24d1527111
speedup section explains the user how to sppedup image build by using tmpfs. The correct user guide to have this section, is the user guide about image building rather than the installation user guide. Change-Id: I96b90bd79df53db4f926a928ae3c86b888315230
89 lines
3.5 KiB
ReStructuredText
89 lines
3.5 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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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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