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Information for community supported and non-endorsed distributions

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Applies to: ✔️ Linux VMs ✔️ Flexible scale sets

The Azure platform SLA applies to virtual machines running the Linux OS only when one of the endorsed distributions is used. For these endorsed distributions, pre-configured Linux images are provided in the Azure Marketplace.

All other, non-Azure Marketplace, distributions running on Azure have a number of prerequisites. This article can't be comprehensive, as every distribution is different. Even if you meet all the criteria below, you may need to significantly tweak your Linux system for it to run properly.

This article focuses on general guidance for running your Linux distribution on Azure.

General Linux Installation Notes

  1. The Hyper-V virtual hard disk (VHDX) format isn't supported in Azure, only fixed VHD. You can convert the disk to VHD format using Hyper-V Manager or the Convert-VHD cmdlet. If you're using VirtualBox, select Fixed size rather than the default (dynamically allocated) when creating the disk.

  2. Azure supports Gen1 (BIOS boot) & Gen2 (UEFI boot) Virtual machines.

  3. The vfat kernel module must be enabled in the kernel

  4. The maximum size allowed for the VHD is 1,023 GB.

  5. When installing the Linux system we recommend that you use standard partitions, rather than Logical Volume Manager (LVM) which is the default for many installations. Using standard partitions will avoid LVM name conflicts with cloned VMs, particularly if an OS disk is ever attached to another identical VM for troubleshooting. LVM or RAID may be used on data disks.

  6. Kernel support for mounting UDF file systems is necessary. At first boot on Azure the provisioning configuration is passed to the Linux VM by using UDF-formatted media that is attached to the guest. The Azure Linux agent must mount the UDF file system to read its configuration and provision the VM.

  7. Linux kernel versions earlier than 2.6.37 don't support NUMA on Hyper-V with larger VM sizes. This issue primarily impacts older distributions using the upstream Red Hat 2.6.32 kernel, and was fixed in Red Hat Enterprise Linux (RHEL) 6.6 (kernel-2.6.32-504). Systems running custom kernels older than 2.6.37, or RHEL-based kernels older than 2.6.32-504 must set the boot parameter numa=off on the kernel command line in grub.conf. For more information, see Red Hat KB 436883.

  8. Don't configure a swap partition on the OS disk. The Linux agent can be configured to create a swap file on the temporary resource disk, as described in the following steps.

  9. All VHDs on Azure must have a virtual size aligned to 1 MB (1024 × 1024 bytes). When converting from a raw disk to VHD you must ensure that the raw disk size is a multiple of 1 MB before conversion, as described in the following steps.

  10. Use the most up-to-date distribution version, packages, and software.

  11. Remove users and system accounts, public keys, sensitive data, unnecessary software and application.

Note

(Cloud-init >= 21.2 removes the udf requirement.) however without the udf module enabled the cdrom will not mount during provisioning preventing custom data from being applied. A workaround for this would be to apply custom data using user data however, unlike custom data user data is not encrypted. https://cloudinit.readthedocs.io/en/latest/topics/format.html

Installing kernel modules without Hyper-V

Azure runs on the Hyper-V hypervisor, so Linux requires certain kernel modules to run in Azure. If you have a VM that was created outside of Hyper-V, the Linux installers may not include the drivers for Hyper-V in the initial ramdisk (initrd or initramfs), unless the VM detects that it's running on a Hyper-V environment. When using a different virtualization system (such as VirtualBox, KVM, and so on) to prepare your Linux image, you may need to rebuild the initrd so that at least the hv_vmbus and hv_storvsc kernel modules are available on the initial ramdisk. This known issue is for systems based on the upstream Red Hat distribution, and possibly others.

The mechanism for rebuilding the initrd or initramfs image may vary depending on the distribution. Consult your distribution's documentation or support for the proper procedure. Here is one example for rebuilding the initrd by using the mkinitrd utility:

  1. Back up the existing initrd image:

    cd /boot
sudo cp initrd-`uname -r`.img  initrd-`uname -r`.img.bak

  2. Rebuild the initrd with the hv_vmbus and hv_storvsc kernel modules:

    sudo mkinitrd --preload=hv_storvsc --preload=hv_vmbus -v -f initrd-`uname -r`.img `uname -r`

Resizing VHDs

VHD images on Azure must have a virtual size aligned to 1 MB. Typically, VHDs created using Hyper-V are aligned correctly. If the VHD isn't aligned correctly, you may receive an error message similar to the following when you try to create an image from your VHD.

