Configuring Device Mapper Multipath (DMMP) tool for hosts that run the Linux operating system

Before you configure the Linux operating system for a host that attaches to the system, ensure that the correct Device Mapper Multipath Tool (DMMP) is installed and configured correctly for the Linux hosts that use DMMP as a multipath driver.Currently, Red Hat Enterprise Linux 5 and later and SUSE Linux Enterprise Server 11 support DMMP as a multipath driver and include it on a distribution disk.

Ensure that the DMMP packages are installed on your Linux host:
In the DMMP configuration file, /etc/multipath.conf, ensure that the default settings have the values that are described in Settings for Linux hosts.
Note: Linux™ usually enables multipathing automatically. However, in some cases such as the RH 6.3 SAN boot installation, the multipathing is not configured properly. If this situation occurs, you must manually edit the /etc/multipath.conf file. For more information about DM-multipath, see the Linux™ Storage Administration Guide or DM-Multipathing Installation and Configuration Guide for your kernel from the Linux™ provider. For more information about Linux™ system configuration, see the following website:
http://www.ibm.com/support/knowledgecenter/linuxonibm/liaab/ic-homepage.htm

Note: If you are using RHEL6 or SUSE Linux Enterprise Server 11 Service Pack 2, use prio "alua" instead of prio_callout "/sbin/mpath_prio_alua /dev/%n" to disable the Using deprecated prio_callout message. This value disables only the error message and does not affect operations. If prio_callout "/sbin/mpath_prio_alua /dev/%n" is set on SUSE Linux Enterprise Server 11 Service Pack 2, the Using deprecated prio_callout message is issued for all devices when the multipath command is used.
To enable DMMP autoload during an operating system boot, issue the following commands as root:
- For Red Hat Enterprise Linux 5 or later, issue one of the following commands:
  - chkconfig multipathd on
  - chkconfig --level 345 multipathd on
    Note: This command enables multipathd service startup at boot.
- For SUSE Linux Enterprise Server 11, issue one of the following commands:
  - chkconfig boot.multipath on
  - chkconfig --level 345 boot.multipath on
    Note: This command enables boot.multipath service startup at boot.
  After you run one of these commands, issue one of the following commands:
  - chkconfig multipathd on
  - chkconfig --level 345 multipathd on
    Note: This command enables multipathd service startup at boot.
Start DMMP manually by completing the following steps:
1. Optional: If you are using SUSE Linux Enterprise Server, run the /etc/init.d/boot.multipath start command before you start the DMMP.
2. Start DMMP daemon by issuing the /etc/init.d/multipathd start command: .
3. Run the commands multipath or multipath -v2 to scan the multipath devices.
4. Issue the multipath -ll command to view the detailed information of the multipath devices. The command has the following output:
```
mpath1 (36005076801860022900000000000019a)  IBM,2145
[size=2.0G][features=0][hwhandler=0]
\_ round-robin 0 [prio=200][ enabled]
\_ 4:0:0:1 sdd  8:48   [active][ready]
\_ 5:0:0:1 sdt  65:48  [active][ready]
\_ round-robin 0 [prio=40][ active]
\_ 4:0:2:1 sdak 66:64  [active][ready]
\_ 5:0:2:1 sdal 66:80  [active][ready]
```

Note: If the new device cannot be found after you issue the multipath command, you must reload the HBA driver or reboot the server, and reissue the multipath command after Linux recognizes the new device in the SCSI layer with a name like sd*. The HBA driver must recognize the new devices before DMMP can recognize them and manage them. To view the detailed status of the multipath devices, issue the multipath -ll command.