Configure MPIO on a StorSimple host running CentOS

This article explains the steps required to configure Multipathing IO (MPIO) on your Centos 6.6 host server. The host server is connected to your Microsoft Azure StorSimple device for high availability via iSCSI initiators. It describes in detail the automatic discovery of multipath devices and the specific setup only for StorSimple volumes.

This procedure is applicable to all the models of StorSimple 8000 series devices.


This procedure cannot be used for a StorSimple Cloud Appliance. For more information, see how to configure host servers for your cloud appliance.

About multipathing

The multipathing feature allows you to configure multiple I/O paths between a host server and a storage device. These I/O paths are physical SAN connections that can include separate cables, switches, network interfaces, and controllers. Multipathing aggregates the I/O paths, to configure a new device that is associated with all of the aggregated paths.

The purpose of multipathing is two-fold:

  • High availability: It provides an alternate path if any element of the I/O path (such as a cable, switch, network interface, or controller) fails.
  • Load balancing: Depending on the configuration of your storage device, it can improve the performance by detecting loads on the I/O paths and dynamically rebalancing those loads.

About multipathing components

Multipathing in Linux consists of kernel components and user-space components as tabulated below.

  • Kernel: The main component is the device-mapper that reroutes I/O and supports failover for paths and path groups.

  • User-space: These are multipath-tools that manage multipathed devices by instructing the device-mapper multipath module what to do. The tools consist of:

    • Multipath: lists and configures multipathed devices.
    • Multipathd: daemon that executes multipath and monitors the paths.
    • Devmap-name: provides a meaningful device-name to udev for devmaps.
    • Kpartx: maps linear devmaps to device partitions to make multipath maps partitionable.
    • Multipath.conf: configuration file for multipath daemon that is used to overwrite the built-in configuration table.

About the multipath.conf configuration file

The configuration file /etc/multipath.conf makes many of the multipathing features user-configurable. The multipath command and the kernel daemon multipathd use information found in this file. The file is consulted only during the configuration of the multipath devices. Make sure that all changes are made before you run the multipath command. If you modify the file afterwards, you will need to stop and start multipathd again for the changes to take effect.

The multipath.conf has five sections:

  • System level defaults (defaults): You can override system level defaults.
  • Blacklisted devices (blacklist): You can specify the list of devices that should not be controlled by device-mapper.
  • Blacklist exceptions (blacklist_exceptions): You can identify specific devices to be treated as multipath devices even if listed in the blacklist.
  • Storage controller specific settings (devices): You can specify configuration settings that will be applied to devices that have Vendor and Product information.
  • Device specific settings (multipaths): You can use this section to fine-tune the configuration settings for individual LUNs.

Configure multipathing on StorSimple connected to Linux host

A StorSimple device connected to a Linux host can be configured for high availability and load balancing. For example, if the Linux host has two interfaces connected to the SAN and the device has two interfaces connected to the SAN such that these interfaces are on the same subnet, then there will be 4 paths available. However, if each DATA interface on the device and host interface are on a different IP subnet (and not routable), then only 2 paths will be available. You can configure multipathing to automatically discover all the available paths, choose a load-balancing algorithm for those paths, apply specific configuration settings for StorSimple-only volumes, and then enable and verify multipathing.

The following procedure describes how to configure multipathing when a StorSimple device with two network interfaces is connected to a host with two network interfaces.


This section details the configuration prerequisites for CentOS server and your StorSimple device.

On CentOS host

  1. Make sure that your CentOS host has 2 network interfaces enabled. Type:


    The following example shows the output when two network interfaces (eth0 and eth1) are present on the host.

     [root@centosSS ~]# ifconfig
     eth0  Link encap:Ethernet  HWaddr 00:15:5D:A2:33:41  
       inet addr:  Bcast:  Mask:
       inet6 addr: 2001:4898:4010:3012:215:5dff:fea2:3341/64 Scope:Global
       inet6 addr: fe80::215:5dff:fea2:3341/64 Scope:Link
      RX packets:36536 errors:0 dropped:0 overruns:0 frame:0
       TX packets:6312 errors:0 dropped:0 overruns:0 carrier:0
       collisions:0 txqueuelen:1000
       RX bytes:13994127 (13.3 MiB)  TX bytes:645654 (630.5 KiB)
     eth1  Link encap:Ethernet  HWaddr 00:15:5D:A2:33:42  
       inet addr:  Bcast:  Mask:
       inet6 addr: 2001:4898:4010:3012:215:5dff:fea2:3342/64 Scope:Global
       inet6 addr: fe80::215:5dff:fea2:3342/64 Scope:Link
       RX packets:25962 errors:0 dropped:0 overruns:0 frame:0
       TX packets:11 errors:0 dropped:0 overruns:0 carrier:0
       collisions:0 txqueuelen:1000
       RX bytes:2597350 (2.4 MiB)  TX bytes:754 (754.0 b)
     loLink encap:Local Loopback  
       inet addr:  Mask:
       inet6 addr: ::1/128 Scope:Host
       UP LOOPBACK RUNNING  MTU:65536  Metric:1
       RX packets:12 errors:0 dropped:0 overruns:0 frame:0
       TX packets:12 errors:0 dropped:0 overruns:0 carrier:0
       collisions:0 txqueuelen:0
       RX bytes:720 (720.0 b)  TX bytes:720 (720.0 b)
  2. Install iSCSI-initiator-utils on your CentOS server. Perform the following steps to install iSCSI-initiator-utils.

