Re: [PATCH 00/13] Parallelizing filesystem writeback

[Christoph Hellwig <hch@lst.de>]

On Wed, Jul 02, 2025 at 11:43:12AM -0700, Darrick J. Wong wrote:
> > On a spinning disk, random IO bandwidth remains unchanged, while sequential
> > IO performance declines. However, setting nr_wb_ctx = 1 via configurable
> > writeback(planned in next version) eliminates the decline.
> > 
> > echo 1 > /sys/class/bdi/8:16/nwritebacks
> > 
> > We can fetch the device queue's rotational property and allocate BDI with
> > nr_wb_ctx = 1 for rotational disks. Hope this is a viable solution for
> > spinning disks?
> 
> Sounds good to me, spinning rust isn't known for iops.
> 
> Though: What about a raid0 of spinning rust?  Do you see the same
> declines for sequential IO?

Well, even for a raid0 multiple I/O streams will degrade performance
on a disk.  Of course many real life workloads will have multiple
I/O streams anyway.

I think the important part is to have:

 a) sane defaults
 b) an easy way for the file system and/or user to override the default

For a) a single thread for rotational is a good default.  For file system
that driver multiple spindles independently or do compression multiple
threads might still make sense.

For b) one big issue is that right now the whole writeback handling is
per-bdi and not per superblock.  So maybe the first step needs to be
to move the writeback to the superblock instead of bdi?  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.