Common tasks and procedures¶
This section shows examples of common configuration procedures and tasks. Before moving on with this section it’s a good idea to read or be familiar with the topics in these pages:
For consistency, we will refer to device mapper multipathing as multipath.
Resizing online multipath devices¶
To resize online multipath devices, first find all the paths to the logical unit number (LUN) that is to be resized by running the following command:
$ 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, reconfigure mpathb
(with wwid = 360014056eee8ec6e1164fcb959086482
) to have 2 GB instead of just 1 GB and check if it 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.
Move root file system from a single path device to a multipath device¶
This is greatly simplified by the use of UUIDs to identify devices with an intrinsic label. To do this, install multipath-tools-boot
and reboot your system. This will rebuild the initial RAM disk and afford multipath the opportunity to build its paths before the root filesystem is mounted by UUID.
Note: Whenever
multipath.conf
is updated, initrd should be updated as well by running:update-initramfs -u -k all
The reason for this is thatmultipath.conf
is copied to the RAM disk, and is integral to determining the available devices to map via itsdenylist
anddevices
sections.
The multipathd daemon¶
If you have trouble implementing a multipath configuration, you should ensure the multipath daemon is running as described in the example configuration page. 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 the multipath command¶
You can use the -l
and -ll
options of the multipath
command to display the current multipath configuration.
-l
: Displays multipath topology gathered from information insysfs
and the device mapper-ll
: 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:
-v0
: Yields no output-v1
: Outputs only the created or updated multipath names, which you can then feed to other tools such askpartx
-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 ofmultipath.conf
The following example shows the output of a sudo multipath -l
command:
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¶
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. Running this command brings up an interactive multipath console where you can enter help
to get a list of available commands, you can enter an interactive command, or you can enter Ctrl+D to quit.
The multipathd
interactive console can be used to troubleshoot problems 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