Re: [PATCH] btrfs: defrag: add under utilized extent to defrag target list

[David Sterba <dsterba@suse.cz>]

On Fri, Jan 05, 2024 at 06:03:40PM +1030, Qu Wenruo wrote:
> [BUG]
> The following script can lead to a very under utilized extent and we
> have no way to use defrag to properly reclaim its wasted space:
> 
>   # mkfs.btrfs -f $dev
>   # mount $dev $mnt
>   # xfs_io -f -c "pwrite 0 128M" $mnt/foobar
>   # sync
>   # btrfs filesystem defrag $mnt/foobar
>   # sync

I don't see what's wrong with this example, as Filipe noted there's a
truncate missing, but still this should be explained better.

Is this the problem when an overwritten and shared extent is partially
overwritten but still occupying the whole range, aka. bookend extent?
If yes, defrag was never meant to deal with that, though we could use
the interface for that.

As Andrei pointed out, this is more like a garbage collection, get rid
of extent that is partially unreachable. Detecting such extent requires
looking for the unreferenced part of the extent while defragmentation
deals with live data. This could be a new ioctl entirely too. But first
I'd like to know if we're talking about the same thing.