Building An Image

updated: 2017-07-26 05:43

Building An Image

Building An Image¶

Now that you have diskimage-builder properly installed you can get started by building your first disk image.

VM Image¶

Our first image is going to be a bootable vm image using one of the standard supported distribution elements (Ubuntu or Fedora).

The following command will start our image build (distro must be either ‘ubuntu’ or ‘fedora’):

disk-image-create <distro> vm

This will create a qcow2 file ‘image.qcow2’ which can then be booted.

Elements¶

It is important to note that we are passing in a list of elements to disk-image-create in our above command. Elements are how we decide what goes into our image and what modifications will be performed.

Some elements provide a root filesystem, such as the ubuntu or fedora element in our example above, which other elements modify to create our image. At least one of these ‘distro elements’ must be specified when performing an image build. It’s worth pointing out that there are many distro elements (you can even create your own), and even multiples for some of the distros. This is because there are often multiple ways to install a distro which are very different. For example: One distro element might use a cloud image while another uses a package installation tool to build a root filesystem for the same distro.

Other elements modify our image in some way. The ‘vm’ element in our example above ensures that our image has a bootloader properly installed. This is only needed for certain use cases and certain output formats and therefore it is not performed by default.

Output Formats¶

By default a qcow2 image is created by the disk-image-create command. Other output formats may be specified using the -t <format> argument. Multiple output formats can also be specified by comma separation. The supported output formats are:

qcow2

tar

tgz

squashfs

vhd

docker

raw

Disk Image Layout¶

The disk image layout (like number of images, partitions, LVM, disk encryption) is something which should be set up during the initial image build: it is mostly not possible to change these things later on.

There are currently two defaults:

When using the vm element a MBR based partition layout is created with exactly one partition that fills up the whole disk and used as root device.
When not using the vm element a plain filesystem image, without any partitioning, is created.

The user can overwrite the default handling by setting the environment variable DIB_BLOCK_DEVICE_CONFIG. This variable must hold YAML structured configuration data.

The default when using the vm element is:

DIB_BLOCK_DEVICE_CONFIG='
  - local_loop:
      name: image0

  - partitioning:
      base: image0
      label: mbr
      partitions:
        - name: root
          flags: [ boot, primary ]
          size: 100%
          mkfs:
            mount:
              mount_point: /
              fstab:
                options: "defaults"
                fsck-passno: 1'

The default when not using the vm element is:

DIB_BLOCK_DEVICE_CONFIG='
  - local_loop:
      name: image0
      mkfs:
        name: mkfs_root
        mount:
          mount_point: /
          fstab:
            options: "defaults"
            fsck-passno: 1'

There are a lot of different options for the different levels. The following sections describe each level in detail.

General Remarks¶

In general each module that depends on another module has a base element that points to the depending base. Also each module has a name that can be used to reference the module.

Tree-Like vs. Complete Digraph Configuration¶

The configuration is specified as a digraph. Each module is a node; a edge is the relation of the current element to its base.

Because the general digraph approach is somewhat complex when it comes to write it down, the configuration can also be given as a tree.

Example: The tree like notation

mkfs:
  name: root_fs
  base: root_part
  mount:
    mount_point: /

is exactly the same as writing

mkfs:
  name: root_fs
  base: root_part

mount:
  name: mount_root_fs
  base: root_fs
  mount_point: /

Non existing name and base entries in the tree notation are automatically generated: the name is the name of the base module prepended by the type-name of the module itself; the base element is automatically set to the parent node in the tree.

In mostly all cases the much simpler tree notation can be used. Nevertheless there are some use cases when the more general digraph notation is needed. Example: when there is the need to combine two or more modules into one new, like combining a couple of physical volumes into one volume group.

Tree and digraph notations can be mixed as needed in a configuration.

Limitations¶

There are a couple of new modules planned, but not yet implemented, like LVM, MD, encryption, …

To provide an interface towards the existing elements, there are currently three fixed keys used - which are not configurable:

root-label: this is the label of the block device that is mounted at /.
image-block-partition: if there is a block device with the name root this is used else the block device with the name image0 is used.
image-path: the path of the image that contains the root file system is taken from the image0.

