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Glossary
RAID
Redundant Array of Independent Disks. A disk subsystem that increases performance and
provides fault tolerance. RAID is a set of two or more hard disks and a specialized disk
controller that contains the RAID functionality.
RAID improves performance by disk striping, which interleaves bytes or groups of bytes
across multiple drives, so more than one disk is reading and writing simultaneously. Fault
tolerance is achieved by mirroring or parity.
RAID 0
Provides disk striping only, which interleaves data across multiple disks for better
performance. It does not provide safeguards against failure.
RAID 1
Uses disk mirroring, which provides 100% duplication of data. Offers highest reliability, but
doubles storage cost.
RAID 2
Bits (rather than bytes or groups of bytes) are interleaved across multiple disks.
RAID 3
Data is striped across three or more drives. Used to achieve the highest data transfer rate,
because all drives operate in parallel. Parity bits are stored on separate, dedicated drives.
RAID 4
Similar to RAID 3, but manages disks independently rather than in unison. Not often used.
RAID 5
Most widely used. Data is striped across three or more drives for performance; parity bits are
used for fault tolerance. The parity bits from two drives are stored on a third drive.
RAID 6
Highest reliability, but not widely used. Similar to RAID 5, but does two different parity
computations or the same computation on overlapping subsets of the data.
RAID 10
Also designated RAID 1,0. It is a combination of RAID 1 and 0 (mirroring and striping).
RCS
See remote copy set.
redundancy
The provision of multiple interchangeable components to perform a single function to cope
with failures and errors. A RAIDset is considered to be redundant when user data is recorded
directly to one member, and all of the other members and associated parity also are recorded.
If a member is missing from the RAIDset, its data can be regenerated as needed, but the
RAIDset is no longer redundant until the missing member is replaced and reconstructed.
Data Replication Manager HSG80 ACS Version 8.7P Scripting User Guide
Glossary–9