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

On Mon, Jul 20, 2026 at 11:11:28AM +0200, Christoph Hellwig wrote:
> On Mon, Jul 20, 2026 at 11:02:15AM +0200, Carlos Maiolino wrote:
> > On Tue, Jun 23, 2026 at 04:21:06PM +0200, Christoph Hellwig wrote:
> > > When using XFS with a main device on an SSD that stores metadata and a RT
> > > device to store data on a HDD, we fail to take the I/O sizes for the RT
> > > device into accounting, leading to up to 5% slower read performance when
> > > using an SSD for metadata vs storing data and metadata on the HDD.
> > > 
> > > Fix this up by taking the RT settings into account.  Note that this
> > > updates the BDI owned by the main device, and leaves those settings in
> > > place even when the file system is unmounted.  This is a bit unexpected
> > > but not different from manual tuning through sysfs (although that is only
> > > possible for the ra_pages value).
> > 
> > Sorry a 'very late reply', but...
> > On a second thought, wouldn't be wise to cache the original value in
> > memory and restore it during unmount/filesystem shutdown?
> > 
> > I could send a patch to complement this one if you guys agree, otherwise
> > just ignore me.
> 
> As mentioned this does seem like a valid option, there's tradeoff
> both ways.  Although we'd also want a flag to avoid restoring them
> if they were changed while mounted so it would get a bit more
> complicated.  Happy to respin it for that if there is a general
> preference.
> 

Sorry coming back late to it, I don't want to give you extra work after
we've already discussed the implementation.

I was going to merge this patch today, but it came to my mind that might
not be a good idea to just quietly update BDI settings while XFS is
using it and just keep whatever change we made persist after the mount.

I can still merge it this week and we decide what to do later or just wait a
bit for extra thoughts on this.
Perhaps let's see if we get other comments soon and we make a decision
later this week.