Recently, I spent some time creating a repeatable method of generating a customized Linux disk image. This disk image is then consumed within an Azure VM Scale Set, (and consequently) in an Azure Devops (ADO) pipeline.

On this disk image we pre-cache a large (20GB) file and repeatedly copy that large files into “work” directories. Copying large files in the default ext4 filesystem is unnecessarily slow, so I decided I to try and utilize a copy-on-write filesystem like btrfs to speed up that operation. I found that it was best to leave the OS disk to something default on the marketplace, and create a separate data disk which I will format and mount in.

Packer allows you to specify additional data disks, but that still leaves you to configure the block device from scratch. On the Microsoft Docs the steps to Attach a data disk to Linux VM are laid out. Below the provisioner JSON block programatically detects our block device (which we created as a 128GB disk - you’ll need to update accordingly for your disk size), formats it to our preferred filesystem btrfs, and updates the /etc/fstab accordingly so it will be mounted again in the future.

{
  "execute_command": "chmod +x ;  sudo -E sh ''",
  "inline": [
    // Print all the block "sd*" block devices (for visual debugging if necessary)
    "lsblk -o NAME,HCTL,SIZE,MOUNTPOINT | grep -i \"sd\"",
    // Use the size of your DATA disk, specified in your packer's `disk_additional_size`, as a key for this grep.
    // Ensure only one block device is expected to be this size!
    "DEVICE=$(lsblk -o NAME,HCTL,SIZE,MOUNTPOINT | grep -i \"sd\" | grep \"128\" | cut -d' ' -f1)",
    "echo \"DEVICE FOUND = $DEVICE\"",
    // Ensure the btrfs module is loaded
    "modprobe btrfs",
    // Make filesystem over the entire block device (no need to partition, like with other filesystems)
    "mkfs.btrfs /dev/$DEVICE -f",
    // Make a directory to mount into. I call it butter
    "mkdir /butter",
    // Mount your directory
    "mount /dev/$DEVICE /butter",
    // Do this so that blkid will pick up the right UUID
    "partprobe /dev/$DEVICE",
    "blkid",
    "UUID=$(blkid | grep \"$DEVICE\" | cut -d' ' -f2 | cut -d\\\" -f2)",
    "echo \"UUID = $UUID\"",
    // You'll need this entry in your /etc/fstab for the device to get remounted again.
    "echo \"UUID=$UUID   /butter   btrfs   defaults,nofail   1   2\" >> /etc/fstab",
    // Overkill on the permissions for this example :')
    "chmod -R 777 /butter",
    // Some output sanity checks
    "df -khT",
    "mount | grep \"/dev\""
  ],
  "inline_shebang": "/bin/bash -xe",
  "type": "shell"
}

Here is the output of the provisioner command above - yours should look similar!

==> azure-arm: Provisioning with shell script: /tmp/packer-shell973735153

    azure-arm: sda     3:0:0:0     128G
    azure-arm: sdb     0:0:0:0      32G
    azure-arm: ├─sdb1             31.9G /
    azure-arm: ├─sdb14               4M
    azure-arm: └─sdb15             106M /boot/efi
    azure-arm: sdc     1:0:1:0      14G
    azure-arm: └─sdc1               14G /mnt

    azure-arm: DEVICE FOUND = sda

    azure-arm: btrfs-progs v4.15.1
    azure-arm: See http://btrfs.wiki.kernel.org for more information.
    azure-arm:
    azure-arm: Label:              (null)
    azure-arm: UUID:               7e8428d8-3fe0-4f20-825a-75c399fd0e56
    azure-arm: Node size:          16384
    azure-arm: Sector size:        4096
    azure-arm: Filesystem size:    128.00GiB
    azure-arm: Block group profiles:
    azure-arm:   Data:             single            8.00MiB
    azure-arm:   Metadata:         DUP               1.00GiB
    azure-arm:   System:           DUP               8.00MiB
    azure-arm: SSD detected:       no
    azure-arm: Incompat features:  extref, skinny-metadata
    azure-arm: Number of devices:  1
    azure-arm: Devices:
    azure-arm:    ID        SIZE  PATH
    azure-arm:     1   128.00GiB  /dev/sda
    azure-arm:

