Red Hat Knowledgebase

DM-Multipath can provide failover in an active/passive configuration. In an active/passive configuration, only half the paths are used at any time for I/O. If any element of an I/O path (the cable, switch, or controller) fails, DM-Multipath switches to an alternate path. DM-Multipath can be configured in active/active mode, where I/O is spread over the paths in a round-robin fashion. In some configurations, DM-Multipath can detect loading on the I/O paths and dynamically re-balance the load.

Before making a configuation about dm-multipath, Please make sure the physical connection is proper. Dm-multipath generally needs at least two HBA cards or one HBA card with two connections or even two fibra switches if possible to a storage.

If the connection is correct, please make sure device-mapper-multipath package is installed by running:

# rpm -q device-mapper-multipath

If the package is installed, please make sure the service in enabled:

# chkconfig multipathd on

Comment out the blacklist in /etc/multipath.conf, the result is shown as below:

devnode_blacklist {
devnode "*"
}

Reboot the server for load dm-multipath module. After the server is online, please run:

# multipath -v3
# multipath -ll

If the module is loaded and the OS has detected the multipath, the output is similar as below:

# multipath �Cll
mpath0 (149455400000000000000000001000000110400000d000000)
[size=12 GB][features="0"][hwhandler="0"]
\_ round-robin 0 [prio=1][active]
\_ 1:0:0:0 sdb 8:16 [active][ready]
\_ round-robin 0 [prio=1][enabled]
\_ 1:0:0:1 sdc 8:32 [active][ready]

Usually the output means multipathed service is successfully started and the multipath group is created. Now some values may be modified as shown below:

defaults {
multipath_tool "/sbin/multipath -v0"
udev_dir /dev
polling_interval 10
default_selector "round-robin 0"
default_path_grouping_policy failover
default_getuid_callout "/sbin/scsi_id -g -u -s /block/%n"
default_prio_callout "/bin/true"
default_features "0"
rr_min_io 100
failback immediate
}

default_path_grouping_policy - It can be changed between failover and multibus. The other value maybe one of these:

failover - 1 path per priority group
multibus - all valid paths in 1 priority group
group_by_serial - 1 priority group per detected serial number
group_by_prio - 1 priority group per path priority value
group_by_node_name - 1 priority group per target node name