Overland Storage SnapServer Administrator's Manual page 261

SnapServer 7.0 Administrator's Guide
RAID
Short for Redundant Array of Independent Disks. A data storage scheme where multiple
hard drives are combined to form a single logical unit which is highly reliable and gives
good performance. Reliability is achieved by mirroring (the copying of data to more than one
disk), striping (the splitting of data across more than one disk) and error correction
(redundant data is stored to enable faults to be detected and corrected).
RAID 0 (Striped)
RAID 0 is ideal for environments in which performance (read and write) is more important
than fault tolerance, or you need the maximum amount of available drive capacity in one
volume.
Data is striped across multiple disks so that it can be read and written in parallel. It
provides higher performance than a single disk, especially when reading or writing large
files, but it is vulnerable to a disk failure. If any disk in the pool fails, the entire pool is
effectively lost. For this reason, RAID 0 pools should only be used in cases where the loss of
the data is unimportant, for example, because it can easily be recreated from another data
source. The capacity of a RAID 0 pool is equal to the total capacity of all the disks making
up the pool
4 TB.
RAID 1 (Mirrored)
RAID 1 is useful for building a fault-tolerant system or data volume, providing excellent
availability without sacrificing performance. However, you lose 50 percent of the assigned
disk capacity.
RAID 1 is also called disk mirroring: data is stored on two identical disks, so that if one disk
fails, the other can still be used to access the data. Write operations are performed in
parallel to both disks, so write performance is identical to that of a single disk; read
operations can be done to either disk, so effectively read performance is doubled.
If one of the disks fails, it should be replaced. When it is replaced, the RAID pool will
automatically be rebuilt by copying all the data from the surviving disk to the new disk.
While the rebuild is occurring, there will be a degradation in performance.
Because disks are mirrored, the usable capacity of a pair of RAID 1 disks is only equal to
the capacity of a single disk, so that a RAID 1 pool made of 2 x 500 GB disks will have a
capacity of 500 GB.
RAID 5 (Striping with Parity)
With a RAID 5 pool, because data is read from many disks in parallel, as for RAID 0, read
performance is good. Write performance is slightly lower because, in addition to writing the
data, parity data has to be calculated and written. If a hardware RAID controller is used,
this will be done using dedicated hardware; if software RAID is used, the work will be done
on the main processor of the storage controller.
The capacity of a RAID 5 pool is reduced by exactly one disks worth of capacity, which is
required to store the parity data. For example, a RAID 5 pool made up of 3 x 500 GB disks
will have a capacity of 1 TB.
In principle, a RAID 5 pool could have a very large number of disks. However, the more
disks there are, the greater the chance of a double disk failure. If a single disk fails, the data
is no longer protected until the disk has been replaced and the pool has been rebuilt by
reconstructing all the data from the failed disk and writing it to the new disk. If the disk
1
Capacity is usually very slightly less because a small but insignificant amount of space is reserved by the
RAID controller to store internal metadata.
10400317-001 10/2011
1
. For example, a RAID 0 pool made up of 4 x 1 TB disks will have a capacity of
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