    1. Log on as root into your CentOS host.
    2. Install the iSCSI-initiator-utils. Type:

      yum install iscsi-initiator-utils

    3. After the iSCSI-Initiator-utils is successfully installed, start the iSCSI service. Type:

      service iscsid start

      On occasions, iscsid may not actually start and the --force option may be needed

    4. To ensure that your iSCSI initiator is enabled during boot time, use the chkconfig command to enable the service.

      chkconfig iscsi on

    5. To verify that that it was properly setup, run the command:

      chkconfig --list | grep iscsi

      A sample output is shown below.

       iscsi   0:off   1:off   2:on3:on4:on5:on6:off
       iscsid  0:off   1:off   2:on3:on4:on5:on6:off

      From the above example, you can see that your iSCSI environment will run on boot time on run levels 2, 3, 4, and 5.

  3. Install device-mapper-multipath. Type:

    yum install device-mapper-multipath

    The installation will start. Type Y to continue when prompted for confirmation.

On StorSimple device

Your StorSimple device should have:

  • A minimum of two interfaces enabled for iSCSI. To verify that two interfaces are iSCSI-enabled on your StorSimple device, perform the following steps in the Azure classic portal for your StorSimple device:

    1. Log into the classic portal for your StorSimple device.
    2. Select your StorSimple Manager service, click Devices and choose the specific StorSimple device. Click Configure and verify the network interface settings. A screenshot with two iSCSI-enabled network interfaces is shown below. Here DATA 2 and DATA 3, both 10 GbE interfaces are enabled for iSCSI.

      MPIO StorsSimple DATA 2 config

      MPIO StorSimple DATA 3 Config

      In the Configure page

      1. Ensure that both network interfaces are iSCSI-enabled. The iSCSI enabled field should be set to Yes.
      2. Ensure that the network interfaces have the same speed, both should be 1 GbE or 10 GbE.
      3. Note the IPv4 addresses of the iSCSI-enabled interfaces and save for later use on the host.
  • The iSCSI interfaces on your StorSimple device should be reachable from the CentOS server. To verify this, you need to provide the IP addresses of your StorSimple iSCSI-enabled network interfaces on your host server. The commands used and the corresponding output with DATA2 ( and DATA3 ( is shown below:

      [root@centosSS ~]# iscsiadm -m discovery -t sendtargets -p,1,1

Hardware configuration

We recommend that you connect the two iSCSI network interfaces on separate paths for redundancy. The figure below shows the recommended hardware configuration for high availability and load-balancing multipathing for your CentOS server and StorSimple device.

MPIO hardware config for StorSimple to Linux host

As shown in the preceding figure:

  • Your StorSimple device is in an active-passive configuration with two controllers.
  • Two SAN switches are connected to your device controllers.
  • Two iSCSI initiators are enabled on your StorSimple device.
  • Two network interfaces are enabled on your CentOS host.

The above configuration will yield 4 separate paths between your device and the host if the host and data interfaces are routable.


  • We recommend that you do not mix 1 GbE and 10 GbE network interfaces for multipathing. When using two network interfaces, both the interfaces should be the identical type.
  • On your StorSimple device, DATA0, DATA1, DATA4 and DATA5 are 1 GbE interfaces whereas DATA2 and DATA3 are 10 GbE network interfaces.|

Configuration steps

The configuration steps for multipathing involve configuring the available paths for automatic discovery, specifying the load-balancing algorithm to use, enabling multipathing and finally verifying the configuration. Each of these steps is discussed in detail in the following sections.

Step 1: Configure multipathing for automatic discovery

The multipath-supported devices can be automatically discovered and configured.

  1. Initialize /etc/multipath.conf file. Type:

    mpathconf --enable

    The above command will create a sample/etc/multipath.conf file.

