Ubuntu – How will union mounts be possible in 12.04

Aufs is the union filesystem used by default for schroot and the LiveCDs. That's what I'd use, but you're right - there's not really an upstream union filesystem solution that I'm aware of.

The kernel used to have a config option for whether to use ATIME or RELATIME; dunno if that also included some option for NOATIME. In any case, that's gone now.

I studied util-linux as well, to see if the mount command had configurable defaults or could be modified, but it did not appear to be the case.

However, it looks like you can patch the kernel to change the default behavior. Modify ./fs/namespace.c, around lines 2334:

long do_mount(...)
...
    /* Default to relatime unless overriden */
    if (!(flags & MS_NOATIME))
            mnt_flags |= MNT_RELATIME;
...

Swap that around:

long do_mount(...)
...
    /* Default to noatime unless overriden */
    if (!(flags & MS_RELATIME))
            mnt_flags |= MNT_NOATIME;
...

And that should do it.