HP ProLiant Storage Server User Guide (440584-001, February 2007)

T o protect against data loss from hard drive failure, storage servers should be configured with fault
tolerance. HP recommends adhering to RAID 5 configurations.
The table below summarizes the impor tant features of the different kinds of RAID suppor ted by the
Smart Array controllers. The decision chart in the following table can help determine which option is
best for different situations.
Table 6 Summary of R AID methods
RAID 0
Striping
(no fault
tolerance)
RAID 1+0
Mirroring
RAID 5
Distributed
Data
Guarding
RAID 6 (ADG)
Maximum number of
hard drives
N/A N/A
14
Storage system
dependent
Tolerant of single hard
drive failu
re?
No
Yes Yes Yes
Tolerant of multiple
simultaneous hard drive
failures?
No
If the failed
drives are not
mirrored to each
other
No
Yes(twodrivescanfail)
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 R AI D 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 stora ge elem ents to file
system
elements. The storage server uses the Window Disk M a nag ement 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 t ypes of management.
Logical drives (LUNs)
While an array is a physical grouping of h ard drives, a logical drive c onsists of comp onents that translate
physical storage elements into file system elements.
It is important to note that a LUN may extend over (span) all physical drives within a storage controller
subsystem, but cannot span multiple storage controller subsystems.
32 Storage management over view