»OMI Builder (chroot)

Type: osc-chroot Artifact BuilderId: oapi.outscale.chroot

The osc-chroot Packer builder is able to create Outscale Machine Images (OMIs) backed by an BSU volume as the root device. For more information on the difference between instance storage and BSU-backed instances, see the "storage for the root device" section in the Outscale documentation.

The difference between this builder and the osc-bsu builder is that this builder is able to build an BSU-backed OMI without launching a new Outscale VM. This can dramatically speed up OMI builds for organizations who need the extra fast build.

The builder does not manage OMIs. Once it creates an OMI and stores it in your account, it is up to you to use, delete, etc., the OMI.

»How Does it Work?

This builder works by creating a new BSU volume from an existing source OMI and attaching it into an already-running Outscale VM. Once attached, a chroot is used to provision the system within that volume. After provisioning, the volume is detached, snapshotted, and an OMI is made.

Using this process, minutes can be shaved off the OMI creation process because a new Outscale VM doesn't need to be launched.

There are some restrictions, however. The host Outscale instance where the volume is attached to must be a similar system (generally the same OS version, kernel versions, etc.) as the OMI being built. Additionally, this process is much more expensive because the Outscale VM must be kept running persistently in order to build OMIs, whereas the other OMI builders start VMs on-demand to build OMIs as needed.

»Configuration Reference

There are many configuration options available for the builder. They are segmented below into two categories: required and optional parameters. Within each category, the available configuration keys are alphabetized.


  • access_key (string) - The access key used to communicate with OUTSCALE. Learn how to set this

  • omi_name (string) - The name of the resulting OMIS that will appear when managing OMIs in the Outscale console or via APIs. This must be unique. To help make this unique, use a function like timestamp (see template engine for more info).

  • secret_key (string) - The secret key used to communicate with Outscale. Learn how to set this

  • source_omi (string) - The initial OMI used as a base for the newly created machine. source_omi_filter may be used instead to populate this automatically.


  • omi_description (string) - The description to set for the resulting OMI(s). By default this description is empty. This is a template engine, see Build template data for more information.

  • omi_account_ids (array of strings) - A list of account IDs that have access to launch the resulting OMI(s). By default no additional users other than the user creating the OMIS has permissions to launch it.

  • omi_virtualization_type (string) - The type of virtualization for the OMI you are building. This option must match the supported virtualization type of source_omi. Can be paravirtual or hvm.

  • chroot_mounts (array of array of strings) - This is a list of devices to mount into the chroot environment. This configuration parameter requires some additional documentation which is in the Chroot Mounts section. Please read that section for more information on how to use this.

  • command_wrapper (string) - How to run shell commands. This defaults to {{.Command}}. This may be useful to set if you want to set environmental variables or perhaps run it with sudo or so on. This is a configuration template where the .Command variable is replaced with the command to be run. Defaults to {{.Command}}.

  • copy_files (array of strings) - Paths to files on the running Outscale VM that will be copied into the chroot environment prior to provisioning. Defaults to /etc/resolv.conf so that DNS lookups work. Pass an empty list to skip copying /etc/resolv.conf. You may need to do this if you're building an image that uses systemd.

  • custom_endpoint_oapi (string) - This option is useful if you use a cloud provider whose API is compatible with Outscale OAPI. Specify another endpoint like this

  • device_path (string) - The path to the device where the root volume of the source OMI will be attached. This defaults to "" (empty string), which forces Packer to find an open device automatically.

  • force_deregister (boolean) - Force Packer to first deregister an existing OMIS if one with the same name already exists. Default false.

  • force_delete_snapshot (boolean) - Force Packer to delete snapshots associated with OMIs, which have been deregistered by force_deregister. Default false.

  • insecure_skip_tls_verify (boolean) - This allows skipping TLS verification of the OAPI endpoint. The default is false.

  • from_scratch (boolean) - Build a new volume instead of starting from an existing OMI root volume snapshot. Default false. If true, source_omi is no longer used and the following options become required: omi_virtualization_type, pre_mount_commands and root_volume_size. The below options are also required in this mode only:

  • omi_block_device_mappings (array of block device mappings) - Add one or more block device mappings to the OMI. These will be attached when booting a new VM from your OMI. To add a block device during the Packer build see launch_block_device_mappings below. Your options here may vary depending on the type of VM you use. The block device mappings allow for the following configuration:

    • delete_on_vm_deletion (boolean) - Indicates whether the BSU volume is deleted on VM termination. Default false. NOTE: If this value is not explicitly set to true and volumes are not cleaned up by an alternative method, additional volumes will accumulate after every build.

    • device_name (string) - The device name exposed to the VM (for example, /dev/sdh or xvdh). Required for every device in the block device mapping.