    azure-arm: /dev/sdc1: UUID="565fc1fa-02ae-4447-8751-d2f3ac7302aa" TYPE="ext4" PARTUUID="24a32a70-01"
    azure-arm: /dev/sdb1: LABEL="cloudimg-rootfs" UUID="37fdcc01-3e3c-465c-b84c-bd2a1c3b8cd7" TYPE="ext4" PARTUUID="9d108ed7-f15f-44ce-b80f-941ffa82141c"
    azure-arm: /dev/sdb15: LABEL="UEFI" UUID="88C1-EAC7" TYPE="vfat" PARTUUID="ffce7944-90d8-4378-bceb-912ef0275ddf"
    azure-arm: /dev/sda: UUID="7e8428d8-3fe0-4f20-825a-75c399fd0e56" UUID_SUB="8bb6a99f-3e93-425c-a163-db477278e7b1" TYPE="btrfs"
    azure-arm: /dev/sdb14: PARTUUID="13b2afac-ab90-472a-b083-f6250f43796a"

    azure-arm: UUID = 7e8428d8-3fe0-4f20-825a-75c399fd0e56

    azure-arm: Filesystem     Type      Size  Used Avail Use% Mounted on
    azure-arm: udev           devtmpfs  3.4G     0  3.4G   0% /dev
    azure-arm: tmpfs          tmpfs     696M  716K  695M   1% /run
    azure-arm: /dev/sdb1      ext4       31G  3.9G   27G  13% /
    azure-arm: tmpfs          tmpfs     3.4G  4.0K  3.4G   1% /dev/shm
    azure-arm: tmpfs          tmpfs     5.0M     0  5.0M   0% /run/lock
    azure-arm: tmpfs          tmpfs     3.4G     0  3.4G   0% /sys/fs/cgroup
    azure-arm: /dev/sdb15     vfat      105M  6.1M   99M   6% /boot/efi
    azure-arm: /dev/sdc1      ext4       14G   41M   13G   1% /mnt
    azure-arm: tmpfs          tmpfs     696M     0  696M   0% /run/user/1000
    azure-arm: /dev/sda       btrfs     128G  3.8M  126G   1% /butter

    azure-arm: udev on /dev type devtmpfs (rw,nosuid,relatime,size=3541800k,nr_inodes=885450,mode=755)
    azure-arm: devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000)
    azure-arm: /dev/sdb1 on / type ext4 (rw,relatime,discard)
    azure-arm: tmpfs on /dev/shm type tmpfs (rw,nosuid,nodev)
    azure-arm: cgroup on /sys/fs/cgroup/devices type cgroup (rw,nosuid,nodev,noexec,relatime,devices)
    azure-arm: hugetlbfs on /dev/hugepages type hugetlbfs (rw,relatime,pagesize=2M)
    azure-arm: mqueue on /dev/mqueue type mqueue (rw,relatime)
    azure-arm: /dev/sdb15 on /boot/efi type vfat (rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro,discard)
    azure-arm: /dev/sdc1 on /mnt type ext4 (rw,relatime,x-systemd.requires=cloud-init.service)
    azure-arm: /dev/sda on /butter type btrfs (rw,relatime,space_cache,subvolid=5,subvol=/)

A full example packer file can be found below.

This configuration…

  • Creates an ordinary Ubuntu 18.04 VM from the Azure marketplace
  • Attaches a 128GB data disk
  • Installs some useful tools (Azure CLI, dotnet, docker, docker-compose, etc)
  • Formats the attached block device as btrfs and mounts it
  • Deprovisions the image
  • Publishes this image to an Azure Image Gallery (to be consumed by an Azure Scaleset, etc…)

Note that I had manually created the resource group and image gallery prior to running this packer file.

