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Chapter 1. Overview of device mapper multipathing

DM Multipath provides:

Redundancy: DM Multipath can provide failover in an active/passive configuration. In an active/passive configuration, only a subset of the paths is used at any time for I/O. If any element of an I/O path such as the cable, switch, or controller fails, DM Multipath switches to an alternate path.

Note

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

Improved Performance: DM Multipath can be configured in an active/active mode, where I/O is spread over the paths in a round-robin fashion. In some configurations, DM Multipath can detect loading on the I/O paths and dynamically rebalance the load.

1.1. Active/Passive multipath configuration with one RAID device

In this configuration, there are two Host Bus Adapters (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. Here, there is one I/O path that goes through hba1, SAN1, and cntrlr1 and a second I/O path that goes through hba2, SAN2, and cntrlr2.

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

1.2. Active/Passive multipath configuration with two RAID devices

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 of the points of the I/O path to either of the RAID devices causes DM Multipath to switch to the alternate I/O path for that device. 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

1.3. Active/Active multipath configuration with one RAID device

In this configuration, there are two HBAs on the server, two SAN switches, and two 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

1.4. DM Multipath components

The following table describes the DM Multipath components.

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. The multipath command

The multipath command is used to detect and combine multiple paths to devices. It provides a variety of options you can use to administer your multipathed devices.

The following table describes some options of the multipath command that you may find useful.

Table 1.2. Useful multipath command options
Option	Description
`-l`	Display the current multipath topology gathered from `sysfs` and the device mapper.
`-ll`	Display the current multipath topology gathered from `sysfs`, the device mapper, and all other available components on the system.
`-f device`	Remove the named multipath device.
`-F`	Remove all unused multipath devices.
`-w device`	Remove the `wwid` of the specified device from the `wwids` file.
`-W`	Reset the `wwids` file to include only the current multipath devices.
`-r`	Force reload of the multipath device.

1.6. Displaying multipath topology

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

Procedure

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

1.7. Path status

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.

1.8. Additional resources

multipath(8) and multipathd(8) man pages
/etc/multipath.conf file

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Chapter 1. Overview of device mapper multipathing

1.1. Active/Passive multipath configuration with one RAID device

1.2. Active/Passive multipath configuration with two RAID devices

1.3. Active/Active multipath configuration with one RAID device

1.4. DM Multipath components

1.5. The multipath command

1.6. Displaying multipath topology

1.7. Path status

1.8. Additional resources

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