Novell LINUX ENTERPRISE SERVER 11 - STORAGE ADMINISTRATION GUIDE 2-23-2010, Administration Manual, Page: 61 / 140

Managing Multipath I/O for Devices 61
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7.6 Configuring Path Failover Policies and
Priorities
In a Linux host, when there are multiple paths to a storage controller, each path appears as a separate
block device, and results in multiple block devices for single LUN. The Device Mapper Multipath
service detects multiple paths with the same LUN ID, and creates a new multipath device with that
ID. For example, a host with two HBAs attached to a storage controller with two ports via a single
unzoned Fibre Channel switch sees four block devices:
/dev/sda
,
/dev/sdb
,
/dev/sdc
, and
/
dev/sdd
. The Device Mapper Multipath service creates a single block device,
/dev/mpath/
mpath1
that reroutes I/O through those four underlying block devices.
This section describes how to specify policies for failover and configure priorities for the paths.
Š
Section 7.6.1, “Configuring the Path Failover Policies,” on page 61
Š
Section 7.6.2, “Configuring Failover Priorities,” on page 61
Š
Section 7.6.3, “Using a Script to Set Path Priorities,” on page 67
Š
Section 7.6.4, “Configuring ALUA,” on page 67
Š
Section 7.6.5, “Reporting Target Path Groups,” on page 69
7.6.1 Configuring the Path Failover Policies
Use the
multipath
command with the -p option to set the path failover policy:
multipath devicename -p policy
Replace policy with one of the following policy options:
Table 7-4
Group Policy Options for the multipath -p Command
7.6.2 Configuring Failover Priorities
You must manually enter the failover priorities for the device in the
/etc/multipath.conf
file.
Examples for all settings and options can be found in the
/usr/share/doc/packages/
multipath-tools/multipath.conf.annotated
file.
Š
“Understanding Priority Groups and Attributes” on page 62
Š “Configuring for Round-Robin Load Balancing” on page 66
Policy Option Description
failover One path per priority group.
multibus All paths in one priority group.
group_by_serial One priority group per detected serial number.
group_by_prio One priority group per path priority value. Priorities are determined by callout
programs specified as a global, per-controller, or per-multipath option in the
/
etc/multipath.conf configuration file.
group_by_node_name One priority group per target node name. Target node names are fetched in the
/sys/class/fc_transport/target*/node_name location.