imgbased

imgbased provides a specific management method to derive writeable filesystem layers from read-only base images. It also takes care that the layer which shall be used can be selected at boot time.

In a nutshell this works by:

• having a boot partition
• and having a default LVM volume group (HostVG)
• which has a thinpool
• each base is kept in a read-only thin logical volume in the thinpool
• for each base at least one writable layer, which is a thin logical volume, is created in the thinpool
• for each layer a boot entry is created which can be used to boot in a specific layer

For more details see below.

Build the tool

How to build the tools.

sudo yum install -y make autoconf automake python
git clean -fdx
./autogen.sh
make dist
rpmbuild -ta imgbased-*.tar.xz

Build an image

The repository also contains some example kickstarts which create an image with the correct LVm layout to get started with this tool.

Note: The imgbase tool is automatically installe inside the image during creation.

# Lorax provides livemedia-creator
pkcon install -y qemu-kvm

# First create the kickstarts
make dist

# Kickoff the image creation
make image-build

# Do some automatic sanity testing on the image:
make check

# Or run the image yourself
# Default password for root: r
qemu-kvm -hda runtime-layout.img -smp 4 -m 1024 -net user -net nic

Using imgbase in the image

The imgbase tool is installed within the example image from the previous section. It can be used to create new layers and install new bases.

# To create the assumed LVM layout
imgbase layout --init

# List existing layers and bases
imgbase layout

# Add a new base
# The `--size` argument specifies the size of the underlying 
# logical volume. It must be at least the size of the filesystem
# contained in `$IMGFILE`.
imgbase base --add --size 1G $IMGFILE

# Get the latest base (which will be used for subsequent layers)
imgbase base --latest

# Add a new layer on the latest base or latest layer of the latest base
imgbase layer --add

# And with more infos
imgbase --debug layer --add

There is also a dry-mode (imgbase --debug --dry ...) which just outputs the commands to run.

Purpose

Provide a more flexible solution for oVirt Node. Mainly a way where we can re-use existing technologies like anaconda.

High-Level Things

• Read-Only bases (see also Drawbacks)
• Write-able layers
• Boot into layers
• No inheritance between bases
• Persistence between bases
  • Copy files between Base-N to Base-(N+1)

Features

• Based on stable things (LVM, ext4)
• A real filesystem (the layer) is modified and is used for boot
  • This solves all sort of problems with early boot
• Sparseness everywhere
  • LVM with thinpool and thinvolume (frees space on discard operation)
  • ext4 FS with discard option (frees space after file removal)
• LiveCD is only the delivery method
  • rootfs image is used at runtime
• More distro agnostic than LiveCD
  • dracut, lvm (with thin volumes) and ext4 are the requirements

Drawbacks

• Not as read-only as LiveCD-everywhere approach
  • The rootfs is kept on a ro LV, if this is changed to rw then the original base lv can be changed. This wasn't possible when the LiveCD was stored, because the rootfs was in a (as by limitation) read-only squashfs.
• The persistence is a copy
  • When a new base image is installed, the persistence of configuration (and other) files from the previous base happens by copying the files. Previously bind mounts were used to achieve this.
• Runtime space requirements are higher compared to LiveCD runtime
  • The LiveCD based delivery will be comparable in size.

LVM Structure

Assumptions about the host:

+ VG
|
+--+ Config (LV)
|
+--+ Base-0 (LV, ro)
|   \
|    \
|     + Base-0.1 (LV, rw)
|     |
|     + Base-0.2 (LV, rw)
|   
|   
+--+ Base-1 (LV, ro)
|   \
|    \
|     + Base-1.1 (LV, rw)
:     :

With a boot entry for each Base-* this allows the user to boot into each Base-LV. Changes are inherited from each Base-* and can also be persisted using the Config LV.

LiveCD Payload

The image is the (in future) intended to be also used as a paylod for LiveCD ISOs and to be deployed via PXE. Because of this we want to minimize the image even further. For this virt-sparsify and squashfs can be used to simulate the size of the image when it is used as a payload.

# To get an idea of the minimized size use
make runtime-layout.squash

The LiveCD will be created the livemedia-creator which is part of lorax.