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Device Mapper Multipathing - Usage & Debug

Device Mapper Multipath will be referred here as multipath only.

Before moving on with this session it is recommended that you read:
1. Device Mapper Multipathing - Introduction
2. Device Mapper Multipathing - Configuration
3. Device Mapper Multipathing - Setup

This section provides step-by-step example procedures for configuring multipath.

It includes the following procedures:

  • Resizing Online Multipath Devices

  • Moving root File System from a Single Path Device to a Multipath Device

  • The Multipath Daemon

  • Issues with queue_if_no_path

  • Multipath Command Output

  • Multipath **Queries **with multipath Command

  • Determining Device Mapper Entries with dmsetup Command

  • Troubleshooting with the multipathd interactive console

Resizing Online Multipath Devices

First find all the paths to the LUN about to be resized:

$ sudo multipath -ll
mpathb (360014056eee8ec6e1164fcb959086482) dm-0 LIO-ORG,lun01
size=1.0G features='0' hwhandler='1 alua' wp=rw
|-+- policy='service-time 0' prio=50 status=active
| `- 7:0:0:1 sde 8:64 active ready running
`-+- policy='service-time 0' prio=50 status=enabled
  `- 8:0:0:1 sdf 8:80 active ready running
mpatha (36001405e3c2841430ee4bf3871b1998b) dm-1 LIO-ORG,lun02
size=1.0G features='0' hwhandler='1 alua' wp=rw
|-+- policy='service-time 0' prio=50 status=active
| `- 7:0:0:2 sdc 8:32 active ready running
`-+- policy='service-time 0' prio=50 status=enabled
  `- 8:0:0:2 sdd 8:48 active ready running

Now I’ll reconfigure mpathb (with wwid = 360014056eee8ec6e1164fcb959086482) to have 2GB instead of just 1Gb and check if has changed:

$ echo 1 | sudo tee /sys/block/sde/device/rescan
1
$ echo 1 | sudo tee /sys/block/sdf/device/rescan
1
$ sudo multipath -ll
mpathb (360014056eee8ec6e1164fcb959086482) dm-0 LIO-ORG,lun01
size=1.0G features='0' hwhandler='1 alua' wp=rw
|-+- policy='service-time 0' prio=50 status=active
| `- 7:0:0:1 sde 8:64 active ready running
`-+- policy='service-time 0' prio=50 status=enabled
  `- 8:0:0:1 sdf 8:80 active ready running
mpatha (36001405e3c2841430ee4bf3871b1998b) dm-1 LIO-ORG,lun02
size=1.0G features='0' hwhandler='1 alua' wp=rw
|-+- policy='service-time 0' prio=50 status=active
| `- 7:0:0:2 sdc 8:32 active ready running
`-+- policy='service-time 0' prio=50 status=enabled
  `- 8:0:0:2 sdd 8:48 active ready running

Not yet! We still need to re-scan the multipath map:

$ sudo multipathd resize map mpathb
ok

And then we are good:

$ sudo multipath -ll
mpathb (360014056eee8ec6e1164fcb959086482) dm-0 LIO-ORG,lun01
size=2.0G features='0' hwhandler='1 alua' wp=rw
|-+- policy='service-time 0' prio=50 status=active
| `- 7:0:0:1 sde 8:64 active ready running
.`-+- policy='service-time 0' prio=50 status=enabled
  `- 8:0:0:1 sdf 8:80 active ready running
mpatha (36001405e3c2841430ee4bf3871b1998b) dm-1 LIO-ORG,lun02
size=1.0G features='0' hwhandler='1 alua' wp=rw
|-+- policy='service-time 0' prio=50 status=active
| `- 7:0:0:2 sdc 8:32 active ready running
`-+- policy='service-time 0' prio=50 status=enabled
  `- 8:0:0:2 sdd 8:48 active ready running

Make sure to run resize2fs /dev/mapper/mpathb to resize the filesystem.

Moving root File System from a Single Path Device to a Multipath Device

This is dramatically simplified by the use of UUIDs to identify devices as an intrinsic label. Simply install multipath-tools-boot and reboot. This will rebuild the initial ramdisk and afford multipath the opportunity to build it’s paths before the root filesystem is mounted by UUID.

Note
Whenever multipath.conf is updated, so should the initrd by executing:
update-initramfs -u -k all
The reason behind is multipath.conf is copied to the ramdisk and is integral to determining the available devices to map via it’s denylist and devices sections.

The Multipath Daemon

If you find you have trouble implementing a multipath configuration, you should ensure the multipath daemon is running as described in Device Mapper Multipathing - Setup. The multipathd daemon must be running in order to use** multipath** devices.

