Re: [PATCH 34/43] xfs: disable reflink for zoned file systems

[Christoph Hellwig <hch@lst.de>]

On Thu, Feb 06, 2025 at 08:31:23PM -0800, Darrick J. Wong wrote:
> On Thu, Feb 06, 2025 at 07:44:50AM +0100, Christoph Hellwig wrote:
> > While the zoned on-disk format supports reflinks, the GC code currently
> > always unshares reflinks when moving blocks to new zones, thus making the
> > feature unusuable.  Disable reflinks until the GC code is refcount aware.
> 
> It might be worth mentioning that I've been working on a refcount-aware
> free space defragmenter that could be used for this kind of thing,
> albeit with the usual problems of userspace can't really stop the
> kernel from filling its brains and starving our defragc process.
> 
> It would be interesting to load up that adversarial thread timing
> sched_ext thing that I hear was talked about at fosdem.

Not sure waht sched_ext thing and how it's relevant.

It's just that refcount awareness is a bit of work and not really
required for the initial use cases.  It might also have fun implications
for the metabtree reservations, but otherwise I don't think it's rocket
science.  Even more so with a pre-existing implementation to steal ideas
from.

Talking about stealing ideas - the in-kernel GC flow should mostly work
for a regular file system as well.  I don't think actually sharing much
code is going to be useful because the block reservations work so
differently, but I suspect doing it in-kernel using the same "check if
the mapping change at I/O completion time" scheme use in GC would work
very well for a freespace defragmenter on a regular file system.  And
doing it in the kernel will be more efficient at operation time and
probably also be a lot less code than doing it with a whole bunch of
userspace to kernel roundtrips.