Re: [PATCH] btrfs: index buffer_tree using node size

[David Sterba <dsterba@suse.cz>]

On Mon, May 12, 2025 at 07:23:20PM +0200, Daniel Vacek wrote:
> So far we are deriving the buffer tree index using the sector size. But each
> extent buffer covers multiple sectors. This makes the buffer tree rather sparse.
> 
> For example the typical and quite common configuration uses sector size of 4KiB
> and node size of 16KiB. In this case it means the buffer tree is using up to
> the maximum of 25% of it's slots. Or in other words at least 75% of the tree
> slots are wasted as never used.
> 
> We can score significant memory savings on the required tree nodes by indexing
> the tree using the node size instead. As a result far less slots are wasted
> and the tree can now use up to all 100% of it's slots this way.

This looks interesting. Is there a way to get xarray stats? I don't see
anything in the public API, e.g. depth, fanout, slack per level. For
debugging purposes we can put it to sysfs or as syslog message,
eventually as non-debugging output to commit_stats.