Avago Technologies MegaRAID Fast Path Software - Manual (Page 22)

Page 22 LSI Corporation Confidential | July 2011
MegaRAID SAS Software User Guide Chapter 2: Introduction to RAID | Components and Features
2.4 Components and
Features
RAID levels describe a system for ensuring the availability and redundancy of data
stored on large disk subsystems. See
Section 2.5, RAID Levels for detailed information
about RAID levels. The following subsections describes the components of RAID drive
groups and RAID levels.
2.4.1 Drive Group A drive group is a group of physical drives. These drives are managed in partitions
known as virtual drives.
2.4.2 Virtual Drive A virtual drive is a partition in a drive group that is made up of contiguous data
segments on the drives. A virtual drive can consist of an entire drive group, more than
one entire drive group, a part of a drive group, parts of more than one drive group, or a
combination of any two of these conditions.
2.4.3 Fault Tolerance Fault tolerance is the capability of the subsystem to undergo a drive failure or failures
without compromising data integrity, and processing capability. The RAID controller
provides this support through redundant drive groups in RAID levels 1, 5, 6, 10, 50, and
60. The system can still work properly even with drive failure in a drive group, though
performance can be degraded to some extent.
In a span of RAID 1 drive groups, each RAID 1 drive group has two drives and can
tolerate one drive failure. The span of RAID 1 drive groups can contain up to 32 drives,
and tolerate up to 16 drive failures - one in each drive group. A RAID 5 drive group can
tolerate one drive failure in each RAID 5 drive group. A RAID 6 drive group can tolerate
up to two drive failures.
Each spanned RAID 10 virtual drive can tolerate multiple drive failures, as long as each
failure is in a separate drive group. A RAID 50 virtual drive can tolerate two drive
failures, as long as each failure is in a separate drive group. RAID 60 drive groups can
tolerate up to two drive failures in each drive group.
NOTE: RAID level 0 is not fault tolerant. If a drive in a RAID 0 drive group fails, the entire
virtual drive (all drives associated with the virtual drive) fails.
Fault tolerance is often associated with system availability because it allows the system
to be available during the failures. However, fault tolerance means that it is also
important for the system to be available during the repair of the problem.
A hot spare is an unused drive that, in case of a disk failure in a redundant RAID drive
group, can be used to rebuild the data and re-establish redundancy. After the hot spare
is automatically moved into the RAID drive group, the data is automatically rebuilt on
the hot spare drive. The RAID drive group continues to handle requests while the
rebuild occurs.
Auto-rebuild allows a failed drive to be replaced and the data automatically rebuilt by
“hot-swapping” the drive in the same drive bay. The RAID drive group continues to
handle requests while the rebuild occurs.