Re: [PATCH] xfs: do not tight-pack write large files

[Christoph Hellwig <hch@lst.de>]

On Mon, Oct 13, 2025 at 03:45:12PM +0900, Damien Le Moal wrote:
> The tick-packing data block allocation which writes blocks of closed
> files in the same zone is very efficient at improving write performance
> on HDDs by reducing, and even suppressing, disk head seeks. However,
> such tight packing does not make sense for large files that require at
> least a full realtime block group (i.e. a zone). If tight-packing
> placement is applied for such files, the VM writeback thread switching
> between inodes result in the large file to be fragmented, thus
> increasing the garbage collection penalty later when the used realtime
> block group/zone needs to be reclaimed.
> 
> This problem can be avoided with a simple heuristic: if the size of the
> inode being written back is at least equal to the realtime block group
> size, do not use tight-packing. Modify xfs_zoned_pack_tight() to always
> return false in this case.
> 
> With this change, a multi-writer workload writing files of 256 MB on a
> file system backed by an SMR HDD with 256 MB zone size sees all files
> occupying exactly one zone, thus completely removing the heavy
> fragmentation observed without this change.
> 
> Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
> ---
>  fs/xfs/xfs_zone_alloc.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/fs/xfs/xfs_zone_alloc.c b/fs/xfs/xfs_zone_alloc.c
> index 1147bacb2da8..c51788550c7c 100644
> --- a/fs/xfs/xfs_zone_alloc.c
> +++ b/fs/xfs/xfs_zone_alloc.c
> @@ -622,6 +622,17 @@ static inline enum rw_hint xfs_inode_write_hint(struct xfs_inode *ip)
>   */
>  static inline bool xfs_zoned_pack_tight(struct xfs_inode *ip)
>  {
> +	struct xfs_mount *mp = ip->i_mount;
> +	size_t zone_capacity =
> +		XFS_FSB_TO_B(mp, mp->m_groups[XG_TYPE_RTG].blocks);
> +
> +	/*
> +	 * Do not pack tight large files that are already using a full group

I'm not a native speaker, but shouldn't this be ordered differently

	  Do not pack large files that are already using a full group (zone)
	  to avoid fragmentation?

Also I'd say either zone or RTG. but not mix both names to avoid confusion.

Otherwise this looks good to me.