9d13084c41
The squashfs format brings a couple of advantages over the other formats. Image is often an order of magnitude smaller and it can be used natively, either as an initrd, either with loop mount. Change-Id: If72940b0c4dafb2504c52dd0429a8eb3f8305751
97 lines
3.7 KiB
ReStructuredText
97 lines
3.7 KiB
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Building An Image
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=================
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Now that you have diskimage-builder properly :doc:`installed <installation>`
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you can get started by building your first disk image.
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VM Image
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--------
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Our first image is going to be a bootable vm image using one of the standard
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supported distribution :doc:`elements <../elements>` (Ubuntu or Fedora).
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The following command will start our image build (distro must be either
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'ubuntu' or 'fedora'):
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::
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disk-image-create <distro> vm
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This will create a qcow2 file 'image.qcow2' which can then be booted.
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Elements
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--------
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It is important to note that we are passing in a list of
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:doc:`elements <../elements>` to disk-image-create in our above command. Elements
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are how we decide what goes into our image and what modifications will be
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performed.
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Some elements provide a root filesystem, such as the ubuntu or fedora element
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in our example above, which other elements modify to create our image. At least
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one of these 'distro elements' must be specified when performing an image
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build. It's worth pointing out that there are many distro elements (you can even
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create your own), and even multiples for some of the distros. This is because
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there are often multiple ways to install a distro which are very different.
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For example: One distro element might use a cloud image while another uses
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a package installation tool to build a root filesystem for the same distro.
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Other elements modify our image in some way. The 'vm' element in our example
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above ensures that our image has a bootloader properly installed. This is only
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needed for certain use cases and certain output formats and therefore it is
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not performed by default.
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Output Formats
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--------------
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By default a qcow2 image is created by the disk-image-create command. Other
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output formats may be specified using the `-t <format>` argument. Multiple
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output formats can also be specified by comma separation. The supported output
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formats are:
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* qcow2
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* tar
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* tgz
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* squashfs
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* vhd
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* docker
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* raw
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Filesystem Caveat
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-----------------
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By default, disk-image-create uses a 4k byte-to-inode ratio when
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creating the filesystem in the image. This allows large 'whole-system'
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images to utilize several TB disks without exhausting inodes. In
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contrast, when creating images intended for tenant instances, this
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ratio consumes more disk space than an end-user would expect (e.g. a
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50GB root disk has 47GB avail.). If the image is intended to run
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within a tens to hundrededs of gigabyte disk, setting the
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byte-to-inode ratio to the ext4 default of 16k will allow for more
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usable space on the instance. The default can be overridden by passing
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``--mkfs-options`` like this::
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disk-image-create --mkfs-options '-i 16384' <distro> vm
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You can also select a different filesystem by setting the ``FS_TYPE``
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environment variable.
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Note ``--mkfs-options`` are options passed to the mfks *driver*,
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rather than ``mkfs`` itself (i.e. after the initial `-t` argument).
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Speedups
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--------
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If you have 4GB of available physical RAM (as reported by /proc/meminfo
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MemTotal), or more, diskimage-builder will create a tmpfs mount to build the
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image in. This will improve image build time by building it in RAM.
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By default, the tmpfs file system uses 50% of the available RAM.
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Therefore, the RAM should be at least the double of the minimum tmpfs
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size required.
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For larger images, when no sufficient amount of RAM is available, tmpfs
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can be disabled completely by passing --no-tmpfs to disk-image-create.
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ramdisk-image-create builds a regular image and then within that image
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creates ramdisk.
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If tmpfs is not used, you will need enough room in /tmp to store two
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uncompressed cloud images. If tmpfs is used, you would still need /tmp space
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for one uncompressed cloud image and about 20% of that image for working files.
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