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Chapter 1. Overview of device mapper multipathing

Device Mapper (DM) Multipath provides redundancy through failover in active/passive configurations and improved performance by spreading I/O across multiple paths.

DM Multipath provides:

Redundancy

DM Multipath supports failover in an active/passive setup, where only some of the available paths handle I/O at any given time. If any active path component, such as a cable, switch, or controller fails, DM Multipath automatically redirects I/O to an alternate path. For more information, see:

  • multipath(8) and multipathd(8) man pages on your system
  • /etc/multipath.conf file
Note

The number of paths is dependent on the setup. Usually, DM Multipath setups have 2, 4, or 8 paths to the storage, but other numbers are also possible.

Improved Performance
DM Multipath also supports an active/active configuration in which I/O is distributed across all available paths by using rotational allocation. Depending on the setup, it can monitor the load on each path and adjust the distribution dynamically to balance I/O more efficiently.

1.1. Active/Passive multipath configuration with one RAID device
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Active/passive configuration provides redundant I/O paths through dual Host Bus Adapters (HBAs) and Storage Area Network (SAN) switches. These paths connect to a single Redundant Array of Independent Disks (RAID) device, enabling automatic failover.

In this configuration, there are two HBAs on the server, two SAN switches, and two RAID controllers. Following are the possible failure in this configuration:

  • HBA failure
  • Fibre Channel cable failure
  • SAN switch failure
  • Array controller port failure

With DM Multipath configured, a failure at any of these points causes DM Multipath to switch to the alternate I/O path. The following image describes the configuration with two I/O paths from the server to a RAID device. One I/O path goes through hba1, SAN1, and cntrlr1. A second I/O path goes through hba2, SAN2, and cntrlr2.

Figure 1.1. Active/Passive multipath configuration with one RAID device

Active/Passive multipath configuration with one RAID device

1.2. Active/Passive multipath configuration with two RAID devices
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Active/passive configuration with dual RAID devices provides redundant paths to multiple storage arrays. This ensures failover protection across separate RAID systems with dedicated controllers.

In this configuration, there are two HBAs on the server, two SAN switches, and two RAID devices with two RAID controllers each. With DM Multipath configured, a failure at any point of the I/O path causes DM Multipath to switch to the alternate I/O path. The following image describes the configuration with two I/O paths to each RAID device. Here, there are two I/O paths to each RAID device.

Figure 1.2. Active/Passive multipath configuration with two RAID device

Active/Passive multipath configuration with two RAID device

1.3. Active/Active multipath configuration with one RAID device
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Active/active configuration enables simultaneous I/O across multiple paths to a Redundant Array of Independent Disks (RAID) device, distributing workload for improved performance while maintaining failover capability.

In this configuration, there are two of each of the following:

  • Host Bus Adapters (HBAs) on the server
  • Storage Area Network (SAN) switches
  • RAID controllers.

The following image describes the configuration with two I/O paths from the server to a storage device. Here, I/O can be spread among these two paths.

Figure 1.3. Active/Active multipath configuration with one RAID device

Active/Active multipath configuration with one RAID device

1.4. DM Multipath components
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Device Mapper (DM) Multipath consists of kernel modules, utilities, and daemons that work together to manage multiple I/O paths, including the multipathd daemon and multipath command tools.

The following table describes the DM Multipath components.

Expand
Table 1.1. Components of DM Multipath

Component

Description

dm_multipath kernel module

Reroutes I/O and supports failover for paths and path groups.

mpathconf utility

Configures and enables device mapper multipathing.

multipath command

Lists and configures the multipath devices. It is also executed by udev whenever a block device is added, to determine if the device should be part of a multipath device or not.

multipathd daemon

Automatically creates and removes multipath devices and monitors paths; as paths fail and come back, it may update the multipath device. Allows interactive changes to multipath devices. Reload the service if there are any changes to the /etc/multipath.conf file.

kpartx command

Creates device mapper devices for the partitions on a device. This command is automatically executed by udev when multipath devices are created to create partition devices on top of them. The kpartx command is provided in its own package, but the device-mapper-multipath package depends on it.

mpathpersist

Sets up SCSI-3 persistent reservations on multipath devices. This command works similarly to the way sg_persist works for SCSI devices that are not multipathed. But it handles setting persistent reservations on all paths of a multipath device. It coordinates with multipathd to ensure that the reservations are set up correctly on paths that are added later. To use this functionality, the reservation_key attribute must be defined in the /etc/multipath.conf file. Otherwise the multipathd daemon will not check for persistent reservations for newly discovered paths or reinstated paths.

