Hardware Reference
In-Depth Information
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RAID Level 0
—Striping-File data is written simultaneously to multiple drives in the array,
which act as a single larger drive. This offers high read/write performance but low reliability.
Requires a minimum of two drives to implement.
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RAID Level 1
—Mirroring-Data written to one drive is duplicated on another, providing
excellent fault tolerance (if one drive fails, the other is used and no is data lost) but no real
increase in performance as compared to a single drive. Requires a minimum of two drives to
implement (same capacity as one drive).
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RAID Level 2
—Bit-level ECC-Data is split one bit at a time across multiple drives, and error
correction codes (ECCs) are written to other drives. This is intended for storage devices that
do not incorporate ECC internally. (All SCSI and ATA drives have internal ECC.) It's a
standard that theoretically provides high data rates with good fault tolerance, but seven or more
drives are required for greater than 50% efficiency, and no commercial RAID 2 controllers or
drives without ECC are available.
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RAID Level 3
—Striped with parity-Combines RAID Level 0 striping with an additional drive
used for parity information. This RAID level is really an adaptation of RAID Level 0 that
sacrifices some capacity, for the same number of drives. However, it also achieves a high level
of data integrity or fault tolerance because data usually can be rebuilt if one drive fails.
Requires a minimum of three drives to implement (two or more for data and one for parity).
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RAID Level 4
—Blocked data with parity—Similar to RAID 3 except data is written in larger
blocks to the independent drives, offering faster read performance with larger files. Requires a
minimum of three drives to implement (two or more for data and one for parity).
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RAID Level 5
—Blocked data with distributed parity—Similar to RAID 4 but offers 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).
There are also
nested
RAID levels created by combining several forms of RAID. The most common
are as follows:
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RAID Level 01: Mirrored stripes
—Drives are first combined in striped RAID 0 sets; 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 of multiple drive failures. This is similar to RAID
