Features/LZMA for Live Images

LZMA for Live Images

Summary

Better compression technology (LZMA) allows us to ship more software on our live images.

Owner

• Name: Bruno Wolff III

• Email: <bruno@wolff.to>

Current status

• Targeted release: Fedora 14
• Last updated: 2010-02-14
• Percentage of completion: 40%

Detailed Description

LZMA compression typically provides better compression than zlib while still providing relatively fast uncompression. Once squashfs file systems are supported by the kernel (which is anticipated in 2.6.34) we can use them for building live images. A test with the games spin resulted in a 10% reduction in size of the resulting live image.

Benefit to Fedora

By allowing more packages to be put on a Live Image, we can make a few of our space constrained images better. For other images, having them be smaller will make them faster to download.

Scope

• Lougher's patches need to be accepted upstream. Right now they are in linux-next and will probably be in 2.6.34.
• Lougher's 4.1 (currently prerelease) version of squashfs-tools needs to be in the release.
  • Bug 504425 documents the current status of that effort.
    • It currently builds.
    • It uses multiple processors for both zlib and lzma compression.
    • A test games spin had a 10% reduction (from almost 4.4 GB to just under 4 GB) in size using lzma compression, saving about 400 MB. (This was a nonfunctional image because of lack of kernel support for lzma squashfs.)
    • A live image was built using this version of squashfs (using the default zlib compression) which functioned properly. No changes were made to livecd-creator to make the image.
    • Currently there is just a scratch build of the dev squashfs-tools.
• livecd-creator needs to be modified to have mksquashfs use lzma compression. (Probably as the default, with other compression methods specifiable as command line options.)
• If possible this should be usable significantly ahead of feature freeze so that spin owners can be pretty safe relying on it for their spins.

How To Test

• We need to make sure live images work.
• We need to make sure live images are smaller using LZMA compression.
• We need to make sure performance is acceptable. (I.e. that LZMA doesn't slow things down significantly on typical systems.)

User Experience

Live images will be smaller or have more included packages.

Dependencies

• squashfs-tools
• kernel (may need to ask for backport)
• livecd-tools

Contingency Plan

• Since the default in the new squashfs is to use zlib compression, we shouldn't have to revert squashfs-tools.
• livecd-creator would need to have at least some of the changes reverted.

Documentation

• Most likely livecd-creator would be getting a new option and would need its help updated along with the code changes.

Release Notes

• Mention the squashfs version change, that it supports LZMA compression, point to its documentation and note that it is supported in the kernel as well.
• Mention that livecd-creator uses LZMA by default, how to get zlib compression if desired for use with older kernels or other reasons.

Comments and Discussion

• See Talk:Features/LZMA_for_Live_Images