Add a disk to a Linux VM

This article shows you how to attach a persistent disk to your VM so that you can preserve your data - even if your VM is reprovisioned due to maintenance or resizing.

Use managed disks

Azure Managed Disks simplifies disk management for Azure VMs by managing the storage accounts associated with the VM disks. You only have to specify the type (Premium or Standard) and the size of disk you need, and Azure creates and manages the disk for you. For more information, see Managed Disks overview.

Attach a new disk to a VM

If you just need a new disk on your VM, use the az vm disk attach command with the --new parameter. If your VM is in an Availability Zone, the disk is automatically created in the same zone as the VM. For more information, see Overview of Availability Zones. The following example creates a disk named myDataDisk that is 50Gb in size:

az vm disk attach -g myResourceGroup --vm-name myVM --disk myDataDisk \
  --new --size-gb 50

Attach an existing disk

In many cases. you attach disks that have already been created. To attach an existing disk, find the disk ID and pass the ID to the az vm disk attach command. The following example queries for a disk named myDataDisk in myResourceGroup, then attaches it to the VM named myVM:

# find the disk id
diskId=$(az disk show -g myResourceGroup -n myDataDisk --query 'id' -o tsv)
az vm disk attach -g myResourceGroup --vm-name myVM --disk $diskId

The output looks something like the following (you can use the -o table option to any command to format the output in):

{
  "accountType": "Standard_LRS",
  "creationData": {
    "createOption": "Empty",
    "imageReference": null,
    "sourceResourceId": null,
    "sourceUri": null,
    "storageAccountId": null
  },
  "diskSizeGb": 50,
  "encryptionSettings": null,
  "id": "/subscriptions/<guid>/resourceGroups/rasquill-script/providers/Microsoft.Compute/disks/myDataDisk",
  "location": "westus",
  "name": "myDataDisk",
  "osType": null,
  "ownerId": null,
  "provisioningState": "Succeeded",
  "resourceGroup": "myResourceGroup",
  "tags": null,
  "timeCreated": "2017-02-02T23:35:47.708082+00:00",
  "type": "Microsoft.Compute/disks"
}

Use unmanaged disks

Unmanaged disks require additional overhead to create and manage the underlying storage accounts. Unmanaged disks are created in the same storage account as your OS disk. To create and attach an unmanaged disk, use the az vm unmanaged-disk attach command. The following example attaches a 50GB unmanaged disk to the VM named myVM in the resource group named myResourceGroup:

az vm unmanaged-disk attach -g myResourceGroup -n myUnmanagedDisk --vm-name myVM \
  --new --size-gb 50

Connect to the Linux VM to mount the new disk

To partition, format, and mount your new disk so your Linux VM can use it, SSH into your Azure VM. For more information, see How to use SSH with Linux on Azure. The following example connects to a VM with the public DNS entry of mypublicdns.westus.cloudapp.azure.com with the username azureuser:

ssh azureuser@mypublicdns.westus.cloudapp.azure.com

Once connected to your VM, you're ready to attach a disk. First, find the disk using dmesg (the method you use to discover your new disk may vary). The following example uses dmesg to filter on SCSI disks:

dmesg | grep SCSI

The output is similar to the following example:

[    0.294784] SCSI subsystem initialized
[    0.573458] Block layer SCSI generic (bsg) driver version 0.4 loaded (major 252)
[    7.110271] sd 2:0:0:0: [sda] Attached SCSI disk
[    8.079653] sd 3:0:1:0: [sdb] Attached SCSI disk
[ 1828.162306] sd 5:0:0:0: [sdc] Attached SCSI disk

Here, sdc is the disk that we want. Partition the disk with fdisk, make it a primary disk on partition 1, and accept the other defaults. The following example starts the fdisk process on /dev/sdc:

sudo fdisk /dev/sdc

The output is similar to the following example:

Device contains neither a valid DOS partition table, nor Sun, SGI or OSF disklabel
Building a new DOS disklabel with disk identifier 0x2a59b123.
Changes will remain in memory only, until you decide to write them.
After that, of course, the previous content won't be recoverable.

Warning: invalid flag 0x0000 of partition table 4 will be corrected by w(rite)

Command (m for help): n
Partition type:
   p   primary (0 primary, 0 extended, 4 free)
   e   extended
Select (default p): p
Partition number (1-4, default 1): 1
First sector (2048-10485759, default 2048):
Using default value 2048
Last sector, +sectors or +size{K,M,G} (2048-10485759, default 10485759):
Using default value 10485759

Create the partition by typing p at the prompt as follows:

Command (m for help): p

Disk /dev/sdc: 5368 MB, 5368709120 bytes
255 heads, 63 sectors/track, 652 cylinders, total 10485760 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x2a59b123

   Device Boot      Start         End      Blocks   Id  System
/dev/sdc1            2048    10485759     5241856   83  Linux

Command (m for help): w
The partition table has been altered!

Calling ioctl() to re-read partition table.
Syncing disks.

