Accusys ExaRAID GUI - Manual (Page 184)

Advanced Functions
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3. Click Apply to apply the settings on the selected MPIO disk.
5.1.5 Linux Multi-Path Solution
Native Linux multi-path I/O support has been added to the Linux 2.6
kernel tree since the release of 2.6.13, and has been back-ported into
Red Hat Enterprise Linux 4 in Update 2 and into Novell SUSE Linux
Enterprise Server 9 in Service Pack 2. It relies on device mapper (DM), a
kernel driver framework that allows add-on drivers to be installed to the
kernel to do I/O manipulation, such as logical volume management,
software RAID, and also in our case, multi-path I/O.
The dm-multipath driver is the implementation of Linux multi-path I/O
based on the device mapper driver. Together with a user-space
program, multipathd, which when started, reads the configuration file, /
etc/multipath.conf, to create multi-path devices at /dev/. It also runs as a
daemon to constantly monitor and recover failed paths.
Because the DM multi-path works above the hardware layer, all HBA
should work. Novell SuSE and RedHat, along with other Linux vendors, are
pushing DM multi-path as the standard multi-path solution. Many RAID
vendors have also adopted DM multi-path as the default multi-path
solution on Linux. You may find extensive related information over the
Internet.
For single-controller RAID systems, native Linux multi-path has everything
you need, and the default configuration file can be used. All you need to
do is to make sure the dm-multipath tool has been installed on your Linux
systems (RHEL 5 requires manual installation of dm-multipath package).
The sections below are examples offered for SUSE SLES 10. For redundant-
controller RAID systems, in addition to the native Linux dm-multipath, you
need also to install the proprietary RPM package and edit the
configuration file.
Command Queue: Given path1 has x IOs in
queue, path2 has y IOs in queue and switch
counter has been set to z IOs. Whenever z Read/
Write IOs have been transferred, the transferred
Read/Write IO Queues on each path will be
checked. If x > y and the primary (active) path is
on path1, the primary path will be switched to
path2 at next IO. Vice versa, if x < y and current
path is path2; the primary path will be switched
back to path1 at next IO.
Round Robin: With switch counter set to z IOs,
whenever every z Read/Write IOs have been
transferred, the primary (active) path will be
switched to another path.
Switch Counter Specify a counter value for path switching. The
range if from 1 to 99.