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For a directory which has millions of files, aufs VDIR consumes
much memory. In this case, RDU (readdir(3) in user-space) is definitely
better.
If you enable CONFIG_AUFS_RDU at compiling aufs, install libau.so from
aufs-util.git, and set some environment variables, then you can use this
feature. When readdir(3) in libau.so receives an aufs dir, it issues
ioctl(2) instead of regular readdir(3).
All merging and whiteout handling are done in userspace.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Provide info about the branches, which will be used from user-space.
This is essentially equivalent to the entries under sysfs
(/sys/fs/aufs/si_*/).
But the ioctl behaviour is atomic and never confuse the matching of the
branch id.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Provide a file descriptor corresponding the specified writable branch.
The file descriptor will be used from user-space such as FHSM and
libau.so. For details, see aufs-util.git.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>