Re: [PATCH v2] btrfs: drop the nossd flag when remounting with -o ssd

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On Tue, Apr 04, 2017 at 12:43:57AM +0200, Hans van Kranenburg wrote:
> On 04/03/2017 02:24 PM, David Sterba wrote:
> > On Fri, Mar 31, 2017 at 10:24:57PM +0200, Hans van Kranenburg wrote:
> >>>> Adding the 'nossd_spread' would be good to have, even if it might be
> >>>> just a marginal usecase.
> >>
> >> Please no, don't make it more complex if not needed.
> > 
> > The only use is when ssd,ssd_spread are on and then I'd just want to
> > disable ssd_spread, without disabling ssd at the same time.
> > 
> > 1. mount -o ssd,ssd_spread
> > 2. mount -o remount,nossd_spread
> > 
> > compared to
> > 
> > 1. mount -o ssd,ssd_spread
> > 2. mount -o remount,nossd
> > 3. mount -o remount,ssd
> > 
> > I'd vote for adding nossd_spread, as the 'no-' options are common and
> > otherwise disabling ssd_spread would be another usage exception.
> 
> Yes, 'nossd_spread' would intuitively be the thing to try to get rid of
> 'ssd_spread' on a mounted fs. But, nossd_spread is not a feature, just
> like noautodefrag isn't. nossd *is* a feature, but also a remount
> option... :o)
> 
> The mount manpage displays the values as a choice between 3 exclusive
> options: ssd|nossd|ssd_spread
> 
> They're like an increasing level of magic that is being applied:
>    nossd < ssd < ssd_spread
> 
> So, that documentation with the | makes me think: I have to choose
> either one. But that's not how it behaves, since some of them can appear

The documentation groups options by functionality, the "|" makes it
indeed confusing as exclusive options in this case. The rendering is not
consistent, I have "|" and also ",".  I hope that the text should
explain more than could fit into the brief option list and I'd like to
avoid complicating the syntax.

> But don't listen to me, I don't know what the best thing is.

Neither do I, so we talk about various aspects and update code or
documentation if we can make it more clear. I appreciate your feedback.
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