[arch-dev-public] aufs / unionfs / aufs2

Thomas Bächler thomas at archlinux.org
Sat Mar 28 17:54:38 EDT 2009

Dan McGee schrieb:
> What one is most likely to land in the kernel? I assume one of these
> has to be on track to actually get in, and we should go with that and
> be done with this mess.

unionfs has been in -mm for ages, and aufs2 has just been submitted for 
mainline inclusion a few weeks ago.

> Why is it that everyone seems to use union filesystems, and yet none
> of them have matured enough to get in the tree?

When unionfs was first submitted, apart from coding style and 
implementation issues, it had design problems, like non-consistent inode 
numbers in some cases (which aufs2 solves, according to the author).
Also, it seems that people weren't happy about the general approach: 
Rather than stacking filesystems, doing copyup and such, it was 
suggested that a special filesystem should be designed that has its own 
on-disk-format for storing differences between an underlying filesystem 
and the actually visible filesystem. unionfs-odf does this, but 
apparently it is not very mature.

For reference, here are the recent LKML discussions (most of the 
information from above is from what I read here some time ago and 

23. Feb: aufs design and first comments
9. Mar: aufs source
16. Mar: aufs source, second try

This is a lot to read and I have no idea if and when this might be included.

However, we should be able to build this as an external module with only 
a non-intrusive patch to the kernel (it exports some symbols that are 
otherwise not exported), while I don't know how intrusive unionfs is.

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