  2. Start multipath service. Type:

    service multipathd start

    You will see the following output:

    Starting multipathd daemon:

  3. Enable automatic discovery of multipaths. Type:

    mpathconf --find_multipaths y

    This will modify the defaults section of your multipath.conf as shown below:

     defaults {
     find_multipaths yes
     user_friendly_names yes
     path_grouping_policy multibus

Step 2: Configure multipathing for StorSimple volumes

By default, all devices are black listed in the multipath.conf file and will be bypassed. You will need to create blacklist exceptions to allow multipathing for volumes from StorSimple devices.

  1. Edit the /etc/mulitpath.conf file. Type:

    vi /etc/multipath.conf

  2. Locate the blacklist_exceptions section in the multipath.conf file. Your StorSimple device needs to be listed as a blacklist exception in this section. You can uncomment relevant lines in this file to modify it as shown below (use only the specific model of the device you are using):

     blacklist_exceptions {
         device {
                    vendor  "MSFT"
                    product "STORSIMPLE 8100*"
         device {
                    vendor  "MSFT"
                    product "STORSIMPLE 8600*"

Step 3: Configure round-robin multipathing

This load-balancing algorithm uses all the available multipaths to the active controller in a balanced, round-robin fashion.

  1. Edit the /etc/multipath.conf file. Type:

    vi /etc/multipath.conf

  2. Under the defaults section, set the path_grouping_policy to multibus. The path_grouping_policy specifies the default path grouping policy to apply to unspecified multipaths. The defaults section will look as shown below.

     defaults {
             user_friendly_names yes
             path_grouping_policy multibus


The most common values of path_grouping_policy include:

  • failover = 1 path per priority group
  • multibus = all valid paths in 1 priority group

Step 4: Enable multipathing

  1. Restart the multipathd daemon. Type:

    service multipathd restart

  2. The output will be as shown below:

     [root@centosSS ~]# service multipathd start
     Starting multipathd daemon:  [OK]

Step 5: Verify multipathing

  1. First make sure that iSCSI connection is established with the StorSimple device as follows:

    a. Discover your StorSimple device. Type:

    iscsiadm -m discovery -t sendtargets -p  <IP address of network interface on the device>:<iSCSI port on StorSimple device>

    The output when IP address for DATA0 is and port 3260 is opened on the StorSimple device for outbound iSCSI traffic is as shown below:,1,1

    Copy the IQN of your StorSimple device,, from the preceding output.

    b. Connect to the device using target IQN. The StorSimple device is the iSCSI target here. Type:

    iscsiadm -m node --login -T <IQN of iSCSI target>

    The following example shows output with a target IQN of The output indicates that you have successfully connected to the two iSCSI-enabled network interfaces on your device.

    Logging in to [iface: eth0, target:, portal:,3260] (multiple)
    Logging in to [iface: eth1, target:, portal:,3260] (multiple)
    Logging in to [iface: eth0, target:, portal:,3260] (multiple)
    Logging in to [iface: eth1, target:, portal:,3260] (multiple)
    Login to [iface: eth0, target:, portal:,3260] successful.
    Login to [iface: eth1, target:, portal:,3260] successful.
    Login to [iface: eth0, target:, portal:,3260] successful.
    Login to [iface: eth1, target:, portal:,3260] successful.

    If you see only one host interface and two paths here, then you need to enable both the interfaces on host for iSCSI. You can follow the detailed instructions in Linux documentation.

  2. A volume is exposed to the CentOS server from the StorSimple device. For more information, see Step 6: Create a volume via the Azure portal on your StorSimple device.

  3. Verify the available paths. Type:

    multipath –l

    The following example shows the output for two network interfaces on a StorSimple device connected to a single host network interface with two available paths.

     mpathb (36486fd20cc081f8dcd3fccb992d45a68) dm-3 MSFT,STORSIMPLE 8100
     size=100G features='0' hwhandler='0' wp=rw
     `-+- policy='round-robin 0' prio=0 status=active
     |- 7:0:0:1 sdc 8:32 active undef running
     `- 6:0:0:1 sdd 8:48 active undef running
     The following example shows the output for two network interfaces on a StorSimple device connected to two host network interfaces with four available paths.
     mpathb (36486fd27a23feba1b096226f11420f6b) dm-2 MSFT,STORSIMPLE 8100
     size=100G features='0' hwhandler='0' wp=rw
     `-+- policy='round-robin 0' prio=0 status=active
     |- 17:0:0:0 sdb 8:16 active undef running
     |- 15:0:0:0 sdd 8:48 active undef running
     |- 14:0:0:0 sdc 8:32 active undef running
     `- 16:0:0:0 sde 8:64 active undef running
     After the paths are configured, refer to the specific instructions on your host operating system (Centos 6.6) to mount and format this volume.

Troubleshoot multipathing

This section provides some helpful tips if you run into any issues during multipathing configuration.

