Re: [PATCH 1/2] Btrfs: make space to keep default mount options

[David Sterba <dave@jikos.cz>]

On Wed, Sep 19, 2012 at 05:31:25PM +0900, Hidetoshi Seto wrote:
> >>  
> >> +	info->mount_opt = info->super_copy->default_mount_opt;
> > 
> > the options have to respect some priority, eg. when I set default
> > options to a filesystem, but mount with a different set, I expect that
> > the explicit flags apply and override the defaults.
> > 
> > I don't remember if this was discussed in the mailinglist or on IRC
> > only, should be easy to dig up if needed.
> 
> At least I don't know whether this was already disscussed or not.

For the subset you've selected, whole-fs default options, I now think we
don't need it, the discussions we had were about option precedence for
per -file, -subvolume, -fs, -mount. At the time of mount only per-mount
and per-fs need to sort their precedence. Any per-file option has to be
evaluated at a specific time eg. when new data are written in case of
compression.

> Now my code gives priority to the default options, and it would not
> be so difficult in case if we have opposing options like "ssd" vs
> "nossd", and "space_cache" vs "no_space_cache"...

Yep, that's what I expect to temporarily disable a specific option, eg.
the space_cache. For that purpose the full set of options with their
no- counterparts would make sense from the usability POV (although there
may be exceptions).

> Or we could use the default options only if there is no options
> specified when it is being mount, but it will make the default
> options useless.

You mean in case of a simple

  mount /dev/ice /mnt

?

This seems like a limited use of the defaults and may be confusing.


david