Hardware Reference
In-Depth Information
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RAID Level 5: Blocked data with distributed parity
—SimilartoRAID4butoffers
improved performance by distributing the parity stripes over a series of hard drives.
Requires a minimum of three drives to implement (two or more for data and one for
parity).
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RAID Level 6: Blocked data with double distributed parity
—Similar to RAID 5
except parity information is written twice using two parity schemes to provide even
better fault tolerance in case of multiple drive failures. Requires a minimum of four
drives to implement (two or more for data and two for parity).
Therearealso
nested
RAIDlevelscreatedbycombiningseveralformsofRAID.Themost
common are as follows:
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RAIDLevel01:Mirroredstripes
—DrivesarefirstcombinedinstripedRAID0sets;
then the RAID 0 sets are mirrored in a RAID 1 configuration. A minimum of four
drives is required, and the total number of drives must be an even number. Most PC
implementations allow four drives only. The total usable storage capacity is equal to
half of the number of drives in the array times the size of the lowest capacity drive.
RAID 01 arrays can tolerate a single drive failure and some (but not all) combinations
of multiple drive failures. This is not generally recommended because RAID 10 offers
more redundancy and performance.
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RAID Level 10: Striped mirrors
—Drives are first combined in mirrored RAID 1
sets; then the RAID 1 sets are striped in a RAID 0 configuration. A minimum of four
drives is required, and the total number of drives must be an even number. Most PC
implementations allow four drives only. The total usable storage capacity is equal to
half of the number of drives in the array times the size of the lowest capacity drive.
RAID 10 arrays can tolerate a single drive failure and many (but not all) combinations
ofmultipledrivefailures.ThisissimilartoRAID01,exceptwithsomewhatincreased
reliability because more combinations of multiple drive failures can be tolerated, and
rebuilding an array after a failed drive is replaced is much faster and more efficient.
Additional custom or proprietary RAID levels exist that were not originally supported by
theRAIDAdvisoryBoard.Forexample,from1993through2004,“RAID7”wasatrade-
marked marketing term used to describe a proprietary RAID implementation released by
the (now defunct) Storage Computer Corp.
When set up for maximum performance, arrays typically run RAID Level 0, which incor-
porates data striping. Unfortunately, RAID 0 also sacrifices reliability such that if any one
drive fails, all data in the array is lost. The advantage is in extreme performance. With
RAID 0, performance generally scales up with the number of drives you add in the array.
For example, with four drives you won't necessarily have four times the performance of
a single drive, but many controllers can come close to that for sustained transfers. Some
