Re: [PATCH 0/2] btrfs-progs: add check and repair ability for super num devices mismatch

[David Sterba <dsterba@suse.cz>]

On Thu, Mar 24, 2022 at 07:30:31AM +0800, Qu Wenruo wrote:
> 
> 
> On 2022/3/24 01:17, David Sterba wrote:
> > On Mon, Feb 28, 2022 at 08:50:06AM +0800, Qu Wenruo wrote:
> >> The patchset can be fetched from github:
> >> https://github.com/adam900710/btrfs-progs/tree/super_num_devs
> >>
> >> The 2nd patch contains a compressed raw image, thus it may be a little
> >> too large for the mail list.
> >
> > The compressed size is 22K, that's fine. I've recompressed it with 'xz
> > --best -e' and it shaved a few hundred bytes but that was just out of
> > curiosity.
> 
> I'm also a little interested in why things like zstd/xz is not that
> efficient in compressing raw btrfs images.

I think it is efficient, unpacked image was 128M and getting that to 22K
is a good result.

> Especially when creating the image I have created the image with all
> zero (fallocated) file in the first place, thus most space should just
> be all zero, and any compression method should be super efficient on them.
> 
> My current guesses are:
> 
> - DUP metadata is not fully detected by the algo
>    Especially when they are mostly quite some ranges away
> 
> - Some older tree blocks from mkfs
> 
> So for further raw image size reduction, we may want to:
> 
> - Use SINGLE metadata/data
> 
> - Fstrim the image first

Yeah that could remove any stale blocks.