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Date: Mon, 20 Jul 2026 11:11:28 +0200
From: Christoph Hellwig <hch@lst.de>
To: Carlos Maiolino <cem@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>, Jan Kara <jack@suse.cz>,
	Filip Blagojevic <filip.blagojevic@wdc.com>,
	Matthew Wilcox <willy@infradead.org>,
	Damien Le Moal <dlemoal@kernel.org>, linux-fsdevel@vger.kernel.org,
	linux-xfs@vger.kernel.org
Subject: Re: [PATCH] xfs: update BDI {io,ra}_pages values based on the RT
 device limits
Message-ID: <20260720091128.GB24062@lst.de>
References: <20260623142109.1838702-1-hch@lst.de> <20260623142109.1838702-2-hch@lst.de> <al3j0GaQaeYj0Ih8@nidhogg.toxiclabs.cc>
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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.