Re: [PATCH v4 4/6] xfs: generic AG set based steering

On Fri, Jul 17, 2026 at 06:25:36PM +0530, Kanchan Joshi wrote:
> Improve allocator concurrency and reduce interleaving by introducing
> fixed sized AG set.
> Use low bits of the inode as a hash to select AG within the AG set.
> Overall, a file will try to use the same AG (and contiguity is maintained),
> but multiple files will be spread across all AGs in the target AG set.
> 
> Suggested-by: Dave Chinner <[email protected]>
> Signed-off-by: Kanchan Joshi <[email protected]>
> Signed-off-by: Anuj Gupta <[email protected]>
> ---
>  fs/xfs/libxfs/xfs_bmap.c | 38 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 38 insertions(+)
> 
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index d64defeda645..fd1a3aa4ad3f 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -3192,6 +3192,36 @@ xfs_bmap_select_minlen(
>  	return args->maxlen;
>  }
>  
> +#define	GENERIC_AG_SET_SZ	(2)

What does this define?

> +
> +static inline xfs_agnumber_t
> +xfs_default_ag_set_size(
> +	struct xfs_inode	*ip)
> +{
> +	struct xfs_mount	*mp = ip->i_mount;
> +
> +	return min_t(xfs_agnumber_t, GENERIC_AG_SET_SZ, mp->m_sb.sb_agcount);

Because I'm not sure what it means on a single-AG filesystem.

> +}
> +
> +static xfs_agnumber_t
> +xfs_ag_to_ag_set(
> +	struct xfs_bmalloca	*ap,
> +	xfs_agnumber_t		base_agno)
> +{
> +	struct xfs_inode	*ip = ap->ip;
> +	struct xfs_mount	*mp = ip->i_mount;
> +	xfs_agnumber_t		set_size;
> +
> +	/* Apply fanning only for regular file data */
> +	if (!(ap->datatype & XFS_ALLOC_USERDATA))
> +		return base_agno;
> +
> +	set_size = xfs_default_ag_set_size(ip);
> +	/* Fan out within the AG set using low bits of the inode */
> +	return (base_agno + (XFS_INO_TO_AGINO(mp, I_INO(ip)) % set_size)) %
> +		mp->m_sb.sb_agcount;
> +}
> +
>  static int
>  xfs_bmap_btalloc_select_lengths(
>  	struct xfs_bmalloca	*ap,
> @@ -3587,8 +3617,16 @@ xfs_bmap_btalloc_best_length(
>  {
>  	xfs_extlen_t		blen = 0;
>  	int			error;
> +	xfs_agnumber_t		target_ag, start_ag;
>  
>  	ap->blkno = XFS_INODE_TO_FSB(ap->ip);
> +
> +	/* fan out initial AG across the generic AG set */
> +	start_ag = XFS_FSB_TO_AGNO(args->mp, ap->blkno);
> +	target_ag = xfs_ag_to_ag_set(ap, start_ag);
> +	if (target_ag != start_ag)
> +		ap->blkno = XFS_AGB_TO_FSB(args->mp, target_ag, 0);

/me wonders, if xfs_bmap_rtalloc looked at ap->blkno for a hint the way
that the data device allocator does, then would it be trivial to have
write streams on the rt device too?

I guess the tricky part would be figuring out what to do if you ever
want to switch a file between rt and data devices -- presumably you'd
just reset the write stream id to the default, but I guess you could
reject such a switch if the id had been set explicitly.

--D

> +
>  	if (!xfs_bmap_adjacent(ap))
>  		ap->eof = false;
>  
> -- 
> 2.25.1
> 
>