Re: update BDI {io,ra}_pages values based on the RT device limits

[Christoph Hellwig <hch@lst.de>]

So, I guess we're stuck with this approach?  Or any other ideas?

On Tue, Jun 23, 2026 at 04:21:05PM +0200, Christoph Hellwig wrote:
> Hi all,
> 
> we ran into a bit of a funny case where adding an SSD to store metadata
> to a (zoned) XFS file system reduced the performance vs using a HDD for
> both data and metadata.  It turns out this is due to readahead code
> looking at the BDI limits, including the io_pages value that can't be
> inspected or tuned from userspace.
> 
> This patch has the simplest fix for that by just updating the values from
> XFS, but for a long-term solution this feels a bit ugly.  Other, a lot
> more invasive options would be:
> 
> 
>   1) support multiple BDIs per file system.
> 
> 	This would work pretty well for XFS, as data for a given file is
> 	always entirely on one device, and the VFS writeback code is not
> 	used for metadata.  But it probably doesn't work well for other
> 	cases
> 
>   2) stop propagating these values through the BDI
> 
> 	Have a way to query these parameters from the file system, either
> 	through a method if we want to be fully dynamic, or through fields
> 	instead of going through the BDI.  The downside would be that
> 	sysfs modifications of the readahead size would not work after
> 	the file system initially queried them.
> 
> Thoughts?
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