Operation Manual

RAID 1
This level provides adequate security for your data, because the data is copied to
another hard disk 1:1. This is known as hard disk mirroring. If one disk is destroyed,
a copy of its contents is available on the other one. All disks but one could be
damaged without endangering your data. However, if the damage is not detected,
the damaged data can be mirrored to the undamaged disk. This could result in the
same loss of data. The writing performance suffers in the copying process compared
to using single disk access (10 to 20 % slower), but read access is signicantly
faster in comparison to any one of the normal physical hard disks. The reason is
that the duplicate data can be parallel-scanned. Generally it can be said that Level 1
provides nearly twice the read transfer rate of single disks and almost the same
write transfer rate as single disks.
RAID 2 and RAID 3
These are not typical RAID implementations. Level 2 stripes data at the bit level
rather than the block level. Level 3 provides byte-level striping with a dedicated
parity disk, and cannot service simultaneous multiple requests. These levels are
rarely used.
RAID 4
Level 4 provides block-level striping just like Level 0 combined with a dedicated
parity disk. In the case of data disk failure, the parity data is used to create a replace-
ment disk. However, the parallel disk may create a bottleneck for write access.
RAID 5
RAID 5 is an optimized compromise between Level 0 and Level 1, in terms of
performance and redundancy. The hard disk space equals the number of disks used
minus one. The data is distributed over the hard disks as with RAID 0. Parity blocks,
created on one of the partitions, exist for security reasons. They are linked to each
other with XOR, enabling the contents to be reconstructed by the corresponding
parity block in case of system failure. With RAID 5, no more than one hard disk
can fail at the same time. If one hard disk fails, it must be replaced as soon as pos-
sible to avoid the risk of losing data.
Other RAID Levels
Several other RAID levels have been developed (RAIDn, RAID 10, RAID 0+1,
RAID 30, RAID 50, etc.), some of them being proprietary implementations created
by hardware vendors. These levels are not very common and therefore are not ex-
plained here.
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