    • iops (number) - The number of I/O operations per second (IOPS) that the volume supports. See the documentation on IOPs for more information

    • no_device (boolean) - Suppresses the specified device included in the block device mapping of the OMI

    • snapshot_id (string) - The ID of the snapshot

    • virtual_name (string) - The virtual device name. See the documentation on Block Device Mapping for more information

    • volume_size (number) - The size of the volume, in GiB. Required if not specifying a snapshot_id

    • volume_type (string) - The volume type. gp2 for General Purpose (SSD) volumes, io1 for Provisioned IOPS (SSD) volumes, and standard for Magnetic volumes

  • root_device_name (string) - The root device name. For example, xvda.

  • mount_path (string) - The path where the volume will be mounted. This is where the chroot environment will be. This defaults to /mnt/packer-amazon-chroot-volumes/{{.Device}}. This is a configuration template where the .Device variable is replaced with the name of the device where the volume is attached.

  • mount_partition (string) - The partition number containing the / partition. By default this is the first partition of the volume, (for example, xvdf1) but you can designate the entire block device by setting "mount_partition": "0" in your config, which will mount xvdf instead.

  • mount_options (array of strings) - Options to supply the mount command when mounting devices. Each option will be prefixed with -o and supplied to the mount command ran by Packer. Because this command is ran in a shell, user discretion is advised. See this manual page for the mount command for valid file system specific options.

  • nvme_device_path (string) - When we call the mount command (by default mount -o device dir), the string provided in nvme_mount_path will replace device in that command. When this option is not set, device in that command will be something like /dev/sdf1, mirroring the attached device name. This assumption works for most instances but will fail with c5 and m5 instances. In order to use the chroot builder with c5 and m5 instances, you must manually set nvme_device_path and device_path.

  • pre_mount_commands (array of strings) - A series of commands to execute after attaching the root volume and before mounting the chroot. This is not required unless using from_scratch. If so, this should include any partitioning and filesystem creation commands. The path to the device is provided by {{.Device}}.

  • post_mount_commands (array of strings) - As pre_mount_commands, but the commands are executed after mounting the root device and before the extra mount and copy steps. The device and mount path are provided by {{.Device}} and {{.MountPath}}.

  • root_volume_size (number) - The size of the root volume in GB for the chroot environment and the resulting OMI. Default size is the snapshot size of the source_omi unless from_scratch is true, in which case this field must be defined.

  • root_volume_type (string) - The type of BSU volume for the chroot environment and resulting OMI. The default value is the type of the source_omi, unless from_scratch is true, in which case the default value is gp2. You can only specify io1 if building based on top of a source_omi which is also io1.

  • root_volume_tags (object of key/value strings) - Tags to apply to the volumes that are launched. This is a template engine, see Build template data for more information.

  • skip_region_validation (boolean) - Set to true if you want to skip validation of the region configuration option. Default false.

  • snapshot_tags (object of key/value strings) - Tags to apply to snapshot. They will override OMI tags if already applied to snapshot. This is a template engine, see Build template data for more information.

  • snapshot_groups (array of strings) - A list of groups that have access to create volumes from the snapshot(s). By default no groups have permission to create volumes from the snapshot(s). all will make the snapshot publicly accessible.

  • snapshot_users (array of strings) - A list of account IDs that have access to create volumes from the snapshot(s). By default no additional users other than the user creating the OMIS has permissions to create volumes from the backing snapshot(s).

  • source_omi_filter (object) - Filters used to populate the source_omi field.

    • filters (map of strings) - filters used to select a source_omi.

    • owners (array of strings) - Filters the images by their owner. You may specify one or more Outscale account IDs, "self" (which will use the account whose credentials you are using to run Packer) or an Outscale owner alias. This option is required for security reasons.


        "source_omi_filter": {
          "filters": {
            "virtualization-type": "hvm",
            "image-name": "ubuntu/images/*ubuntu-xenial-16.04-amd64-server-*",
            "root-device-type": "ebs"
          "owners": ["099720109477"]
      {  "source_omi_filter": {    "filters": {      "virtualization-type": "hvm",      "image-name": "ubuntu/images/*ubuntu-xenial-16.04-amd64-server-*",      "root-device-type": "ebs"    },    "owners": ["099720109477"]  }}

      This selects an Ubuntu 16.04 HVM BSU OMIS from Canonical. NOTE: This will fail unless exactly one OMIS is returned. In the above example, most_recent will cause this to succeed by selecting the newest image.

      You may set this in place of source_omi or in conjunction with it. If you set this in conjunction with source_omi, the source_omi will be added to the filter. The provided source_omi must meet all of the filtering criteria provided in source_omi_filter; this pins the OMI returned by the filter, but will cause Packer to fail if the source_omi does not exist.