The VHD http:\//\<mystorageaccount>.blob.core.windows.net/vhds/MyLinuxVM.vhd has an unsupported virtual size of 21475270656 bytes. The size must be a whole number (in MBs).

In this case, resize the VM using either the Hyper-V Manager console or the Resize-VHD PowerShell cmdlet. If you aren't running in a Windows environment, we recommend using qemu-img to convert (if needed) and resize the VHD.

Note

There is a known bug in qemu-img versions >=2.2.1 that results in an improperly formatted VHD. The issue has been fixed in QEMU 2.6. We recommend using either qemu-img 2.2.0 or lower, or 2.6 or higher.

  1. Resizing the VHD directly using tools such as qemu-img or vbox-manage may result in an unbootable VHD. We recommend first converting the VHD to a RAW disk image. If the VM image was created as a RAW disk image (the default for some hypervisors such as KVM), then you may skip this step.

    qemu-img convert -f vpc -O raw MyLinuxVM.vhd MyLinuxVM.raw

  2. Calculate the required size of the disk image so that the virtual size is aligned to 1 MB. The following bash shell script uses qemu-img info to determine the virtual size of the disk image, and then calculates the size to the next 1 MB.

    rawdisk="MyLinuxVM.raw"
vhddisk="MyLinuxVM.vhd"

MB=$((1024*1024))
size=$(qemu-img info -f raw --output json "$rawdisk" | \
gawk 'match($0, /"virtual-size": ([0-9]+),/, val) {print val[1]}')

rounded_size=$(((($size+$MB-1)/$MB)*$MB))

echo "Rounded Size = $rounded_size"

  3. Resize the raw disk using $rounded_size as set above.

    qemu-img resize MyLinuxVM.raw $rounded_size

  4. Now, convert the RAW disk back to a fixed-size VHD.

    qemu-img convert -f raw -o subformat=fixed,force_size -O vpc MyLinuxVM.raw MyLinuxVM.vhd

    Or, with qemu versions before 2.6, remove the force_size option.

    qemu-img convert -f raw -o subformat=fixed -O vpc MyLinuxVM.raw MyLinuxVM.vhd

Linux Kernel Requirements

The Linux Integration Services (LIS) drivers for Hyper-V and Azure are contributed directly to the upstream Linux kernel. Many distributions that include a recent Linux kernel version (such as 3.x) have these drivers available already, or otherwise provide backported versions of these drivers with their kernels. These drivers are constantly being updated in the upstream kernel with new fixes and features, so when possible we recommend running an endorsed distribution that includes these fixes and updates.

If you're running a variant of Red Hat Enterprise Linux versions 6.0 to 6.3, then you'll need to install the latest LIS drivers for Hyper-V. Beginning with RHEL 6.4+ (and derivatives) the LIS drivers are already included with the kernel and so no additional installation packages are needed.

If a custom kernel is required, we recommend a recent kernel version (such as 3.8+). For distributions or vendors who maintain their own kernel, you'll need to regularly backport the LIS drivers from the upstream kernel to your custom kernel. Even if you're already running a relatively recent kernel version, we highly recommend keeping track of any upstream fixes in the LIS drivers and backport them as needed. The locations of the LIS driver source files are specified in the MAINTAINERS file in the Linux kernel source tree:

    F:    arch/x86/include/asm/mshyperv.h
    F:    arch/x86/include/uapi/asm/hyperv.h
    F:    arch/x86/kernel/cpu/mshyperv.c
    F:    drivers/hid/hid-hyperv.c
    F:    drivers/hv/
    F:    drivers/input/serio/hyperv-keyboard.c
    F:    drivers/net/hyperv/
    F:    drivers/scsi/storvsc_drv.c
    F:    drivers/video/fbdev/hyperv_fb.c
    F:    include/linux/hyperv.h
    F:    tools/hv/

The following patches must be included in the kernel. This list can't be complete for all distributions.

The Azure Linux Agent

The Azure Linux Agent waagent provisions a Linux virtual machine in Azure. You can get the latest version, file issues, or submit pull requests at the Linux Agent GitHub repo.

