IBM DS8000 User Manual page 234

Hewlett-Packard that provides for
multiple paths to a volume, as well as
static load balancing.
R
R0 See track-descriptor record.
rack See enclosure.
RAID See redundant array of independent disks.
RAID is also commonly expanded to
redundant array of inexpensive disks. See
also array.
RAID 5
A type of RAID that optimizes
cost-effective performance while
emphasizing use of available capacity
through data striping. RAID 5 provides
fault tolerance for up to two failed disk
drives by distributing parity across all the
drives in the array plus one parity disk
drive. The DS8000 automatically reserves
spare disk drives when it assigns arrays
to a device adapter pair (DA pair). See
also device adapter, RAID 10, and redundant
array of independent disks.
RAID 6
Any form of RAID that can continue to
process read and write requests to all of
an array's virtual disks in the presence of
two concurrent disk failures. See also
device adapter, RAID 5, RAID 10, and
redundant array of independent disks.
RAID 10
A type of RAID that optimizes high
performance while maintaining fault
tolerance for up to two failed disk drives
by striping volume data across several
disk drives and mirroring the first set of
disk drives on an identical set. The
DS8000 automatically reserves spare disk
drives when it assigns arrays to a device
adapter pair (DA pair). See also device
adapter, RAID 5, and redundant array of
independent disks.
random access
A mode of accessing data on a medium in
a manner that requires the storage device
to access nonconsecutive storage locations
on the medium.
rank One or more arrays that are combined to
create a logically contiguous storage
space.
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redundant array of independent disks (RAID)
A methodology of grouping disk drives
for managing disk storage to insulate data
from a failing disk drive.
refresh FlashCopy target volume
An option (previously called incremental
FlashCopy) of the DS8000 that creates a
point-in-time data copy without copying
an entire volume for each point-in-time
copy.
Remote Mirror and Copy
A feature of a storage server that
constantly updates a secondary copy of a
logical volume to match changes made to
a primary logical volume. The primary
and secondary volumes can be on the
same storage server or on separate
storage servers. See also Global Mirror,
Metro Mirror and Global Copy.
remote technical assistance information network
(RETAIN)
The initial service tracking system for
IBM service support, which captures
heartbeat and call-home records. See also
support catcher and support catcher telephone
number.
REQ/ACK
See request for acknowledgment and
acknowledgment.
request for acknowledgment and
acknowledgment (REQ/ACK)
A cycle of communication between two
data transport devices for the purpose of
verifying the connection, which starts
with a request for acknowledgment from
one of the devices and ends with an
acknowledgment from the second device.
The REQ and ACK signals help to
provide uniform timing to support
synchronous data transfer between an
initiator and a target. The objective of a
synchronous data transfer method is to
minimize the effect of device and cable
delays.
reserved allegiance
For System z, a relationship that is
created in a control unit between a device
and a channel path, or path group, when
the device completes a Sense Reserve
command. The allegiance causes the
control unit to guarantee access (that is,
busy status is not presented) to the
device. Access is over the set of channel