Accusys ExaRAID GUI - Manual (Page 175)

Advanced Functions
5-2
side software RAID0 because RAID0 forces I/O to be truncated into data
stripes, resulting in overhead and limited size of per-transaction data
transfer.
• Higher bandwidth utilization
With statically assigning paths to logical devices on a controller, the
bandwidth of all connections cannot be fully utilized if the loading on
different logical devices is uneven. By transferring data over all paths,
bandwidth utilization is more efficient, and ROI is improved.
• Easier management
With dynamic load balancing, the users need not to worry about either
bandwidth planning during the installation or the reconfiguration for
performance tuning. When there is new HBA or new connection is
added, the bandwidth created can be utilized easily. Therefore, MPIO
largely reduces the management efforts and the TCO (Total Cost of
Ownership).
5.1.3 Configuring MPIO Hosts and RAID Controller
A path is defined as the link from host to a virtual disk presented by the
RAID controller, and it includes the HBA, cables, and optionally a storage
switch. To build multiple I/O paths, there have to be multiple links
between the host computer and the RAID system, and the virtual disks in
the RAID system have to be exported to multiple host-interface ports. The
multi-path software on the host computer can access the virtual disks
through any of the exported LUNs over the links. Because multiple HBAs
(or multiple ports on single HBA) on a host computer are required to
access the same set of LUN mappings, grouping these HBAs into a host
group and using symmetric storage presentation for exporting LUNs would
it ease your configuration tasks.
If directly attaching the host-interface ports to the HBAs, you can easily
know the number of paths for single virtual disk by counting the physical
links. But if a switch is used to build a fabric, you need to multiply the
number of HBA ports and the number of host-interface ports of the
controller to derive the number of paths. For example, if a host has three
HBA ports connecting to a switch, which links to a RAID system with four
host-interface ports, there will be twelve paths for the host to the RAID
system.
For regular operating systems, a path is perceived as a physical disk, and
accessing a LUN through different paths simultaneously without multi-
path software could cause data corruption. It is the multi-path software
to scan all detected physical disks (paths) and map the physical disks to
single logical disk (MPIO disk) if these physical disks present consistent
information to indicate that they belong to the same virtual disk. The
applications then can access the LUN via the MPIO disk.
After completing the physical connections between hosts and RAID
systems, please follow the steps below: