Re: [PATCH v3 0/6] AG aware parallel writeback for XFS

[Kundan Kumar <kundan.kumar@samsung.com>]

On 2/6/2026 11:55 AM, Christoph Hellwig wrote:
> I fear we're deep down a rabbit hole solving the wrong problem here.
> Traditionally block allocation, in XFS and in general, was about finding
> the "best" location to avoid seeks.  With SSDs the seeks themselves are
> kinda pointless, although large sequential write streams are still very
> useful of course, as is avoiding both freespace and bmap fragmentation.
> On the other hand avoiding contention from multiple writers is a good
> thing.  (this is discounting the HDD case, where the industry is very
> rapidly moving to zoned device, for which zoned XFS has a totally
> different allocator)
> 
> With multi-threaded writeback this become important for writeback, but
> even before this would be useful for direct and uncached I/O.
> 
> So I think the first thing I'd look into it to tune the allocator to
> avoid that contention, by by spreading different allocation streams from
> different core to different AGs, and relax the very sophisticated and
> detailed placement done by the XFS allocator.

When you say "coarse-grained sharding", do you mean tracking a single
"home AG" per inode (no per-folio tagging) and using it as a best-effort
hint for writeback routing?

If so, we can align with that and keep the threading model generic by
relying on bdi writeback contexts. Concretely, XFS would set up a 
bounded number of bdi wb contexts at mount time, and route each inode to 
its home shard. Does this align with what you have in mind?

We had implemented a similar approach in earlier versions of this
series[1]. But the feedback[2] that we got was that mapping high level
writeback to eventual AG allocation can be sometimes inaccurate (aged
filesystems, inode spanning accross multiple AGs, etc.), so we moved to
per folio tagging to make the routing decision closer to the actual IO.
That said, I agree that the per folio approach is complex.

Darrick, does this direction look reasonable to you as well?

[1] 
https://lore.kernel.org/all/20251014120845.2361-1-kundan.kumar@samsung.com/
[2] https://lore.kernel.org/all/20251107133742.GA5596@lst.de/