Re: [PATCH v2 00/16] Parallelizing filesystem writeback

[Christoph Hellwig <hch@lst.de>]

On Fri, Nov 07, 2025 at 02:54:42PM +0530, Kundan Kumar wrote:
> Predicting the Allocation Group (AG) for aged filesystems and passing
> this information to per-AG writeback threads appears to be a complex
> task.

Yes.  But in the end aged file systems are what will see most usage.
Fresh file systems look nice in benchmarks, but they aren't what
users will mostly deal with.

> To segregate these I/O requests by AG, it is necessary to associate
> AG-specific information with the pages/folios in the page cache. Two
> possible approaches are:
> (1) storing AG information in the folio->private field, or
> (2) introducing new markers in the xarray to track AG-specific data.
> 
> The AG-affined writeback thread processes specific pages from the page 
> cache marked for its AG. Is this a viable approach, or are there 
> alternative solutions that could be more effective?

Or maybe the per-AG scheme isn't that great after all and we just
need some other simple sharding scheme?  Of course lock contention
will be nicer on a per-AG basis, but as you found out actually
mapping high-level writeback to AGs is pretty hard.