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Table 2 Summary of RAID methods
Maximum number of hard
drives
Tolerant of single hard
drive failure?
Tolerant of multiple
simultaneous hard drive
failures?
Online spares
Further protection against data loss can be achieved by assigning an online spare (or hot spare)
to any configuration except RAID 0. This hard drive contains no data and is contained within the
same storage subsystem as the other drives in the array. When a hard drive in the array fails, the
controller can then automatically rebuild information that was originally on the failed drive onto
the online spare. This quickly restores the system to full RAID level fault tolerance protection.
However, unless RAID Advanced Data Guarding (ADG) is being used, which can support two
drive failures in an array, in the unlikely event that a third drive in the array should fail while data
is being rewritten to the spare, the logical drive still fails.
Logical storage elements
Logical storage elements consist of those components that translate the physical storage elements
to file system elements. The storage system uses the Window Disk Management utility to manage
the various types of disks presented to the file system. There are two types of LUN presentation:
basic disk and dynamic disk. Each of these types of disk has special features that enable different
types of management.
Logical drives (LUNs)
While an array is a physical grouping of hard drives, a logical drive consists of components that
translate physical storage elements into file system elements. A LUN may also be referred to as a
virtual disk.
It is important to note that a LUN may span all physical drives within a storage controller subsystem,
but cannot span multiple storage controller subsystems.
Figure 1 1 Two arrays (A1, A2) and five logical drives (L1 through L5) spread over five physical
drives
NOTE:
This type of configuration may not apply to all storage systems and serves only as an
example.
Through the use of basic disks, you can create primary partitions or extended partitions. Partitions
can only encompass one LUN. Through the use of dynamic disks, you can create volumes that
30
Storage management overview
RAID 0 Striping
RAID 1+0
(no fault
Mirroring
tolerance)
N/A
N/A
No
Yes
No
If the failed
drives are not
mirrored to each
other
RAID 5
RAID 6 (ADG)
Distributed Data
Guarding
14
Storage system dependent
Yes
Yes
No
Yes (two drives can fail)
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