Configuring Device Mapper Multipath Tool (DMMP) for hosts running the Linux operating system

Before configuring the Linux® operating system for a host that attaches to the SAN Volume Controller, you should ensure that the correct Device Mapper Multipath Tool (DMMP) has been installed and configured correctly for the Linux hosts which use DMMP as a multipath driver. Currently Red Hat Enterprise Linux 4 or later and SUSE Linux Enterprise Server 9 or later support DMMP as a multipath driver and ship it on a distribution disc.

Ensure that the DMMP packages are installed on your Linux host:
- For Red Hat Enterprise Linux 4 or later, install device-mapper and device-mapper-multipath.
- For SUSE Linux Enterprise Server 9 or later, install device-mapper and multipath-tools.
In the DMMP configuration file, /etc/multipath.conf, ensure that the default settings have the following values:
```
defaults {
      polling_interval       30
      failback               immediate
      no_path_retry          5
      rr_min_io              100
      path_checker           tur
      user_friendly_names    yes
      }
      # SVC 
           device {
                 vendor                  "IBM"
                 product                 "2145"
                 path_grouping_policy    group_by_prio
                 prio_callout            "/sbin/mpath_prio_alua /dev/%n"

							    }
```
Note: If you are using SUSE Linux Enterprise Server 10 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 10 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 4 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 9 or later, 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 running one of these commands, issue one of these commands:
  - chkconfig multipathd on
  - chkconfig --level 345 multipathd on
    Note: This command enables multipathd service startup at boot.
Manually start DMMP by completing the following steps:
1. Optional: If you are using SUSE Linux Enterprise Server, run the following command before starting the DMMP: /etc/init.d/boot.multipath start.
2. Start DMMP daemon by issuing the following command: /etc/init.d/multipathd start.
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 issuing 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.