Q. I do not see the changes in multipath.conf file taking effect.

A. If you have made any changes to the multipath.conf file, you will need to restart the multipathing service. Type the following command:

service multipathd restart

Q. I have enabled two network interfaces on the StorSimple device and two network interfaces on the host. When I list the available paths, I see only two paths. I expected to see four available paths.

A. Make sure that the two paths are on the same subnet and routable. If the network interfaces are on different vLANs and not routable, you will see only two paths. One way to verify this is to make sure that you can reach both the host interfaces from a network interface on the StorSimple device. You will need to contact Microsoft Support as this verification can only be done via a support session.

Q. When I list available paths, I do not see any output.

A. Typically, not seeing any multipathed paths suggests a problem with the multipathing daemon, and it’s most likely that any problem here lies in the multipath.conf file.

It would also be worth checking that you can actually see some disks after connecting to the target, as no response from the multipath listings could also mean you don’t have any disks.

  • Use the following command to rescan the SCSI bus:

    $ of sg3_utils package)

  • Type the following commands:

    $ dmesg | grep sd*


    $ fdisk –l

    These will return details of recently added disks.

  • To determine whether it is a StorSimple disk, use the following commands:

    cat /sys/block/<DISK>/device/model

    This will return a string, which will determine if it’s a StorSimple disk.

A less likely but possible cause could also be stale iscsid pid. Use the following command to log off from the iSCSI sessions:

iscsiadm -m node --logout -p <Target_IP>

Repeat this command for all the connected network interfaces on the iSCSI target, which is your StorSimple device. Once you have logged off from all the iSCSI sessions, use the iSCSI target IQN to reestablish the iSCSI session. Type the following command:

iscsiadm -m node --login -T <TARGET_IQN>

Q. I am not sure if my device is whitelisted.

A. To verify whether your device is whitelisted, use the following troubleshooting interactive command:

multipathd –k
multipathd> show devices
available block devices:
ram0 devnode blacklisted, unmonitored
ram1 devnode blacklisted, unmonitored
ram2 devnode blacklisted, unmonitored
ram3 devnode blacklisted, unmonitored
ram4 devnode blacklisted, unmonitored
ram5 devnode blacklisted, unmonitored
ram6 devnode blacklisted, unmonitored
ram7 devnode blacklisted, unmonitored
ram8 devnode blacklisted, unmonitored
ram9 devnode blacklisted, unmonitored
ram10 devnode blacklisted, unmonitored
ram11 devnode blacklisted, unmonitored
ram12 devnode blacklisted, unmonitored
ram13 devnode blacklisted, unmonitored
ram14 devnode blacklisted, unmonitored
ram15 devnode blacklisted, unmonitored
loop0 devnode blacklisted, unmonitored
loop1 devnode blacklisted, unmonitored
loop2 devnode blacklisted, unmonitored
loop3 devnode blacklisted, unmonitored
loop4 devnode blacklisted, unmonitored
loop5 devnode blacklisted, unmonitored
loop6 devnode blacklisted, unmonitored
loop7 devnode blacklisted, unmonitored
sr0 devnode blacklisted, unmonitored
sda devnode whitelisted, monitored
dm-0 devnode blacklisted, unmonitored
dm-1 devnode blacklisted, unmonitored
dm-2 devnode blacklisted, unmonitored
sdb devnode whitelisted, monitored
sdc devnode whitelisted, monitored
dm-3 devnode blacklisted, unmonitored

For more information, go to use troubleshooting interactive command for multipathing.

List of useful commands

Type Command Description
iSCSI service iscsid start Start iSCSI service
  service iscsid stop Stop iSCSI service
  service iscsid restart Restart iSCSI service
  iscsiadm -m discovery -t sendtargets -p <TARGET_IP> Discover available targets on the specified address
  iscsiadm -m node --login -T <TARGET_IQN> Log in to the iSCSI target
  iscsiadm -m node --logout -p <Target_IP> Log out from the iSCSI target
  cat /etc/iscsi/initiatorname.iscsi Print iSCSI initiator name
  iscsiadm –m session –s <sessionid> -P 3 Check the state of the iSCSI session and volume discovered on the host
  iscsi –m session Shows all the iSCSI sessions established between the host and the StorSimple device
Multipathing service multipathd start Start multipath daemon
  service multipathd stop Stop multipath daemon
  service multipathd restart Restart multipath daemon
  chkconfig multipathd on
mpathconf –with_chkconfig y
Enable multipath daemon to start at boot time
  multipathd –k Start the interactive console for troubleshooting
  multipath –l List multipath connections and devices
  mpathconf --enable Create a sample mulitpath.conf file in /etc/mulitpath.conf

Next steps

As you are configuring MPIO on Linux host, you may also need to refer to the following CentoS 6.6 documents: