Re: btrfs-progs confusing message

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2016-04-20 16:23, Konstantin Svist wrote:
> Pretty much all commands print out the usage message when no device is
> specified:
>
> [root@host ~]# btrfs scrub start
> btrfs scrub start: too few arguments
> usage: btrfs scrub start [-BdqrRf] [-c ioprio_class -n ioprio_classdata]
> <path>|<device>
> ...
>
> However, balance doesn't
>
> [root@host ~]# btrfs balance start
> ERROR: can't access 'start': No such file or directory

And this is an example of why backwards comparability can be a pain. 
The original balance command was 'btrfs filesystem balance', and had no 
start, stop, or similar sub-commands.  This got changed to the current 
incarnation when the support for filters was added.  For backwards 
compatibility reasons, we decided to still accept balance with no 
arguments other than the path as being the same as running 'btrfs 
balance start' on that path, and then made the old name an alias to the 
new one, with the restriction that you can't pass in filters through 
that interface.  What is happening here is that balance is trying to 
interpret start as a path, not a command, hence the message about not 
being able to access 'start'.