Now, write a file system to the partition with the mkfs command. Specify your filesystem type and the device name. The following example creates an ext4 filesystem on the /dev/sdc1 partition that was created in the preceding steps:

sudo mkfs -t ext4 /dev/sdc1

The output is similar to the following example:

mke2fs 1.42.9 (4-Feb-2014)
Discarding device blocks: done
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
327680 inodes, 1310464 blocks
65523 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=1342177280
40 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks:
    32768, 98304, 163840, 229376, 294912, 819200, 884736
Allocating group tables: done
Writing inode tables: done
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done

Now, create a directory to mount the file system using mkdir. The following example creates a directory at /datadrive:

sudo mkdir /datadrive

Use mount to then mount the filesystem. The following example mounts the /dev/sdc1 partition to the /datadrive mount point:

sudo mount /dev/sdc1 /datadrive

To ensure that the drive is remounted automatically after a reboot, it must be added to the /etc/fstab file. It is also highly recommended that the UUID (Universally Unique IDentifier) is used in /etc/fstab to refer to the drive rather than just the device name (such as, /dev/sdc1). If the OS detects a disk error during boot, using the UUID avoids the incorrect disk being mounted to a given location. Remaining data disks would then be assigned those same device IDs. To find the UUID of the new drive, use the blkid utility:

sudo -i blkid

The output looks similar to the following example:

/dev/sda1: UUID="11111111-1b1b-1c1c-1d1d-1e1e1e1e1e1e" TYPE="ext4"
/dev/sdb1: UUID="22222222-2b2b-2c2c-2d2d-2e2e2e2e2e2e" TYPE="ext4"
/dev/sdc1: UUID="33333333-3b3b-3c3c-3d3d-3e3e3e3e3e3e" TYPE="ext4"
Note

Improperly editing the /etc/fstab file could result in an unbootable system. If unsure, refer to the distribution's documentation for information on how to properly edit this file. It is also recommended that a backup of the /etc/fstab file is created before editing.

Next, open the /etc/fstab file in a text editor as follows:

sudo vi /etc/fstab

In this example, we use the UUID value for the /dev/sdc1 device that was created in the previous steps, and the mountpoint of /datadrive. Add the following line to the end of the /etc/fstab file:

UUID=33333333-3b3b-3c3c-3d3d-3e3e3e3e3e3e   /datadrive   ext4   defaults,nofail   1   2
Note

Later removing a data disk without editing fstab could cause the VM to fail to boot. Most distributions provide either the nofail and/or nobootwait fstab options. These options allow a system to boot even if the disk fails to mount at boot time. Consult your distribution's documentation for more information on these parameters.

The nofail option ensures that the VM starts even if the filesystem is corrupt or the disk does not exist at boot time. Without this option, you may encounter behavior as described in Cannot SSH to Linux VM due to FSTAB errors

TRIM/UNMAP support for Linux in Azure

Some Linux kernels support TRIM/UNMAP operations to discard unused blocks on the disk. This feature is primarily useful in standard storage to inform Azure that deleted pages are no longer valid and can be discarded, and can save money if you create large files and then delete them.

There are two ways to enable TRIM support in your Linux VM. As usual, consult your distribution for the recommended approach:

  • Use the discard mount option in /etc/fstab, for example:

    UUID=33333333-3b3b-3c3c-3d3d-3e3e3e3e3e3e   /datadrive   ext4   defaults,discard   1   2
    
  • In some cases, the discard option may have performance implications. Alternatively, you can run the fstrim command manually from the command line, or add it to your crontab to run regularly:

    Ubuntu

    sudo apt-get install util-linux
    sudo fstrim /datadrive
    

    RHEL/CentOS

    sudo yum install util-linux
    sudo fstrim /datadrive
    

Troubleshooting

When adding data disks to a Linux VM, you may encounter errors if a disk does not exist at LUN 0. If you are adding a disk manually using the azure vm disk attach-new command and you specify a LUN (--lun) rather than allowing the Azure platform to determine the appropriate LUN, take care that a disk already exists / will exist at LUN 0.

Consider the following example showing a snippet of the output from lsscsi:

[5:0:0:0]    disk    Msft     Virtual Disk     1.0   /dev/sdc 
[5:0:0:1]    disk    Msft     Virtual Disk     1.0   /dev/sdd 

The two data disks exist at LUN 0 and LUN 1 (the first column in the lsscsi output details [host:channel:target:lun]). Both disks should be accessbile from within the VM. If you had manually specified the first disk to be added at LUN 1 and the second disk at LUN 2, you may not see the disks correctly from within your VM.

Note

The Azure host value is 5 in these examples, but this may vary depending on the type of storage you select.

This disk behavior is not an Azure problem, but the way in which the Linux kernel follows the SCSI specifications. When the Linux kernel scans the SCSI bus for attached devices, a device must be found at LUN 0 in order for the system to continue scanning for additional devices. As such:

  • Review the output of lsscsi after adding a data disk to verify that you have a disk at LUN 0.
  • If your disk does not show up correctly within your VM, verify a disk exists at LUN 0.

Next steps

  • Remember, that your new disk is not available to the VM if it reboots unless you write that information to your fstab file.
  • To ensure your Linux VM is configured correctly, review the Optimize your Linux machine performance recommendations.
  • Expand your storage capacity by adding additional disks and configure RAID for additional performance.