Multipath Device Identifiers

Each multipath device has a World Wide Identifier (WWID), which is guaranteed to be globally unique and unchanging. By default, the name of a multipath device is set to its WWID. For example, a node with two HBAs attached to a storage controller with two ports via a single unzoned FC switch sees four devices: /dev/sda, /dev/sdb, /dev/sdc, and /dev/sdd. DM-Multipath creates a single device with a unique WWID that reroutes I/O to those four underlying devices according to the multipath configuration

# multipath -ll
360080e50001818f80000138754ee12d6 dm-6 IBM,1814      FAStT
size=500G features='1 queue_if_no_path' hwhandler='1 rdac' wp=rw
|-+- policy='round-robin 0' prio=14 status=active
| `- 1:0:0:18 sdk  8:160  active ready running
`-+- policy='round-robin 0' prio=9 status=enabled
  `- 1:0:1:18 sdy  65:128 active ready running

Alternately, you can set the user_friendly_names option in the multipath configuration file, which causes DM-Multipath to use a node-unique alias of the form mpathn as the name. The name of the multipath device is set to mpathn. When new devices are brought under the control of DM-Multipath, the new devices may be seen in two different places under the /dev directory: /dev/mapper/mpathn and /dev/dm-n.

  • The devices in /dev/mapper are created early in the boot process. Use these devices to access the multipathed devices, for example when creating logical volumes
  • Any devices of the form /dev/dm-n are for internal use only and should never be used.
For information on the multipath configuration defaults, including the user_friendly_names configuration option, see Section , Configuration File Defaults.
Also, you can also set the name of a multipath device to a name of your choosing by using the alias option in the multipaths section of the multipath configuration file.

Multipath Devices in Logical Volumes

After creating multipath devices, you can use the multipath device names just as you would use a physical device name when creating an LVM physical volume.

Creating pv using multipath device

For example, if /dev/mapper/mpatha is the name of a multipath device, the following command will mark /dev/mapper/mpatha as a physical volume.

# pvcreate /dev/mapper/lun1
You can use the resulting LVM physical device when you create an LVM volume group just as you would use any other LVM physical device.

If you attempt to create an LVM physical volume on a whole device on which you have configured partitions, the pvcreate command will fail.

When you create an LVM logical volume that uses active/passive multipath arrays as the underlying physical devices, you should include filters in the lvm.conf to exclude the disks that underlie the multipath devices. This is because if the array automatically changes the active path to the passive path when it receives I/O, multipath will failover and failback whenever LVM scans the passive path if these devices are not filtered. For active/passive arrays that require a command to make the passive path active, LVM prints a warning message when this occurs.

Filter all physical devices underlying multipath devices

To filter all SCSI devices in the LVM configuration file (lvm.conf), include the following filter in the devices section of the file.

filter = [ "r/block/", "r/disk/", "r/sd.*/", "a/.*/" ]
After updating /etc/lvm.conf, it's necessary to update the initrd so that this file will be copied there, where the filter matters the most, during boot. Perform:
update-initramfs -u -k all

Every time either /etc/lvm.conf or /etc/multipath.conf is updated, the initrd should be rebuilt to reflect these changes. This is imperative when blacklists and filters are necessary to maintain a stable storage configuration.