Technical data

GL–14 Glossary
RAID level 1 A RAID storageset of two or more physical disks that maintains a complete
and independent copy of the entire virtual disk’s data. This type of
storageset has the advantage of being highly reliable and extremely tolerant
of device failure. Raid level 1 storagesets are sometimes referred to as
mirrorsets.
RAID level 3 A RAID storageset that transfers data parallel across the array’s disk drives
a byte at a time, causing individual blocks of data to be spread over several
disks serving as one enormous virtual disk. A separate redundant check disk
for the entire array stores parity on a dedicated disk drive within the
storageset. See also RAID level 5.
RAID Level 5 A RAID storageset that, unlike RAID level 3, stores the parity information
across all of the disk drives within the storageset. See also RAID level 3.
RAID level 3/5 A DIGITAL-developed RAID storageset that stripes data and parity across
three or more members in a disk array. A RAIDset combines the best
characteristics of RAID level 3 and RAID level 5. A RAIDset is the best
choice for most applications with small to medium I/O requests, unless the
application is write intensive. A RAIDset is sometimes called parity RAID.
Raid level 3/5 storagesets are sometimes referred to as RAIDsets.
RAIDset See RAID level 3/5.
RAM Random access memory.
read ahead caching A caching technique for improving performance of synchronous sequential
reads by prefetching data from disk.
read caching A cache management method used to decrease the subsystem’s response
time to a read request by allowing the controller to satisfy the request from
the cache memory rather than from the disk drives.
reconstruction The process of regenerating the contents of a failed member’s data. The
reconstruct process writes the data to a spareset disk and then incorporates
the spareset disk into the mirrorset, striped mirrorset, or RAIDset from
which the failed member came. See also regeneration.
reduced Indicates that a mirrorset or RAIDset is missing one member because the
member has failed or has been physically removed.