Additional information for users of multipath-tools from Debian.

What are these weird numbers in /dev/mapper?
============================================
Modern Fibre-Channel connected storage devices are identified by
World-Wide-Numbers. If multipath-tools detects one of these devices,
it uses that as the default identifier in /dev/mapper.
If you want multipath-tools to use easier-to-read names, set
  user_friendly_names = yes
in the 'defaults' section of /etc/multipath.conf.

Note: While widely used (including the Debian Installer, which depends on it),
user_friendly_names can lead to locking issues with large numbers of LUNs
resulting in some maps not having friendly_names [1].

Where did my FC-connected filesystem go?
========================================
If you were previously mounting a device connected to your system by
Fibre Channel and then installed multipath-tools, you need to change
the way you mount the device. The device must now be accessed using
the identifier by which device-mapper knows it.

For example if you have in /dev/mapper a file like this:
   brw-rw---- 1 root disk 254,  8 2009-01-05 14:35 /dev/mapper/36000393000007d3901000000fef00a2d

then you can mount the filesystem like this:
  mount /dev/mapper/36000393000007d3901000000fef00a2d /mnt
or this
  mount /dev/disk/by-id/scsi-36000393000007d3901000000fef00a2d /mnt
You should prefer the latter, as this will work whether or not
multipath-tools is installed.

Note that with multipath-tools installed you cannot use the device's node
in /dev, e.g.
  # mount /dev/sdc1 /mnt
  mount: /dev/sdc1 already mounted or /mnt busy
The device is 'busy' because it is part of a multipath map.
See the output of 'multipath -l' to confirm this.

Preventing multipath-tools mapping any devices
==============================================
You may wish to prevent multipath-tools mapping any of your storage devices.
To do this replace the contents of /etc/multipath.conf with:

  blacklist {
         devnode "*"
  }

Booting from multipathed devices
================================
To enable booting from multipathed devices install the multipath-tools-boot
package. Be sure to update your bootloader configuration (e.g.
/boot/grub/menu.lst) to point to the multipathed root device afterwards.

By default this will be

  /dev/disk/by-id/scsi-<wwid>-part<N>

If you set user_friendly_names = yes in /etc/multipath.conf, the path
to the device will be

  /dev/mapper/mpath<X>-part<N>

If you make changes in multipath.conf, remember to run update-initramfs so the
updated version of the configuration gets included in the initramfs.

 -- Guido Günther <agx@sigxcpu.org>  Tue, 20 Feb 2008 13:49:18 +0100

-- 
[1] https://bugzilla.redhat.com/show_bug.cgi?id=445268