Multipath Command Output

When you create, modify, or list a multipath device, you get a printout of the current device setup. The format is as follows. For each multipath device:

action_if_any: alias (wwid_if_different_from_alias) dm_device_name_if_known vendor,product
   size=size features='features' hwhandler='hardware_handler' wp=write_permission_if_known

For each path group:

  -+- policy='scheduling_policy' prio=prio_if_known
  status=path_group_status_if_known

For each path:

   `- host:channel:id:lun devnode major:minor dm_status_if_known path_status
  online_status

For example, the output of a multipath command might appear as follows:

mpathb (360014056eee8ec6e1164fcb959086482) dm-0 LIO-ORG,lun01
size=2.0G features='0' hwhandler='1 alua' wp=rw
|-+- policy='service-time 0' prio=50 status=active
| `- 7:0:0:1 sde 8:64 active ready running
`-+- policy='service-time 0' prio=50 status=enabled
  `- 8:0:0:1 sdf 8:80 active ready running

If the path is up and ready for I/O, the status of the path is ready or ghost. If the path is down, the status is faulty or shaky. The path status is updated periodically by the multipathd daemon based on the polling interval defined in the /etc/multipath.conf file.

The dm_status is similar to the path status, but from the kernel’s point of view. The dm_status has two states: failed, which is analogous to faulty, and active, which covers all other path states. Occasionally, the path state and the dm state of a device will temporary not agree.

The possible values for online_status are running and offline. A status of offline means that the SCSI device has been disabled.

Multipath **Queries **with multipath Command

You can use the -l and -ll options of the multipath command to display the current multipath configuration. The -l option displays multipath topology gathered from information in sysfs and the device mapper. The -ll option displays the information the -l displays in addition to all other available components of the system.

When displaying the multipath configuration, there are three verbosity levels you can specify with the -v option of the multipath command. Specifying -v0 yields no output. Specifying**-v1** outputs the created or updated multipath names only, which you can then feed to other tools such as kpartx. Specifying -v2 prints all detected paths, multipaths, and device maps.

Note
The default verbosity level of multipath is 2 and can be globally modified by defining the verbosity attribute in the defaults section of multipath.conf.

The following example shows the output of a multipath -l command.

$ sudo multipath -l
mpathb (360014056eee8ec6e1164fcb959086482) dm-0 LIO-ORG,lun01
size=2.0G features='0' hwhandler='1 alua' wp=rw
|-+- policy='service-time 0' prio=0 status=active
| `- 7:0:0:1 sde 8:64 active undef running
`-+- policy='service-time 0' prio=0 status=enabled
  `- 8:0:0:1 sdf 8:80 active undef running
mpatha (36001405e3c2841430ee4bf3871b1998b) dm-1 LIO-ORG,lun02
size=1.0G features='0' hwhandler='1 alua' wp=rw
|-+- policy='service-time 0' prio=0 status=active
| `- 7:0:0:2 sdc 8:32 active undef running
`-+- policy='service-time 0' prio=0 status=enabled
  `- 8:0:0:2 sdd 8:48 active undef running

Determining Device Mapper Entries with dmsetup Command

You can use the dmsetup command to find out which device mapper entries match the multipathed devices. The following command displays all the device mapper devices and their major and minor numbers. The minor numbers determine the name of the dm device. For example, a minor number of 1 corresponds to the multipathd device /dev/dm-1.

$ sudo dmsetup ls
mpathb  (253:0)
mpatha  (253:1)

$ ls -lahd /dev/dm*
brw-rw---- 1 root disk 253, 0 Apr 27 14:49 /dev/dm-0
brw-rw---- 1 root disk 253, 1 Apr 27 14:47 /dev/dm-1

Troubleshooting with the multipathd interactive console

The multipathd -k command is an interactive interface to the multipathd daemon. Entering this command brings up an interactive multipath console. After entering this command, you can enter help to get a list of available commands, you can enter a interactive command, or you can enter CTRL-D to quit.

The multipathd interactive console can be used to troubleshoot problems you may be having with your system. For example, the following command sequence displays the multipath configuration, including the defaults, before exiting the console.

$ sudo multipathd -k
  > show config
  > CTRL-D

The following command sequence ensures that multipath has picked up any changes to the multipath.conf,

$ sudo multipathd -k
> reconfigure
> CTRL-D

Use the following command sequence to ensure that the path checker is working properly.

$ sudo multipathd -k
> show paths
> CTRL-D

Commands can also be streamed into multipathd using STDIN like so:

$ echo 'show config' | sudo multipathd -k

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