Re: [PATCH 00/13] Parallelizing filesystem writeback

Christoph Hellwig
2025-07-07T14:28:15Z

On Fri, Jul 04, 2025 at 12:32:51PM +0530, Kundan Kumar wrote:
> bdi is tied to the underlying block device, and helps for device
> bandwidth specific throttling, dirty ratelimiting etc. Making it per
> superblock will need duplicating the device specific throttling, ratelimiting
> to superblock, which will be difficult.

Yes, but my point is that compared to actually having a high performing
writeback code that doesn't matter. What is the use case for actually
having production workloads (vs just a root fs and EFI partition) on
a single SSD or hard disk?

> > If someone
> > uses partitions and multiple file systems on spinning rusts these
> > days reducing the number of writeback threads isn't really going to
> > save their day either.
> >
>
> in this case with single wb thread multiple partitions/filesystems use the
> same bdi, we fall back to base case, will that not help ?

If you multiple file systems sharing a BDI, they can have different
and potentially very different requirements and they can trivially
get in the way. Or in other words we can't do anything remotely
smart without fully having the file system in charge.