{
  "variables": {
    "image_version": "0.0.1",
    "rg": "YOUR_RG"
  },
  "builders": [
    {
      "type": "azure-arm",
      "use_azure_cli_auth": true,
      "os_type": "Linux",
      "os_disk_size_gb": 32,
      "image_publisher": "Canonical",
      "image_offer": "UbuntuServer",
      "image_sku": "18.04-LTS",
      "build_resource_group_name": "",
      "managed_image_resource_group_name": "",
      "managed_image_name": "local-packer",
      "managed_image_storage_account_type": "Premium_LRS",
      "vm_size": "Standard_DS2_v2",
      "disk_additional_size": [128],
      "azure_tags": {
        "localversion": ""
      },
      "shared_image_gallery_destination": {
        "resource_group": "",
        "gallery_name": "local_shared_image_gallery",
        "image_name": "local-packed",
        "image_version": "",
        "replication_regions": ["West US 2"]
      }
    }
  ],
  "provisioners": [
    {
      "execute_command": "chmod +x ;  sudo -E sh ''",
      "inline": [
        "export DEBIAN_FRONTEND=noninteractive",
        "apt-get update",
        "apt-get upgrade -y",
        "apt-get install -y jq dirmngr gnupg apt-transport-https ca-certificates",
        "echo \"deb [arch=amd64] https://packages.microsoft.com/repos/azure-cli/ $(lsb_release -cs) main\" > /etc/apt/sources.list.d/azure-cli.list",
        "curl -sL https://packages.microsoft.com/keys/microsoft.asc | (OUT=$(apt-key add - 2>&1) || echo $OUT)",
        "apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF",
        "'echo \"deb https://download.mono-project.com/repo/ubuntu stable-bionic main\" > /etc/apt/sources.list.d/mono-official-stable.list'",
        "apt-get update",
        "apt-get install -y azure-cli mono-complete",
        "mono --version",
        "az --version",
        "DISTRO=$(lsb_release -is | tr '[:upper:]' '[:lower:]')",
        "CODENAME=$(lsb_release -cs)",
        "curl -s https://packages.microsoft.com/keys/microsoft.asc | (OUT=$(apt-key add - 2>&1) || echo $OUT)",
        "echo \"deb [arch=amd64] https://packages.microsoft.com/repos/microsoft-${DISTRO}-${CODENAME}-prod ${CODENAME} main\" > /etc/apt/sources.list.d/microsoft.list",
        "apt-get update",
        "apt-get -y install --no-install-recommends moby-cli moby-engine",
        "LATEST_COMPOSE_VERSION=$(curl -sSL \"https://api.github.com/repos/docker/compose/releases/latest\" | grep -o -P '(?<=\"tag_name\": \").+(?=\")')",
        "curl -sSL \"https://github.com/docker/compose/releases/download/${LATEST_COMPOSE_VERSION}/docker-compose-$(uname -s)-$(uname -m)\" -o /usr/local/bin/docker-compose",
        "chmod +x /usr/local/bin/docker-compose",
        "wget https://dot.net/v1/dotnet-install.sh",
        "chmod +x ./dotnet-install.sh",
        "apt-get install -y dotnet-sdk-3.1",
        "./dotnet-install.sh  -v 3.1.404 --install-dir /usr/share/dotnet"
      ],
      "inline_shebang": "/bin/sh -xe",
      "type": "shell"
    },
    {
      "execute_command": "chmod +x ;  sudo -E sh ''",
      "inline": [
        "lsblk -o NAME,HCTL,SIZE,MOUNTPOINT | grep -i \"sd\"",
        "DEVICE=$(lsblk -o NAME,HCTL,SIZE,MOUNTPOINT | grep -i \"sd\" | grep \"128\" | cut -d' ' -f1)",
        "echo \"DEVICE FOUND = $DEVICE\"",
        "modprobe btrfs",
        "mkfs.btrfs /dev/$DEVICE -f",
        "mkdir /butter",
        "mount /dev/$DEVICE /butter",
        "partprobe /dev/$DEVICE",
        "blkid",
        "UUID=$(blkid | grep \"$DEVICE\" | cut -d' ' -f2 | cut -d\\\" -f2)",
        "echo \"UUID = $UUID\"",
        "echo \"UUID=$UUID   /butter   btrfs   defaults,nofail   1   2\" >> /etc/fstab",
        "chmod -R 777 /butter",
        "df -khT",
        "mount | grep \"/dev\""
      ],
      "inline_shebang": "/bin/bash -xe",
      "type": "shell"
    },
    // ...ANY OTHER PROVISIONERS YOU WANT HERE...
    // ...
    {
      "execute_command": "chmod +x ;  sudo -E sh ''",
      "inline": [
        "/usr/sbin/waagent -force -deprovision+user && export HISTSIZE=0 && sync"
      ],
      "inline_shebang": "/bin/sh -xe",
      "type": "shell"
    }
  ]
}