Level 0¶

Module: Local Loop¶

This module generates a local image file and uses the loop device to create a block device from it. The symbolic name for this module is local_loop.

Configuration options:

name: (mandatory) The name of the image. This is used as the name for the image in the file system and also as a symbolic name to be able to reference this image (e.g. to create a partition table on this disk).
size: (optional) The size of the disk. The size can be expressed using unit names like TiB (1024^4 bytes) or GB (1000^3 bytes). Examples: 2.5GiB, 12KB. If the size is not specified here, the size as given to disk-image-create (–image-size) or the automatically computed size is used.
directory: (optional) The directory where the image is created.

Example:

local_loop:
  name: image0

local_loop:
  name: data_image
  size: 7.5GiB
  directory: /var/tmp

This creates two image files and uses the loop device to use them as block devices. One image file called image0 is created with default size in the default temp directory. The second image has the size of 7.5GiB and is created in the /var/tmp folder.

Level 1¶

Module: Partitioning¶

This module generates partitions on existing block devices. This means that it is possible to take any kind of block device (e.g. LVM, encrypted, …) and create partition information in it.

The symbolic name for this module is partitioning.

Currently the only supported partitioning layout is Master Boot Record MBR.

It is possible to create primary or logical partitions or a mix of them. The numbering of the primary partitions will start at 1, e.g. /dev/vda1; logical partitions will typically start with 5, e.g. /dev/vda5 for the first partition, /dev/vda6 for the second and so on.

The number of logical partitions created by this module is theoretical unlimited and it was tested with more than 1000 partitions inside one block device. Nevertheless the Linux kernel and different tools (like parted, sfdisk, fdisk) have some default maximum number of partitions that they can handle. Please consult the documentation of the appropriate software you plan to use and adapt the number of partitions.

Partitions are created in the order they are configured. Primary partitions - if needed - must be first in the list.

There are the following key / value pairs to define one partition table:

base: (mandatory) The base device where to create the partitions in.
label: (mandatory) Possible values: ‘mbr’ This uses the Master Boot Record (MBR) layout for the disk. (There are currently plans to add GPT later on.)
align: (optional - default value ‘1MiB’) Set the alignment of the partition. This must be a multiple of the block size (i.e. 512 bytes). The default of 1MiB (~ 2048 * 512 bytes blocks) is the default for modern systems and known to perform well on a wide range of targets. For each partition there might be some space that is not used - which is align - 512 bytes. For the default of 1MiB exactly 1048064 bytes (= 1 MiB - 512 byte) are not used in the partition itself. Please note that if a boot loader should be written to the disk or partition, there is a need for some space. E.g. grub needs 63 * 512 byte blocks between the MBR and the start of the partition data; this means when grub will be installed, the align must be set at least to 64 * 512 byte = 32 KiB.
partitions: (mandatory) A list of dictionaries. Each dictionary describes one partition.

The following key / value pairs can be given for each partition:

name

(mandatory) The name of the partition. With the help of this name, the partition can later be referenced, e.g. when creating a file system.

flags

(optional) List of flags for the partition. Default: empty. Possible values:

boot: Sets the boot flag for the partition
primary: Partition should be a primary partition. If not set a logical partition will be created.

size

(mandatory) The size of the partition. The size can either be an absolute number using units like 10GiB or 1.75TB or relative (percentage) numbers: in the later case the size is calculated based on the remaining free space.

Example:

 - partitioning:
    base: image0
    label: mbr
    partitions:
      - name: part-01
        flags: [ boot ]
        size: 1GiB
      - name: part-02
        size: 100%

- partitioning:
    base: data_image
    label: mbr
    partitions:
      - name: data0
        size: 33%
      - name: data1
        size: 50%
      - name: data2
        size: 100%

On the image0 two partitions are created. The size of the first is 1GiB, the second uses the remaining free space. On the data_image three partitions are created: all are about 1/3 of the disk size.

