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RAID (Redundant
Array of Independent Disks)
RAID Level 0: Striped Disk Array without Fault Tolerance
A RAID 0 is created by striping data across two or more drives.
This striping scheme creates no redundancy to protect the data.
However, because the drives share load equally, RAID 0 provides
the best read and write performance of any RAID type. The failure
of just one drive will result in all data in an array being
lost
An example of a RAID 0 array is shown below:
RAID Level 1: Mirroring & Duplexing
A RAID 1 array is created by copying, or mirroring, all data
from one drive onto a second drive. This mirroring provides
redundancy, ensuring that if one drive fails, no data is lost.
100% redundancy of data means no rebuild is necessary in case
of a disk failure, just a copy to the replacement disk. Redundancy
also means that only half of the total capacity is available.
RAID 1 offers no write performance advantage over a simple volume
configuration. However, because the drives share read load equally,
RAID 1 provides improved read performance. (Twice the Read transaction
rate of single disks, same Write transaction rate as single
disks.) An example of a RAID 1 array is shown below.
RAID Level 1/0: High
Data Transfer Performance with Data Protection
This dual-level RAID type is created by using two or more equalsized
RAID 1 arrays to create a RAID 0. Both read and write
performance are improved due to array striping. However, the
performance improvement costs additional storage space due to
the mirroring of the arrays. Excellent solution for sites that
need high performance but are not concerned with achieving maximum
reliability.
RAID Level 5: Independent Data Disks with Distributed Parity
Blocks
This RAID type requires at least three drives. Like RAID 0,
data is striped across the drives. However, in RAID 5, parity
is stored separately from data, and occupies the equivalent
of one drive. The controller generates parity every time data
is written to the array, and it is distributed in stripes
across all the drives. In the event of a drive failure, the
contents of the failed drive can be rebuilt from the data
and parity on the remaining drives.
Using parity minimizes the storage cost of redundancy. Because
only one drive is used to store parity, a three-drive array,
which is the worst case, uses only one-third of the total
capacity for redundancy. In larger arrays, the storage cost
of redundancy decreases proportionately RAID 5 write performance
is limited by the need to generate parity data for every write.
Read performance is good because the load is spread equally
across all the drives.
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