Enabling and Disabling Multipathing in the Linux Operating System

This section describes how to enable and disable multipathing in supported versions of the Linux operating system. The following subsections are included:

• About Multipathing

• To Enable Multipathing in Linux

• To Disable Multipathing in Linux

About Multipathing

After cabling your server for multipath, you will see two copies of each disk from the OS since you are using two separate array paths (SAS A and B). If you want to have multiple hosts accessing disks in the array, you must first set up zoning per host as described in Chapter 3, Adding and Zoning Array Storage Using CAM.

For example, if you have created a zone in each of the array's SAS domains that includes three disks, when entering the lsscsi command before MPxIO is installed, you will see two of each multipathed disk.

# lsscsi

[1:0:0:0] disk SEAGATE ST330055SSUN300G 0B92 /dev/sda
[1:0:1:0] disk SEAGATE ST330055SSUN300G 0B92 /dev/sdb
[1:0:2:0] disk SEAGATE ST330055SSUN300G 0B92 /dev/sdc
[1:0:3:0] enclosu SUN Storage J4500 3R21 -
[2:0:0:0] disk SEAGATE ST330055SSUN300G 0B92 /dev/sdd
[2:0:1:0] disk SEAGATE ST330055SSUN300G 0B92 /dev/sde
[2:0:2:0] disk SEAGATE ST330055SSUN300G 0B92 /dev/sdf
[2:0:3:0] enclosu SUN Storage J4500 3R21 -

Once the multipath daemon is started on the host, you can see multipath details using the multipath command.

# multipath -ll

35000c5000357625b dm-2 SEAGATE,ST340008SSUN0.4
[size=373G][features=0][hwhandler=0]
    \_ round-robin 0 [prio=2][active]
    \_ 1:0:1:0  sdb 8:0    [active][ready]
    \_ 2:0:1:0  sde 8:192  [active][ready]

To correlate the dm-2 dual path disks with what's displayed in CAM (in the Host details page, see Two Hosts With Zoned Disks in CAM), use the lsscsi command with the -g option.

# lsscsi -g

[1:0:0:0] disk SEAGATE ST330055SSUN300G 0B92 /dev/sda /dev/sg0
[1:0:1:0] disk SEAGATE ST330055SSUN300G 0B92 /dev/sdb /dev/sg1
[1:0:2:0] disk SEAGATE ST330055SSUN300G 0B92 /dev/sdc /dev/sg2
[1:0:3:0] enclosu SUN Storage J4500 3R21 - /dev/sg3
[2:0:0:0] disk SEAGATE ST330055SSUN300G 0B92 /dev/sdd /dev/sg4
[2:0:1:0] disk SEAGATE ST330055SSUN300G 0B92 /dev/sde /dev/sg5
[2:0:2:0] disk SEAGATE ST330055SSUN300G 0B92 /dev/sdf /dev/sg6
[2:0:3:0] enclosu SUN Storage J4500 3R21 - /dev/sg7

For each disk, CAM will report the device names from the last column in the /dev/sgN format.

To Enable Multipathing in Linux

1. Attach a J4500 array to a server with a supported version of Linux installed.
2. On the server, edit or create the /etc/multipath.conf file.
3. Reboot the server.
4. After the reboot, make sure that the OS discovers all the disks in the J4500 array either by using the Linux commands, fdisk or lsscsi.
5. Partition any disks you want to the desired sizes.
6. Use the Linux command modprobe to add the loadable kernel modules dm-multipath and dm-round-robin.

   # modprobe dm-multipath

   # modprobe dm-round-robin

7. Start the multipathd daemon.

   For Linux SUSE 9, use the following command:

   # multipathd -v0

   For other supported Linux versions, use the following command:

   # service multipathd start

8. Start the multipathing device mapper target autoconfig.

   # multipath -v2

9. List the multipath devices that have been created.

   # multipath -ll

   The output should list the same number of devices as there are disks in the J4500 array. The following is an example of output:

   35000c5000357625b dm-2 SEAGATE,ST340008SSUN0.4
   [size=373G][features=0][hwhandler=0]
       \_ round-robin 0 [prio=2][active]
       \_ 0:0:0:0  sda 8:0    [active][ready]
       \_ 1:0:0:0  sdm 8:192  [active][ready]

To Disable Multipathing in Linux

1. If a RAID volume, LVM volume, or volume mount have been placed over the device node of the multipathed disk, quiesce the volume.
2. Use the multipath -f command to disable multipathing to a specific device.

   # multipath -f mpath1

3. Use the multipath -F command to disable multipathing on all multipathed devices.

   # multipath -F

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   Note - If the message map in use appears for a device when you attempt to disable multipathing, the device is still in use. You must unmount or otherwise quiesce the device before you can disable multipathing. If you cannot quiesce the device, edit the /etc/multipath.conf file to exclude the device and then reboot the server.

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