linux-kernel - Re: [PATCH] btrfs: index buffer

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Message-ID: <20250512202054.GX9140@twin.jikos.cz>
Date: Mon, 12 May 2025 22:20:54 +0200
From: David Sterba <dsterba@...e.cz>
To: Daniel Vacek <neelx@...e.com>
Cc: Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
	David Sterba <dsterba@...e.com>, linux-btrfs@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] btrfs: index buffer_tree using node size

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.