This document describes how to use persistent device naming on your Linux VM.

For VMs that use a Linux operating system, device names, for example/dev/sda,might change after you perform procedures such as the following:

• Starting and stopping a VM
• Detaching and reattaching disks
• Changing machine types

This device name change occurs because device names are assigned from anavailable range once a VM starts or a device is attached. Detaching a device orstopping the VM frees up the device name. When the device is reattached or theVM restarted a new device name is then assigned from the available range. TheLinux kernel does not guarantee device ordering across reboots.

A device name change might cause any applications or scripts that depend onthe original device name to not work properly or it might cause the VM to notboot after a restart.

It is recommended that you use persistent device naming when referencing disksand partitions on your Linux VMs to avoid this issue. You can also usesymlinks.

Before you begin

• Review device management for your Linux operating system:
  • Using UUIDs with Ubuntu
  • Red Hat Device Management
• If you haven't already, then set up authentication.Authentication is the process by which your identity is verified for access to Google Cloud services and APIs. To run code or samples from a local development environment, you can authenticate to Compute Engine by selecting one of the following options:
  

  Select the tab for how you plan to use the samples on this page:

  Console

  When you use the Google Cloud console to access Google Cloud services and APIs, you don't need to set up authentication.

  gcloud

  1. Afterinstalling the Google Cloud CLI,initialize it by running the following command:

     gcloudinit

     If you're using an external identity provider (IdP), you must first sign in to the gcloud CLI with your federated identity.

     Note: If you installed the gcloud CLI previously, make sure you have the latest version by runninggcloud components update.
  2. Set a default region and zone.

Device naming on Linux VMs

The Linux device names for the disks attached to your VM depend on the interfacethat you choose when creating the disks. When you use thelsblk operatingsystem command to view your disk devices, it displays the prefixnvme fordisks attached with the NVMe interface, and the prefixsd for disks attachedwith the SCSI interface.

The ordering of the disk numbers or NVMe controllers is not predictable orconsistent across VMs restarts. On the first boot, a persistent disk might benvme0n1 (orsda for SCSI). On the second boot, the device name for the samepersistent disk might benvme2n1 ornvme0n3 (orsdc for SCSI).

When accessing attached disks, you should use the symbolic links created in/dev/disk/by-id/ instead. These names persist across reboots.For more information about symlinks, seeSymbolic links for disks attached to a VM.

SCSI device names

The format of a SCSI-attached disk device issda for the first attached disk.The disk partitions appear assda1. Each additional diskuses a sequential letter, such assdb andsdc. Whensdz isreached, the next disks added have the names such assdaa,sdab, andsdac,up tosddx.

NVMe device names

The format of a NVMe-attached disk device in Linux operating systems isnvmenumbernnamespace. Thenumber represents the NVMe disk controller number andnamespace is an NVMe namespace ID that is assigned by the NVMe diskcontroller. For partitions, pn is appended to the device name,wheren is a number, starting with 1, that denotes then^th partition.

The controller number starts at0. A single NVMe disk attached to your computeinstance has a device name ofnvme0n1. Most machine types use a single NVMedisk controller. The NVMe device names are thennvme0n1,nvme0n2,nvme0n3,and so on.

Local SSD disks attached tothird generation machine seriesor later instances have a separate NVMe controller for each disk. On these VMs,the Local SSD NVMe-attached device names look likenvme0n1,nvme1n1, andnvme2n1. The number of attached Local SSD disks depends on the machine typeof your VM.

Compute instances based on third generation machine series or later use NVMe forboth Persistent Disk and Google Cloud Hyperdisk, and also Local SSD disks. Each VMhas 1 NVMe controller for Persistent Disk and Hyperdisk and 1 NVMecontroller for each Local SSD disk. The Persistent Disk and HyperdiskNVMe controller has a single NVMe namespace for all attached disks. So, a thirdgeneration machine series instance with one Persistent Disk and oneHyperdisk (each with 2 partitions), and 2 unformatted Local SSDdisks uses the following device names:

• nvme0n1 - Persistent Disk
• nvme0n1p1
• nvme0n1p2
• nvme0n2 - Hyperdisk
• nvme0n2p1
• nvme0n2p2
• nvme1n1 - first Local SSD
• nvme2n1 - second Local SSD

Use persistent device naming

To configure a persistent device name, you assign a mount point name for thedisk device in thefstab file. There are three ways to configure a persistentdevice name.

• By using alabel.This option requires that the file system supports labels and that you add alabel to the disk partitions.
• By using a partition or diskUUID.A UUID is generated when a disk is created with a partition table, and theUUID is unique per partition.
• By using a persistent disk ID (/dev/disk/by-id) for Persistent Disk orGoogle Cloud Hyperdisk, or asymlink, thatis based on the disk resource name.

We recommend using the partition UUID or the symlink for Linux VMs.