Wednesday, September 12, 2012

Customizing a Tinycore Linux Image

In preparing the backend network for our new turn-based strategy game, Just Tactics, we architected it to be able to easily scale up as the game becomes more popular. Step one was designing an automated netboot operating system environment, to allow us to bring up new servers without having to sit through an install procedure, and remove our dependence on mechanical storage devices.

As such a minimal Linux environment, Tinycore provides a great starting point for a low-overhead server, but some applications will inevitably need to be added to the base install. They distribute packages as Tinycore Extensions in compressed .tcz files, and provide some great methods to dynamically load them on boot or as needed. This works great for a desktop or virtual machine appliance, but for a constantly-running server, this provides an unnecessary level of complication and overhead.

Initial Attempt

Initially, we started using the recommended Dynamic Root Filesystem Remastering method. This method decompresses the extensions into a second initial ramdisk, which is loaded by one of the Syslinux bootloaders alongside the tinycore ramdisk. This is how it worked in our environment:
  • TFTP load and boot the tinycore ramdisk
  • mount an nfs share inside the tinycore ramdisk
  • load the secondary ramdisk from the NFS share
We were having problems with consistently mounting the NFS share inside the tinycore ramdisk, so we decided to simplify, and integrate the extensions directly into the primary image.

Final Config

We ended up using the method outlined in this guide to decompress the tinycore image and all relevant extensions, and rebuild it with the additional packages integrated. This way, everything is integrated into the root filesystem, just like it would be with a standard local install.

Download necessary extensions:

All tinycore extensions for the current release are listed here. I manually downloaded extensions for all necessary packages, including the dependencies listed in .dep files.

Unpack extensions:

After installing squashfs-tools, you can unzip each .tcz extension using the following command:
unsquashfs -f filename.tcz
The extensions will be unzipped into a filesystem tree in squashfs-root in the current directory, for example /tmp/squashfs-root

Unpack tinycore:

You will want to unpack tinycore into it's own directory, for example /tmp/tinycore. Unpack the tinycore ramdisk using this command:
sudo zcat tinycore.gz | cpio -i -H newc -d

Merge everything:

To integrate all of the extensions into the base install, you simply copy the uncompressed files into the right place. This will give you one filesystem tree with tinycore files and extensions in the same places. Sudo is necessary to preserve permissions in some of the system files.
sudo cp -Rp /tmp/squashfs-root/* /tmp/tinycore

Recompressing the Tinycore Image:

I use the following commands to update the dynamic linker and build the image. The advdef command  is optional - it simply compresses the image a little more.
sudo ldconfig -r /tmp/tinycore
sudo find | sudo cpio -o -H newc | gzip -2 > /tmp/tinycore.gz
cd /tmp && advdef -z4 tinycore.gz

Additional References:
Integrating Extensions into an ISO


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