Re: Are the btrfs mount options inconsistent?

[David Sterba <dsterba@suse.cz>]

On Tue, Aug 21, 2018 at 02:43:35PM +0100, David Howells wrote:
> Qu Wenruo <quwenruo.btrfs@gmx.com> wrote:
> 
> > But to be more clear, NOSSD shouldn't be a special case.
> > In fact currently NOSSD only affects whether we will output the message
> > "enabling ssd optimization", no real effect if I didn't miss anything.

There is a real effect.

> That's not quite true.  In:
> 
> 	if (!btrfs_test_opt(fs_info, NOSSD) &&
> 	    !fs_info->fs_devices->rotating) {
> 		btrfs_set_and_info(fs_info, SSD, "enabling ssd optimizations");
> 	}
> 
> the call to btrfs_set_and_info() will turn on SSD.
> 
> What this seems to me is that, normally, SSD will be turned on automatically
> unless at least one of the devices is a rotating medium - but this appears to
> be explicitly suppressed by the NOSSD option.

Right. So expected behaviour:

- nothing: auto-detect non-rotating devices, enable SSD mount option in turn
- nossd: disable auto-detection of non-rotating devices
- ssd: enable SSD optimizations uconditionally
- ssd_spread: implies SSD and affects some allocator decisions regarding
              new extent alignments