Avago Technologies MegaRAID Fast Path Software - Manual (Page 23)

LSI Corporation Confidential | July 2011 Page 23
MegaRAID SAS Software User Guide Chapter 2: Introduction to RAID | Components and Features
2.4.3.1 Multipathing The firmware provides support for detecting and using multiple paths from the RAID
controllers to the SAS devices that are in enclosures. Devices connected to enclosures
have multiple paths to them. With redundant paths to the same port of a device, if one
path fails, another path can be used to communicate between the controller and the
device. Using multiple paths with load balancing, instead of a single path, can increase
reliability through redundancy.
Applications show the enclosures and the drives connected to the enclosures. The
firmware dynamically recognizes new enclosures added to a configuration along with
their contents (new drives). In addition, the firmware dynamically adds the enclosure
and its contents to the management entity currently in use.
Multipathing provides the following features:
 Support for failover, in the event of path failure
 Auto-discovery of new or restored paths while the system is online, and reversion to
system load-balancing policy
 Measurable bandwidth improvement to the multi-path device
 Support for changing the load-balancing path while the system is online
The firmware determines whether enclosure modules (ESMs) are part of the same
enclosure. When a new enclosure module is added (allowing multi-path) or removed
(going single path), an Asynchronous Event Notification (AEN) is generated. AENs
about drives contain correct information about the enclosure, when the drives are
connected by multiple paths. The enclosure module detects partner ESMs and issues
events appropriately.
In a system with two ESMs, you can replace one of the ESMs without affecting the
virtual drive availability. For example, the controller can run heavy I/Os, and when you
replace one of the ESMs, I/Os should not stop. The controller uses different paths to
balance the load on the entire system.
In the MegaRAID Storage Manager utility, when multiple paths are available to a drive,
the drive information shows only one enclosure. The utility shows that a redundant
path is available to a drive. All drives with a redundant path display this information.
The firmware supports online replacement of enclosure modules.
2.4.4 Consistency Check The consistency check operation verifies correctness of the data in virtual drives that
use RAID levels 1, 5, 6, 10, 50, and 60. (RAID 0 does not provide data redundancy.) For
example, in a system with parity, checking consistency means computing the data on
one drive and comparing the results to the contents of the parity drive.
NOTE: It is recommended that you perform a consistency check at least once a month.
2.4.5 Copyback The copyback feature allows you to copy data from a source drive of a virtual drive to a
destination drive that is not a part of the virtual drive. Copyback is often used to create
or restore a specific physical configuration for a drive group (for example, a specific
arrangement of drive group members on the device I/O buses). Copyback can be run
automatically or manually.