In the aufs manpage of the current version of aufs-utils, only the br: syntax for branch options is documented, the br= syntax is not mentioned. But lxc only supports the br= syntax, and it fails without any explanation if you try to use the other syntax.
Most mount errors are accompanied at least by an explanatory WARN level log message, but none of the "goto err" cases in aufs_mkdir() or ovl_mkdir() result in a log message, either directly or in the caller mount_entry_create_dir_file(). So all you get is a vague message like
lxc-start: conf.c: lxc_setup: 3920 failed to setup the mount entries for 'container-name'
If the mount is optional, you don't even get that much.
Please tell us which kernel / aufs module source you are using. Lxc has always used br=. It sounds like br= and br: should be equivalent (see discussion in #969), so switching may be ok, but i don't want to break long-time users if we can help it.
Hi, so I think we'd like to avoid further logging in smaller helper functions for now. We report SYSERROR() back on mount failure and a WARN() when we fail to create a aufs/overlay helper dir.
If the syntax for br: and br= is equivalent it probably be best to stick with br= for simplicity and backwards compatibility; otherwise we'd have to special case br: in there again.
Closing this for now.
Sorry, I didn't realise I would have to spell this out. Yes, the kernel module supports both. lxc should similarly support both. #969 would break lxc for all existing users and I certainly would not recommend it. It is not difficult to support both. I'll submit a pull request if you like.
Oh, I see. Not at all how I interpreted your original email.
As you say that should be easy enough. A PR would be great.