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RAID
disk systems consist of multiple disks, or arrays, which appear
to the users' applications as a single large disk, or virtual disk.
This single virtual storage capacity is commonly referred to as
a Logical
Drive or Logical
Volume. The following information is written
to comply with the explicit specifications set out by the RAID
Advisory Board (RAB) which oversees and publishes
the requirements for each level together with specifications for
hardware tolerance and operating criteria.
Other
RAID Levels
This guide ignores many proprietary standards implemented by suppliers
or manufacturers of RAID or Array technology. These proprietary
standards are usually simply a combination of standard RAID Levels
that have been incorporated into a RAID controller as a single RAID
Level. As such the underlying technology remains the same and the
pattern of data
parity and storage are the same. Mostly
this is done for marketing purposes in a bid to offer 'higher' RAID
specifications. In reality these rarely exist - to the point we
have never discovered a single solution offered with a proprietary
RAID level or designation that did not already have a either a single
RAB equivalent, or was simply a combination of two separate levels
given a higher-sounding specification simply for marketing purposes.
Some of these proprietary RAID Levels offer a simple and stable
solution by combining the best RAID Levels into a single logical
volume. There are several manufacturers who have a controller that
uses a mirrored pair
as a cache volume for quick read/write access and later migrate
this data onto a standard Level 5 parity
stripe set when user access is lower.
This type of solution is only proprietary only in the fact that
the manufacturer's own controller must always be used to access
the data and rebuild from the two RAID Level volumes. Such a solution
may be termed 'RAID Level 50' (Level
5 + Level
0).
The RAB sets out two variants on array systems. Simplistically,
RAID itself comprises of redundant disks storing parity information,
and EDAP describes and sets standards for the physical implementation
of the RAID storage system and is a series of criterion to which
a RAID solution may be compared to judge the protection level.
How
EDAP and RAID works
The concepts behind the standards
RAID
Definition
The RAB specifications for a RAID or Array sub-system
EDAP Definition
The RAB specifications for EDAP sub-systems.
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