  • tags (object of key/value strings) - Tags applied to the OMI. This is a template engine, see Build template data for more information.

»Basic Example

Here is a basic example. It is completely valid except for the access keys:

  "type": "osc-chroot",
  "access_key": "YOUR KEY HERE",
  "secret_key": "YOUR SECRET KEY HERE",
  "source_omi": "ami-3e158364",
  "omi_name": "packer-outscale-chroot {{timestamp}}"
{  "type": "osc-chroot",  "access_key": "YOUR KEY HERE",  "secret_key": "YOUR SECRET KEY HERE",  "source_omi": "ami-3e158364",  "omi_name": "packer-outscale-chroot {{timestamp}}"}

»Chroot Mounts

The chroot_mounts configuration can be used to mount specific devices within the chroot. By default, the following additional mounts are added into the chroot by Packer:

These default mounts are usually good enough for anyone and are sane defaults. However, if you want to change or add the mount points, you may using the chroot_mounts configuration. Here is an example configuration which only mounts /proc and /dev:

  "chroot_mounts": [
    ["proc", "proc", "/proc"],
    ["bind", "/dev", "/dev"]
{  "chroot_mounts": [    ["proc", "proc", "/proc"],    ["bind", "/dev", "/dev"]  ]}

chroot_mounts is a list of a 3-tuples of strings. The three components of the 3-tuple, in order, are:

  • The filesystem type. If this is "bind", then Packer will properly bind the filesystem to another mount point.

  • The source device.

  • The mount directory.


A quick note on parallelism: it is perfectly safe to run multiple separate Packer processes with the osc-chroot builder on the same Outscale VM. In fact, this is recommended as a way to push the most performance out of your OMI builds.

Packer properly obtains a process lock for the parallelism-sensitive parts of its internals such as finding an available device.


»Unmounting the Filesystem

One of the difficulties with using the chroot builder is that your provisioning scripts must not leave any processes running or packer will be unable to unmount the filesystem.

For debian based distributions you can setup a policy-rc.d file which will prevent packages installed by your provisioners from starting services:

  "type": "shell",
  "inline": [
    "echo '#!/bin/sh' > /usr/sbin/policy-rc.d",
    "echo 'exit 101' >> /usr/sbin/policy-rc.d",
    "chmod a+x /usr/sbin/policy-rc.d"
  "type": "shell",
  "inline": ["rm -f /usr/sbin/policy-rc.d"]
({  "type": "shell",  "inline": [    "echo '#!/bin/sh' > /usr/sbin/policy-rc.d",    "echo 'exit 101' >> /usr/sbin/policy-rc.d",    "chmod a+x /usr/sbin/policy-rc.d"  ]},{  "type": "shell",  "inline": ["rm -f /usr/sbin/policy-rc.d"]})

»Ansible provisioner

Running ansible against osc-chroot requires changing the Ansible connection to chroot and running Ansible as root/sudo.

»Building From Scratch

This example demonstrates the essentials of building an image from scratch. A 15G gp2 (SSD) device is created (overriding the default of standard/magnetic). The device setup commands partition the device with one partition for use as an HVM image and format it ext4. This builder block should be followed by provisioning commands to install the os and bootloader.

  "type": "osc-chroot",
  "ami_name": "packer-from-scratch {{timestamp}}",
  "from_scratch": true,
  "ami_virtualization_type": "hvm",
  "pre_mount_commands": [
    "parted {{.Device}} mklabel msdos mkpart primary 1M 100% set 1 boot on print",
    "mkfs.ext4 {{.Device}}1"
  "root_volume_size": 15,
  "root_device_name": "xvdf",
  "ami_block_device_mappings": [
      "device_name": "xvdf",
      "delete_on_termination": true,
      "volume_type": "gp2"
{  "type": "osc-chroot",  "ami_name": "packer-from-scratch {{timestamp}}",  "from_scratch": true,  "ami_virtualization_type": "hvm",  "pre_mount_commands": [    "parted {{.Device}} mklabel msdos mkpart primary 1M 100% set 1 boot on print",    "mkfs.ext4 {{.Device}}1"  ],  "root_volume_size": 15,  "root_device_name": "xvdf",  "ami_block_device_mappings": [    {      "device_name": "xvdf",      "delete_on_termination": true,      "volume_type": "gp2"    }  ]}

»Build template data

In configuration directives marked as a template engine above, the following variables are available:

  • BuildRegion - The region (for example eu-west-2) where Packer is building the OMI.
  • SourceOMI - The source OMIS ID (for example ami-a2412fcd) used to build the OMI.
  • SourceOMIName - The source OMIS Name (for example ubuntu-390) used to build the OMI.
  • SourceOMITags - The source OMIS Tags, as a map[string]string object