is not of primary importance, that is, data that you can re-create easily. You also might want to use a level 0 logical drive when the work you are doing requires maximum capacity. RAID level 0 provides the largest capacity of the three RAID levels offered, because no room is taken up for redundant data or data parity storage.

RAID Level 1: RAID level 1 provides 100% data redundancy and requires two hard disk drives. With RAID level 1, the first stripe is the data stripe, the second stripe is the mirror (copy) of the first stripe, but written to another drive. If one of the hard disk drives fails, the ServeRAID controller switches read and write requests to the remaining functional drive in the array.
Because the data is mirrored, the capacity of the logical drive when assigned level 1 is 50% of the physical capacity of the grouping of hard disk drives in the array. RAID level 1 requires two drives. When you group more than two drives into a RAID level 1 array, the ServeRAID controller automatically assigns the Enhanced RAID level 1.

The Enhanced RAID level 1 stripes data and copies of the data across all the drives in the array. As with the standard RAID level 1, the data is mirrored, and the capacity of the logical drive is 50% of the physical capacity of the grouping of hard disk drives in the array.

The following illustration shows data arranged in an array with three hard disk drives. The logical drive is assigned the Enhanced level 1. Notice that the mirror of the first data stripe (XXX YYY ZZZ) is shifted one drive. The other data stripes in level 1 follow the same pattern.

If you have only two drives available, the second drive is a mirror copy of the first drive.

RAID Level 5: RAID level 5 stripes data and parity across all drives in the array. When an array is assigned level 5, the capacity of the logical drive is reduced by one drive (for data parity storage).

RAID level 5 is generally the most desirable choice, because it offers both data protection and increased throughput. RAID level 5 gives you higher capacity than level 1, but level 1 offers better performance.

If you want to have a hot-spare drive (that is, a drive that can be automatically used to replace a similar drive that fails) and also assign RAID level 5, you must have at least four hard disk drives in the server.

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