  • The Linux agent is released under the Apache 2.0 license. Many distributions already provide RPM or .deb packages for the agent, and these packages can easily be installed and updated.
  • The Azure Linux Agent requires Python v2.6+.
  • The agent also requires the python-pyasn1 module. Most distributions provide this module as a separate package to be installed.
  • In some cases, the Azure Linux Agent may not be compatible with NetworkManager. Many of the RPM/deb packages provided by distributions configure NetworkManager as a conflict to the waagent package. In these cases, it will uninstall NetworkManager when you install the Linux agent package.
  • The Azure Linux Agent must be at or above the minimum supported version.

Note

Make sure 'udf' and 'vfat' modules are enabled. Disabling the UDF module will cause a provisioning failure. Disabling the VFAT module will cause both provisioning and boot failures. Cloud-init >= 21.2 can provision VMs without requiring UDF if: 1) the VM was created using SSH public keys and not password and 2) no custom data was provided.

General Linux System Requirements

  1. Modify the kernel boot line in GRUB or GRUB2 to include the following parameters, so that all console messages are sent to the first serial port. These messages can assist Azure support with debugging any issues.

    GRUB_CMDLINE_LINUX="rootdelay=300 console=ttyS0 earlyprintk=ttyS0 net.ifnames=0"

    We also recommend removing the following parameters if they exist.

    rhgb quiet crashkernel=auto

    Graphical and quiet boot isn't useful in a cloud environment, where we want all logs sent to the serial port. The crashkernel option may be left configured if needed, but note that this parameter reduces the amount of available memory in the VM by at least 128 MB, which may be problematic for smaller VM sizes.

  2. After you are done editing /etc/default/grub, run the following command to rebuild the grub configuration:

    sudo grub2-mkconfig -o /boot/grub2/grub.cfg

  3. Add Hyper-V modules both initrd and initramfs instructions (Dracut).

  4. Rebuild initrd or initramfs.

    Initramfs

    cp /boot/initramfs-$(uname -r).img /boot/initramfs-[latest kernel version ].img.bak 
dracut -f -v /boot/initramfs-[latest kernel version ].img  [depending on the version of grub] 
grub-mkconfig -o /boot/grub/grub.cfg 
grub2-mkconfig -o /boot/grub2/grub.cfg

    Initrd

    mv /boot/[initrd kernel] /boot/[initrd kernel]-old 
mkinitrd /boot/initrd.img-[initrd kernel]-generic /boot/[initrd kernel]-generic-old 
update-initramfs -c -k [initrd kernel] 
update-grub

  5. Ensure that the SSH server is installed, and configured to start at boot time. This configuration is usually the default.

  6. Install the Azure Linux Agent. The Azure Linux Agent is required for provisioning a Linux image on Azure. Many distributions provide the agent as an RPM or .deb package (the package is typically called WALinuxAgent or walinuxagent). The agent can also be installed manually by following the steps in the Linux Agent Guide.

    Note

    Make sure 'udf' and 'vfat' modules are enable. Blocklisting or removing the udf module will cause a provisioning failure. Blocklisting or removing vfat module will cause both provisioning and boot failures. (_Cloud-init >= 21.2 removes the udf requirement. Please read top of document for more detail)

    Install the Azure Linux Agent, cloud-init and other necessary utilities by running the following command:

    Redhat/Centos

    sudo yum install -y [waagent] cloud-init cloud-utils-growpart gdisk hyperv-daemons

    Ubuntu/Debian

    sudo apt install walinuxagent cloud-init cloud-utils-growpart gdisk hyperv-daemons

    Suse

    sudo zypper install python-azure-agent cloud-init cloud-utils-growpart gdisk hyperv-daemons

    Then enable the agent and cloud-init on all distributions using:

    sudo systemctl enable waagent.service
sudo systemctl enable cloud-init.service

  7. Swap: Do not create swap space on the OS disk.

    The Azure Linux Agent or Cloud-init can be used to configure swap space using the local resource disk. This resource disk is attached to the VM after provisioning on Azure. The local resource disk is a temporary disk, and might be emptied when the VM is deprovisioned. The following blocks show how to configure this swap.

    Azure Linux Agent Modify the following parameters in /etc/waagent.conf

    ResourceDisk.Format=y
ResourceDisk.Filesystem=ext4
ResourceDisk.MountPoint=/mnt/resource
ResourceDisk.EnableSwap=y
ResourceDisk.SwapSizeMB=2048    ## NOTE: Set this to your desired size.

