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StorView RAID Module
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6.1.2 Terminology
The following describes some of the terminology used when creating disk array’s and logical drives.
Term
Description
Array
A group of disk drives that are combined together to create a single
large storage area. Up to 64 arrays are supported, each containing up
to 16 drives per array. There is no capacity limit for the arrays.
Back-off Percent
In order to allow drives from a different family or manufacturer to be
used as a replacement for a drive in an array, it is recommended that
a small percentage of the drive’s capacity be reserved when creating
the array. This is user selectable, from 0 to 10 percent. This is
sometimes known as Reserved Capacity.
Cache Flush Array
This is the array that is used to automatically flush cache data in a
situation where power has failed to some of the drives.
Chunk Size
This is the amount of data that is written on a single drive before the
controller moves to the next drive in the stripe.
Initialization
RAID 5, 6, and 50 disk arrays must have consistent parity before they
can be used to protect data. However, StorView will initialize all array
RAID types regardless. If the user chooses the Trust option during
array creation or stops the initialization, the array will be trusted. Note
that any drive failure in a trusted array will result in data corruption.
It is possible to perform the initialization later. This recalculates the
parity based on current data, ensuring data and parity are consistent.
Logical Drive Availability
To accommodate hosts with multiple ports and multiple host systems,
it is possible to restrict a logical drive’s availability to a particular HBA
or controller port. Access can be enabled or disabled for each host port
of each controller.
Mapped LUN Number
Each logical drive is presented to the host system with a unique LUN.
In certain cases (such as after deleting another logical drive) it may be
desirable to change the number that a logical drive is presented as.
This can be done at any time, bearing in mind that any attached host
systems may need to be rebooted or re-configured to maintain access
to the logical drive.
RAID Level 0
RAID 0 is defined as disk striping where data is striped or spread
across one or more drives in parallel. RAID 0 is ideal for environments
in which performance (read and write) is more important than fault
tolerance or you need the maximum amount of available drive capacity
in one volume. Drive parallelism increases throughput because all
disks in the stripe set work together on every I/O operation. For
greatest efficiency, all drives in the stripe set must be the same
capacity. Because all drives are used in every operation, RAID 0 allows
for single-threaded I/O only (i.e., one I/O operation at a time).
Environments with many small simultaneous transactions (e.g., order
entry systems) will not get the best possible throughput.
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