Level 2¶

Module: Mkfs¶

This module creates file systems on the block device given as base. The following key / value pairs can be given:

base: (mandatory) The name of the block device where the filesystem will be created on.
name: (mandatory) The name of the partition. This can be used to reference (e.g. mounting) the filesystem.
type: (mandatory) The type of the filesystem, like ext4 or xfs.
label: (optional - defaults to the name) The label of the filesystem. This can be used e.g. by grub or in the fstab.
opts: (optional - defaults to empty list) Options that will passed to the mkfs command.
uuid: (optional - no default / not used if not givem) The UUID of the filesystem. Not all file systems might support this. Currently there is support for ext2, ext3, ext4 and xfs.

Example:

- mkfs:
    name: mkfs_root
    base: root
    type: ext4
    label: cloudimage-root
    uuid: b733f302-0336-49c0-85f2-38ca109e8bdb
    opts: "-i 16384"

Level 3¶

Module: Mount¶

This module mounts a filesystem. The options are:

base: (mandatory) The name of the filesystem that will be mounted.
name: (mandatory) The name of the mount point. This can be used for reference the mount (e.g. creating the fstab).
mount_point: (mandatory) The mount point of the filesystem.

There is no need to list the mount points in the correct order: an algorithm will automatically detect the mount order.

Example:

- mount:
    name: root_mnt
    base: mkfs_root
    mount_point: /

Level 4¶

Module: fstab¶

This module creates fstab entries. The following options exists. For details please consult the fstab man page.

base: (mandatory) The name of the mount point that will be written to fstab.
name: (mandatory) The name of the fstab entry. This can be used later on as reference - and is currently unused.
options: (optional, defaults to default) Special mount options can be given. This is used as the fourth field in the fstab entry.
dump-freq: (optional, defaults to 0 - don’t dump) This is passed to dump to determine which filesystem should be dumped. This is used as the fifth field in the fstab entry.
fsck-passno: (optional, defaults to 2) Determines the order to run fsck. Please note that this should be set to 1 for the root file system. This is used as the sixth field in the fstab entry.

Example:

- fstab:
    name: var_log_fstab
    base: var_log_mnt
    options: nodev,nosuid
    dump-freq: 2

Filesystem Caveat¶

By default, disk-image-create uses a 4k byte-to-inode ratio when creating the filesystem in the image. This allows large ‘whole-system’ images to utilize several TB disks without exhausting inodes. In contrast, when creating images intended for tenant instances, this ratio consumes more disk space than an end-user would expect (e.g. a 50GB root disk has 47GB avail.). If the image is intended to run within a tens to hundrededs of gigabyte disk, setting the byte-to-inode ratio to the ext4 default of 16k will allow for more usable space on the instance. The default can be overridden by passing --mkfs-options like this:

disk-image-create --mkfs-options '-i 16384' <distro> vm

You can also select a different filesystem by setting the FS_TYPE environment variable.

Note --mkfs-options are options passed to the mfks driver, rather than mkfs itself (i.e. after the initial -t argument).

Speedups¶

If you have 4GB of available physical RAM (as reported by /proc/meminfo MemTotal), or more, diskimage-builder will create a tmpfs mount to build the image in. This will improve image build time by building it in RAM. By default, the tmpfs file system uses 50% of the available RAM. Therefore, the RAM should be at least the double of the minimum tmpfs size required.

For larger images, when no sufficient amount of RAM is available, tmpfs can be disabled completely by passing –no-tmpfs to disk-image-create. ramdisk-image-create builds a regular image and then within that image creates ramdisk.

If tmpfs is not used, you will need enough room in /tmp to store two uncompressed cloud images. If tmpfs is used, you would still need /tmp space for one uncompressed cloud image and about 20% of that image for working files.

Chosing an Architecture¶

If needed you can specify an override the architecture selection by passing a -a argument like:

disk-image-create -a <arch> ...

Notes about PowerPC Architectures¶

PowerPC can operate in either Big or Little Endian mode. ppc64 always refers to Big Endian operation. When running in little endian mode it can be referred to as ppc64le or ppc64el.

Typically ppc64el refers to a .deb based distribution architecture, and ppc64le refers to a .rpm based distribution. Regardless of the distribution the kernel architecture is always ppc64le.

updated: 2017-07-26 05:43

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