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Summary

We use union filesystems to implement the live cd experience. In Jaunty this was implemented using AUFS. However this is not expected to ever make it upstream. The currently favourite implementation of this style of functionality is VFS union-mount. The purpose of this spec is to track both the progress upstream of the required functionality as well as enumerating the other options.

Release Note

None required as there should be no user visible change.

Rationale

The current AUFS support is buggy and not likely to make it upstream. We should be trying to move to the upstream solution as it becomes available. Any help and testing we can provide them can only help its integration.

Assumptions

VFS union-mount will be sufficiently stable, performant, and feature complete to allow it to replace the current AUFS implementation.

Implementation

The Karmic kernel already has dropped AUFS as it did not port forward without work. This leaves us with no liveCD.

Evaluate the alternative options:

1. DM COW snapshots
2. fuse union mount
3. VFS union mount
4. AUFS
5. UnionFS

Retool to use the selected solution.

DM COW snapshots

These work but the filesystems the create are not rsync friendly. This limitation is very problematic for QA who use this feature to make ISO testing cheaper.

FUSE Union Mount

This was tested and does seem to work, but performance is poor. This work will be retained for the

VFS Union-Mount

This seems to be the communities preferred platform.

http://lkml.org/lkml/2009/5/18/289 http://people.ubuntu.com/~scott/vfs-union/ http://valerieaurora.org/union/

AUFS

UnionFS

Test/Demo Plan

TBD

Unresolved issues

It is unclear the VFS union-mount will be merged in time for 2.6.31 which would mean carrying it as a patch set much as we carry AUFS now.

CategorySpec