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In /proc/mounts, my system used to show /dev/mapper/sda2_crypt as /. After used for some days, including kernel and software upgrading, the line changed to /dev/dm-0 as /. What happened? I booted into the old kernel and it was still /dev/dm-0.

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It's normal when you are using a multipath device. 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. 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. 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. When the user_friendly_names configuration option is set to yes in /etc/multipath.conf, 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.

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.

For more information see this The DM-Multipath Configuration File

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  • Sorry, I am still confused. I do not have multipath related packages installed. And I do not have /etc/multipath.conf, either. I also blacklisted dm_multipath.ko. So why the internal mount point changed?
    – Magicloud
    Dec 15, 2015 at 14:03
  • Another point is that, as you said sda1_crypt is created early, is it earlier than dm-0? I see sda1_crypt is just a link to dm-0.
    – Magicloud
    Dec 15, 2015 at 14:04
  • What version of Ubuntu do you have?
    – Tung Tran
    Dec 15, 2015 at 14:10
  • It is Trusty. Everything updated to latest and the new kernel is 4.3.2, old one is 4.3.0.
    – Magicloud
    Dec 15, 2015 at 15:12
  • I may have found the reason. Some changes in 4.3 on BTRFS made the difference. I mean by my testing, only BTRFS + 4.3 kernel changed from /dev/mapper/something to /dev/dm-x.
    – Magicloud
    Dec 20, 2015 at 9:16

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