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

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.

> 
> Reported-by: Filip Blagojevic <[email protected]>
> Signed-off-by: Christoph Hellwig <[email protected]>
> ---
>  fs/xfs/xfs_super.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index e6781f86d867..88edd3822872 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -546,6 +546,29 @@ xfs_open_devices(
>  	return error;
>  }
>  
> +/*
> + * When using a RT device some or all data I/O is using the RT device, but
> + * the BDI is inherited from the main data device.  When the underlying block
> + * device for the RT device has larger I/O sizes, the BDI settings might be
> + * incorrect, which is especially bad if the main device is a SSD and the
> + * RT device is a HDD, as the io_opt fixup in blk_apply_bdi_limits is missing
> + * for this case.
> + *
> + * Update the BDI values to the max of the data and RT device to cover our
> + * bases.
> + */
> +static void
> +xfs_update_bdi_rahead(
> +	struct xfs_mount	*mp)
> +{
> +	struct backing_dev_info	*rt_bdi =
> +		mp->m_rtdev_targp->bt_bdev->bd_disk->bdi;
> +	struct backing_dev_info	*sb_bdi = mp->m_super->s_bdi;
> +
> +	sb_bdi->ra_pages = max(sb_bdi->ra_pages, rt_bdi->ra_pages);
> +	sb_bdi->io_pages = max(sb_bdi->io_pages, rt_bdi->io_pages);
> +}
> +
>  /*
>   * Setup xfs_mount buffer target pointers based on superblock
>   */
> @@ -583,6 +606,7 @@ xfs_setup_devices(
>  				mp->m_sb.sb_sectsize, mp->m_sb.sb_rblocks);
>  		if (error)
>  			return error;
> +		xfs_update_bdi_rahead(mp);
>  	}
>  
>  	return 0;
> -- 
> 2.53.0
>