Re: [PATCH 3/3] btrfs: defrag: remove the physical adjacent extents rejection in defrag_check_next_extent()

[Filipe Manana <fdmanana@kernel.org>]

On Wed, Jan 26, 2022 at 08:58:50AM +0800, Qu Wenruo wrote:
> There is a rejection for physically adjacent extents in
> defrag_check_next_extent() from the very beginning.
> 
> The check will reject physically adjacent extents which are also large
> enough.
> 
> The extent size threshold check is now a generic check, and the
> benefit of rejecting physically adjacent extents is unsure.
> 
> Sure physically adjacent extents means no extra seek time, thus
> defragging them may not bring much help.
> 
> On the other hand, btrfs also benefits from reduced number of extents
> (which can reduce the size of extent tree, thus reduce the mount time).

And also reduce the number of file extent items in the subvolume's tree.

I think it's fine.

Reviewed-by: Filipe Manana <fdmanana@suse.com>

Thanks.

> 
> So such rejection is not a full win.
> 
> Remove such check, and policy on which extents should be defragged is
> mostly on @extent_thresh and @newer_than parameters.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>  fs/btrfs/ioctl.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index 2911df12fc48..0929d08bb378 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -1069,9 +1069,6 @@ static bool defrag_check_next_extent(struct inode *inode, struct extent_map *em,
>  	/* Extent is already large enough */
>  	if (next->len >= extent_thresh)
>  		goto out;
> -	/* Physically adjacent */
> -	if ((em->block_start + em->block_len == next->block_start))
> -		goto out;
>  	ret = true;
>  out:
>  	free_extent_map(next);
> -- 
> 2.34.1
>