Chapter 2. Multipath devices

Multipath devices use specific naming conventions that determine how they appear in the file system. They can be identified by their World Wide Identifier (WWID) or user-friendly names like mpathN, depending on your configuration preferences.

When new devices are under the control of DM Multipath, these devices are created in the /dev/mapper/ and /dev/ directory.

The following describes multipath device names:

For example, a node with two HBAs attached to a storage controller with two ports by means of 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.

In addition to the user_friendly_names and alias options, a multipath device also has other attributes. You can modify these attributes for a specific multipath device by creating an entry for that device in the multipaths section of the /etc/multipath.conf file. For more information, see:

Multipath devices integrate seamlessly with LVM, allowing you to create physical volumes, volume groups, and logical volumes by using multipath device names instead of individual disk paths.

After creating multipath devices, you can use the multipath device names as you would use a physical device name when creating an Logical volume manager (LVM) physical volume. For example, if /dev/mapper/mpatha is the name of a multipath device, the pvcreate /dev/mapper/mpatha command marks /dev/mapper/mpatha as a physical volume.

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.

To filter all the sd devices in the /etc/lvm/lvm.conf file, add the filter = [ "r/block/", "r/disk/", "r/sd./", "a/./" ] filter in the devices section of the file. For more information see the lvm.conf man page on your system.

When you create an LVM logical volume that uses active/passive multipath arrays as the underlying physical devices, you can optionally include filters in the /etc/lvm/lvm.conf file to exclude the disks that underline 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.

The kernel changes the active/passive state by automatically detecting the correct hardware handler to use. For active/passive paths that require intervention to change their state, multipath automatically uses this hardware handler to do so as necessary. If the kernel does not automatically detect the correct hardware handler to use, you can configure which hardware handler to use in the multipath.conf file with the "hardware_handler" option. For active/passive arrays that require a command to make the passive path active, LVM prints a warning message when this occurs.

Depending on your configuration, LVM may print any of the following messages:

The following are the reasons for the mentioned errors: