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JBOD
Just A Bunch of Disks (Drives), a number of disk drives,
usually in an enclosure. JBOD implies that the disks do not
use RAID technology and function independently.
L
Latency
The time between the making of an I/O request and
completion of the request's execution.
Short for rotational latency, the time between the
completion of a seek and the instant of arrival of the first
block of data to be transferred at the disk's read/write head.
Logical Drive
The logical devices that are presented to the operating
system. System drives are presented as available disk
drives, each with a capacity specified by the RAID
controller.
LUN
Logical Unit Number, a SCSI representation of a system
drive on a given channel and target ID. This may be a
single device or an array of devices configured to behave
as a single device.
LUN Mapping
A method whereby a LUN ID is assigned to a system drive,
allowing a LUN to be made accessible through specific
controllers and ports using system drive affinity.
M
Mirrored Cache
A cache memory that has duplicate data from another
controller. In the event of failure of the original controller,
the second controller can take the cached data and place it
on the disk array.
Mirrored Hard Drive
Two hard drives the computer sees as one unit. Information
is stored simultaneously on each drive. If one hard disk
drive fails, the other contains all of the cached data and the
system can continue operating.
Mirroring
230
Refers to the complete duplication of data on one disk
drive to another disk drive, this duplication occurs
simultaneously with each write operation: each disk will be
the mirror image of the other (also known as RAID Level
1).
MORE
Mylex Online RAID Expansion, an advanced
configuration mode that allows expansion of any
unconfigured or hot spare drive into the expandable drive
group while the controller is online with the host. For
example, a system using a five-disk-drive RAID set can
add another disk drive to create a six-disk-drive RAID set.
The MORE operation can be performed on all RAID levels
except JBOD. During the RAID set expansion process,
which includes re-striping data from the old (smaller)
RAID set to the new (expanded) RAID set, the controller
continues to service host I/O requests.
NVRAM
Non-Volatile Random Access Memory, a memory unit
equipped with a battery so that the data stays intact even
after the main power had been switched off.
P
Parity
A method of providing complete data redundancy while
requiring only a fraction of the storage capacity of
mirroring. The data and parity blocks are divided between
the disk drives in such a way that if any single disk drive is
removed or fails, the data on it can be reconstructed using
the data on the remaining disk drives. The parity data may
exist on only one disk drive or be distributed between all
disk drives in a RAID group.
Parity Check
a function used to verify the integrity of data on a system
drive. It verifies that mirror or parity information matches
the stored data on the redundant arrays. If the parity block
information is inconsistent with the data blocks, the
controller corrects the inconsistencies.
Partitioning
When the full usable storage capacity of a disk or array of
disks appears to an operating environment in the form of
Spheras Storage Director Installation & User Guide
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