User guide

44 Configuring a Storage Solution
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 desir-
able 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 transac
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tions (e.g., order entry systems) will not get the best possible throughput.
RAID Level 1 RAID 1 is defined as disk mirroring where one drive is an exact copy of the
other. RAID 1 is useful for building a fault-tolerant system or data volume,
providing excellent availability without sacrificing performance. However,
you lose 50 percent of the assigned disk capacity. Read performance is
somewhat higher than write performance.
RAID Level 5 RAID 5 is defined as disk striping with parity where the parity data is
distributed across all drives in the volume. Normal data and parity data
are written to drives in the stripe set in a round-robin algorithm. RAID 5
is multi-threaded for both reads and writes because both normal data
and parity data are distributed round-robin. This is one reason why RAID
5 offers better overall performance in server applications. Random I/O
benefits more from RAID 5 than does sequential I/O, and writes take a
performance hit because of the parity calculations. RAID 5 is ideal for data
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base applications.
RAID Level 10 RAID 10 is defined as mirrored stripe sets or also known as RAID 0+1. You
can build RAID 10 either directly through the RAID controller (depending on
the controller) or by combining software mirroring and controller striping,
or vice versa (called RAID 01).
RAID Level 50 This RAID level is a combination of RAID level 5 and RAID level 0. Indi
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vidual smaller RAID 5 arrays are striped, to give a single RAID 50 array. This
can increase the performance by allowing the controller to more efficiently
cluster commands together. Fault tolerance is also increased, as one drive
can fail in each individual array.
Stripe The process of separating data for storage on more than one disk. For
example, bit striping stores bits 0 and 4 of all bytes on disk 1, bits 1 and 5
on disk 2, etc.
Stripe Size This is the number of data drives multiplied by the chunk size.
Sub-array In RAID 50 applications, this is the name given to the individual RAID 5
arrays that are striped together. Each sub-array has one parity drive.
Unassigned Free Space The controller keeps a map of all the space that is not assigned to any
logical drive. This space is available for creation or expansion. Each unas
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signed region is individually listed.