Re: [PATCH v2] btrfs: allow defrag to convert inline extents to regular extents

[David Sterba <dsterba@suse.cz>]

On Mon, May 09, 2022 at 08:00:53PM +0800, Qu Wenruo wrote:
> Btrfs defaults to max_inline=2K to make small writes inlined into
> metadata.
> 
> The default value is always a win, as even DUP/RAID1/RAID10 doubles the
> metadata usage, it should still cause less physical space used compared
> to a 4K regular extents.
> 
> But since the introduce of RAID1C3 and RAID1C4 it's no longer the case,
> users may find inlined extents causing too much space wasted, and want
> to convert those inlined extents back to regular extents.
> 
> Unfortunately defrag will unconditionally skip all inline extents, no
> matter if the user is trying to converting them back to regular extents.
> 
> So this patch will add a small exception for defrag_collect_targets() to
> allow defragging inline extents, if and only if the inlined extents are
> larger than max_inline, allowing users to convert them to regular ones.
> 
> This also allows us to defrag extents like the following:
> 
> 	item 6 key (257 EXTENT_DATA 0) itemoff 15794 itemsize 69
> 		generation 7 type 0 (inline)
> 		inline extent data size 48 ram_bytes 4096 compression 1 (zlib)
> 	item 7 key (257 EXTENT_DATA 4096) itemoff 15741 itemsize 53
> 		generation 7 type 1 (regular)
> 		extent data disk byte 13631488 nr 4096
> 		extent data offset 0 nr 16384 ram 16384
> 		extent compression 1 (zlib)
> 
> Previously we're unable to do any defrag, since the first extent is
> inlined, and the second one has no extent to merge.
> 
> Now we can defrag it to just one single extent, saving 48 bytes metadata
> space.
> 
> 	item 6 key (257 EXTENT_DATA 0) itemoff 15810 itemsize 53
> 		generation 8 type 1 (regular)
> 		extent data disk byte 13635584 nr 4096
> 		extent data offset 0 nr 20480 ram 20480
> 		extent compression 1 (zlib)
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
> Changelog:
> v2:
> - Re-phase why we add the inline extent without checking @next_mergeable
> 
> - Add some commit message on the new ability to handle mixed inline and
>   regular extents

Added to misc-next, with the updated comment, thanks.