FAQ

Frequently asked questions

See also Troubleshooting.

Why do device names change after a hotplug or a reboot?

Question

A controller is hot-unplugged or the host reboots. Why does a block device such as /dev/nvme3n1 still appear in nvme list after the controller nvme3 is gone? Why do the controller numbers in nvme list and nvme list-subsys change?

Answer

This is the expected behavior of the Linux NVMe driver.

1. Controller numbers are not persistent. The kernel does not keep the instance numbers of controllers (nvme0, nvme1, ...) or the names of block devices (/dev/nvmeXnY) across a reboot or a hotplug (PCIe hotplug, DLPAR). A controller that comes back gets the next free number. For example, nvme3 becomes nvme4.

2. Multipath namespaces belong to the subsystem, not to a controller. With native NVMe multipath, the block devices in nvme list (/dev/nvmeSnN) and the generic character devices (/dev/ngSnN) are multipath namespace heads. Their number is the number of the subsystem (nvme-subsysS), not of a controller:

   • /dev/nvme3n1 belongs to nvme-subsys3.
   • /dev/nvme2n1 belongs to nvme-subsys2.

   A subsystem can have several controllers (paths). If one controller goes away, for example nvme3, the other controllers stay, for example nvme1. While at least one path to nvme-subsys3 is live, the subsystem and its namespace heads stay, and their device nodes do not disappear.

Best practices

• Do not use the names /dev/nvmeX or /dev/nvmeXnY in scripts, automation or tests.
• Run nvme list -v or nvme list-subsys -v to see the subsystems, the controllers and the namespaces together.
• Use persistent names to find and mount devices:
  • the links in /dev/disk/by-id/, for example nvme-eui.* and nvme-nvme.*
  • the subsystem NQN (nvme-subsysX - NQN=...)