Re: [PATCH 2/2] xfs: don't extend the FITRIM range if the rt device does not support discard

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Aug 19, 2024 at 05:08:04PM +0200, Christoph Hellwig wrote:
> On Mon, Aug 19, 2024 at 08:00:30AM -0700, Darrick J. Wong wrote:
> > > This works.  OTOH it will break again with the zoned RT subvolume
> > > which can't support FITRIM even on devices that claim it.  And for
> > > actual users that care (and not just xfstests) these kinds of hacks
> > > don't seem very palatable..
> > 
> > What does discard do on a zoned device?  Is that how you reset the write
> > pointer?  And does that mean that either you tell the device to discard
> > everything it's written in a zone, or it will do nothing?
> 
> On an actual zone device it will probably do nothing.  But at least for
> NVMe the command used to implement discard is mandatory, so all
> devices will show support.  We also support the zoned mode on
> conventional devices, but instead of through FITRIM we want to issue
> it instad of a zone reset when the whole rtg has been garbage collected.
> 
> > Hmm.  No manpage for FITRIM.  Why don't we return the number of bytes
> > in the space map that we iterated as range.len?  Or perhaps leave it
> > unchanged?
> 
> The above would seem sensible.  Not sure if we can still pull it
> off, though.

It seems to have survived testing on TOT overnight, so I'll bake it into
djwong-dev when I go through and remove the rtgroups/rtsb feature bits
today.  And I guess the rtgroups xarray conversion too.

--D