1.5. Displaying multipath topology
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To effectively monitor paths, troubleshoot multipath issues, or check whether the multipath configurations are set correctly, you can display the multipath topology.

Procedure

  1. Display the multipath device topology: 

    # multipath -ll
     
    mpatha (3600d0230000000000e13954ed5f89300) dm-4 WINSYS,SF2372
size=233G features='1 queue_if_no_path' hwhandler='0' wp=rw
`-+- policy='service-time 0' prio=1 status=active
  `- 6:0:0:0 sdf 8:80 active ready running

    The output can be split into three parts. Each part displays information for the following group:

    • Multipath device information:

      • mpatha (3600d0230000000000e13954ed5f89300): alias (wwid if it’s different from the alias)
      • dm-4: dm device name
      • WINSYS,SF2372: vendor, product
      • size=233G: size
      • features='1 queue_if_no_path': features
      • hwhandler='0': hardware handler
      • wp=rw: write permissions

    • Path group information:

      • policy='service-time 0': scheduling policy
      • prio=1: path group priority
      • status=active: path group status

    • Path information:

      • 6:0:0:0: host:channel:id:lun
      • sdf: devnode
      • 8:80: major:minor numbers
      • active: dm status
      • ready: path status

      • running: online status

        For more information about the dm, path and online status, see Path status.

        Other multipath commands, which are used to list, create, or reload multipath devices, also display the device topology. However, some information might be unknown and shown as undef in the output. This is normal behavior. Use the multipath -ll command to view the correct state.

        Note

        In certain cases, such as creating a multipath device, the multipath topology displays a parameter, which represents if any action was taken. For example, the following command output shows the create: parameter to represent that a multipath device was created: 

        create: mpatha (3600d0230000000000e13954ed5f89300) undef WINSYS,SF2372
size=233G features='1 queue_if_no_path' hwhandler='0' wp=undef
`-+- policy='service-time 0' prio=1 status=undef
  `- 6:0:0:0 sdf 8:80 undef ready running

1.6. Path status
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Path status indicates the current state of individual storage paths, ranging from ready and functional to faulty or delayed, helping determine multipath device health and behavior.

The path status is updated periodically by the multipathd daemon based on the polling interval defined in the /etc/multipath.conf file. In terms of the kernel, the dm status is similar to the path status. The dm state will retain its current status until the path checker has completed.

Path status
ready, ghost
The path is up and ready for I/O.
faulty, shaky
The path is down.
i/o pending
The checker is actively checking this path, and the state will be updated shortly.
i/o timeout
The checker did not return success/failure before the timeout period. This is treated the same as faulty.
removed
The path has been removed from the system, and will shortly be removed from the multipath device. This is treated the same as faulty.
wild
multipathd was unable to run the path checker, because of an internal error or configuration issue. This is treated the same as faulty, except multipath will skip many actions on the path.
unchecked
The path checker has not run on this path, either because it has just been discovered, it does not have an assigned path checker, or the path checker encountered an error. This is treated the same as wild.
delayed
The path checker returns that the path is up, but multipath is delaying the reinstatement of the path because the path has recently failed multiple times and multipath has been configured to delay paths in this case. This is treated the same as faulty.
Dm status
Active
Maps to the ready and ghost path status.
Failed
Maps to all other path status, except i/o pending that does not have an equivalent dm state.
Online status
Running
The device is enabled.
Offline
The device has been disabled.
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