    Cloud-init Configure cloud-init to handle the provisioning:

    sed -i 's/Provisioning.Agent=auto/Provisioning.Agent=cloud-auto/g' /etc/waagent.conf
sed -i 's/ResourceDisk.Format=y/ResourceDisk.Format=n/g' /etc/waagent.conf
sed -i 's/ResourceDisk.EnableSwap=y/ResourceDisk.EnableSwap=n/g' /etc/waagent.conf

    Configure Cloud-init to create swap.

    To format and create swap you have 2 options either:

    1. Pass this in as a cloud-init config every time you create a VM through customdata. This is the recommended method.

    2. Use a cloud-init directive baked into the image that will do this every time the VM is created.

    Create cfg file to configure swap using Cloud-init:

    echo 'DefaultEnvironment="CLOUD_CFG=/etc/cloud/cloud.cfg.d/00-azure-swap.cfg"' >> /etc/systemd/system.conf
cat > /etc/cloud/cloud.cfg.d/00-azure-swap.cfg << EOF
#cloud-config
# Generated by Azure cloud image build
disk_setup:
  ephemeral0:
    table_type: mbr
    layout: [66, [33, 82]]
    overwrite: True
fs_setup:
  - device: ephemeral0.1
    filesystem: ext4
  - device: ephemeral0.2
    filesystem: swap
mounts:
  - ["ephemeral0.1", "/mnt"]
  - ["ephemeral0.2", "none", "swap", "sw,nofail,x-systemd.requires=cloud-init.service,x-systemd.device-timeout=2", "0", "0"]
EOF

  8. Configure cloud-init to handle the provisioning:

    1. Configure waagent for cloud-init: 
      sed -i 's/Provisioning.Agent=auto/Provisioning.Agent=cloud-init/g' /etc/waagent.conf
sed -i 's/ResourceDisk.Format=y/ResourceDisk.Format=n/g' /etc/waagent.conf
sed -i 's/ResourceDisk.EnableSwap=y/ResourceDisk.EnableSwap=n/g' /etc/waagent.conf
      If you are migrating a specific virtual machine and do not wish to create a generalized image, set Provisioning.Agent=disabled in the /etc/waagent.conf config.
    2. Configure mounts: 
      echo "Adding mounts and disk_setup to init stage"
sed -i '/ - mounts/d' /etc/cloud/cloud.cfg
sed -i '/ - disk_setup/d' /etc/cloud/cloud.cfg
sed -i '/cloud_init_modules/a\\ - mounts' /etc/cloud/cloud.cfg
sed -i '/cloud_init_modules/a\\ - disk_setup' /etc/cloud/cloud.cfg
    3. Configure Azure datasource: 
      echo "Allow only Azure datasource, disable fetching network setting via IMDS"
cat > /etc/cloud/cloud.cfg.d/91-azure_datasource.cfg <<EOF
datasource_list: [ Azure ]
datasource:
   Azure:
     apply_network_config: False
EOF
    4. If configured, remove existing swapfile: 
      if [[ -f /mnt/resource/swapfile ]]; then
echo "Removing swapfile" #RHEL uses a swapfile by defaul
swapoff /mnt/resource/swapfile
rm /mnt/resource/swapfile -f
fi

  9. Configure cloud-init logging:

    echo "Add console log file"
cat >> /etc/cloud/cloud.cfg.d/05_logging.cfg <<EOF

# This tells cloud-init to redirect its stdout and stderr to
# 'tee -a /var/log/cloud-init-output.log' so the user can see output
# there without needing to look on the console.
output: {all: '| tee -a /var/log/cloud-init-output.log'}
EOF

  10. Deprovision.

    Caution

    If you are migrating a specific virtual machine and do not wish to create a generalized image, skip the deprovision step. Running the command waagent -force -deprovision+user will render the source machine unusable, this step is intended only to create a generalized image.

    Run the following commands to deprovision the virtual machine.

    # sudo rm -f /var/log/waagent.log
# sudo cloud-init clean
# waagent -force -deprovision+user
# rm -f ~/.bash_history
# export HISTSIZE=0
# logout

    Note

    On Virtualbox you may see the following error after running waagent -force -deprovision that says [Errno 5] Input/output error. This error message is not critical and can be ignored.

  11. Shut down the virtual machine and upload the VHD to Azure.

Next Steps

Create a Linux VM from a custom disk with the Azure CLI.