Re: [PATCH 0/3] fanotify support for btrfs sub-volumes

Christoph Hellwig
2023-11-06T12:25:46Z

On Mon, Nov 06, 2023 at 06:48:11PM +1030, Qu Wenruo wrote:
> > st_dev has only been very historically about treating something as
> > a device. For userspae the most important part is that it designates
> > a separate domain for inode numbers. And that's something that's simply
> > broken in btrfs.
>
> In fact, I'm not sure if the "treating something as a device" thing is even
> correct long before btrfs.

It never really has been. There's just two APIs that ever did this,
ustat and the old quotactl. Both have been deprecated a long time ago
and never hid wide use.

> For example, for an EXT4 fs with external log device. Thankfully it's still
> more or less obvious we would use the device number of the main fs, not the
> log device, but we already had such examples.

More relevant (as the log device never has persistent data) is the XFS
realtime device.