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

Thanks, I’m going to restate what I think you are suggesting to make
sure I'm tracking correctly.

We will step back from per-folio tagging and instead align coarse
sharding with a simpler allocation policy:

- Create a bounded number of bdi wb contexts at mount time (capped,
e.g. ≤ agcount).
- Store a per-inode stream/shard id (no per-folio state).
- Assign the stream id once and use it to select the wb context for
writeback.
- In the delalloc allocator, bias AG selection from the stream id by
partitioning AG space into per-stream "bands" and rotating the start
AG within that band; fall back to the existing allocator when
allocation can't be satisfied.

Does this align with what you have in mind?