Re: [PATCH 1/2] mkfs: enable new features by default

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

On Tue, Dec 02, 2025 at 10:31:22PM -0800, Christoph Hellwig wrote:
> On Tue, Dec 02, 2025 at 04:53:45PM -0800, Darrick J. Wong wrote:
> > On Mon, Dec 01, 2025 at 11:38:46PM -0800, Christoph Hellwig wrote:
> > > On Mon, Dec 01, 2025 at 05:28:16PM -0800, Darrick J. Wong wrote:
> > > > From: Darrick J. Wong <djwong@kernel.org>
> > > > 
> > > > Since the LTS is coming up, enable parent pointers and exchange-range by
> > > > default for all users.  Also fix up an out of date comment.
> > > 
> > > Do you have any numbers that show the overhead or non-overhead of
> > > enabling rmap?  It will increase the amount of metadata written quite
> > > a bit.
> > 
> > I'm assuming you're interested in the overhead of *parent pointers* and
> > not rmap since we turned on rmap by default back in 2023?
> 
> Yes, sorry.
> 
> > I see more or less the same timings for the nine subsequent runs for
> > each parent= setting.  I think it's safe to say the overhead ranges
> > between negligible and 10% on a cold new filesystem.
> 
> Should we document this cleary?  Because this means at least some
> workloads are going to see a performance decrease.

Yep.  But first -- all those results are inaccurate because I forgot
that fsstress quietly ignores everything after the first op=freq
component of the optarg, so all that benchmark was doing was creating
millions of files in a single directory and never deleting anything.
That's why the subsequent runs were much faster -- most of those files
were already created.

So I'll send a patch to fstests to fix that behavior.  With that, the
benchmark that I alleged I was running produces these numbers when
creating a directory tree of only empty files:

naming   =version 2              bsize=4096   ascii-ci=0, ftype=1, parent=1
real    0m12.742s
user    0m28.074s
sys     0m10.839s

real    0m13.469s
user    0m25.827s
sys     0m11.816s

real    0m11.352s
user    0m22.602s
sys     0m11.275s

naming   =version 2              bsize=4096   ascii-ci=0, ftype=1, parent=0
real    0m12.782s
user    0m28.892s
sys     0m8.897s

real    0m13.591s
user    0m25.371s
sys     0m9.601s

real    0m10.012s
user    0m20.849s
sys     0m9.018s

Almost no difference here!  If I add in write=1 then there's a 5%
decrease going to parent=1:

naming   =version 2              bsize=4096   ascii-ci=0, ftype=1, parent=1
real    0m15.020s
user    0m22.358s
sys     0m14.827s

real    0m17.196s
user    0m22.888s
sys     0m15.586s

real    0m16.668s
user    0m21.709s
sys     0m15.425s

naming   =version 2              bsize=4096   ascii-ci=0, ftype=1, parent=0
real    0m14.808s
user    0m22.266s
sys     0m12.843s

real    0m16.323s
user    0m22.409s
sys     0m13.695s

real    0m15.562s
user    0m21.740s
sys     0m12.927s

--D