I'd like to post this here for posterity's sake. This seems to be a common point of confusion, and I occasionally see folks using the dm-n
devices, which they should not be doing.
When new devices are brought under the control of DM-Multipath, the new devices may be seen in three different places under the /dev directory: /dev/mapper/mpathn, /dev/mpath/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.
- The devices in /dev/mpath are provided as a convenience so that all multipathed devices can be seen in one directory. These devices are created by the udev device manager and may not be available on startup when the system needs to access them. Do not use these devices for creating logical volumes or filesystems.
- Any devices of the form /dev/dm-n are for internal use only and should never be used.
[ some formatting changes made by me. ]