5e9c451d5f
End user docs would benefit from a section about the byte-to-inode ratio, and why it's set the way it is. This update explains why and how to manipulate the ratio depending on the intended use. Change-Id: Iffb5ef6f4c7c74f4aa6e25912d4991d7a611c8fe Closes-bug: 1512841
73 lines
2.6 KiB
ReStructuredText
73 lines
2.6 KiB
ReStructuredText
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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* 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 creating the
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filesystem in the image. This allows large 'whole-system' images to utilize
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several TB disks without exhausting inodes. In contrast, when creating images
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intended for tenant instances, this ratio consumes more disk space than an
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end-user would expect (e.g. a 50GB root disk has 47GB avail.). If the image is
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intended to run 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 usable space
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on the instance. The default can be overridden by passing --mkfs-options like
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this::
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disk-image-create --mkfs-options '-i 16384' <distro> vm
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