Re: [PATCH 2/3] writeback: allow the file system to override MIN_WRITEBACK_PAGES

Dave Chinner
2025-10-16T08:23:19Z

On Thu, Oct 16, 2025 at 06:39:58AM +0200, Christoph Hellwig wrote:
> On Thu, Oct 16, 2025 at 07:49:20AM +1100, Dave Chinner wrote:
> > On Wed, Oct 15, 2025 at 03:27:15PM +0900, Christoph Hellwig wrote:
> > > The relatively low minimal writeback size of 4MiB leads means that
> > > written back inodes on rotational media are switched a lot. Besides
> > > introducing additional seeks, this also can lead to extreme file
> > > fragmentation on zoned devices when a lot of files are cached relative
> > > to the available writeback bandwidth.
> > >
> > > Add a superblock field that allows the file system to override the
> > > default size.
> >
> > Hmmm - won't changing this for the zoned rtdev also change behaviour
> > for writeback on the data device? i.e. upping the minimum for the
> > normal data device on XFS will mean writeback bandwidth sharing is a
> > lot less "fair" and higher latency when we have a mix of different
> > file sizes than it currently is...
>
> In theory it is. In practice with a zoned file system the main device
> is:
>
> a) typically only used for metadata
> b) a fast SSD when not actually on the same device
>
> So I think these concerns are valid, but not really worth replacing the
> simple superblock field with a method to query the value. But I'll write
> a comment documenting these assumptions as that is useful for future
> readers of the code.

That sounds reasonable to me. Eventually we might want to explore
per-device BDIs, but for the moment documenting the trade-off being
made is good enough.

-Dave.
--
Dave Chinner
david@fromorbit.com