linux-kernel - Re: [PATCH v2] btrfs: index buffer

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Message-ID: <20250623142501.GA28944@suse.cz>
Date: Mon, 23 Jun 2025 16:25:02 +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>, Qu Wenruo <wqu@...e.com>,
	linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] btrfs: index buffer_tree using node size

On Mon, Jun 23, 2025 at 04:04:39PM +0200, Daniel Vacek wrote:
> On Fri, 20 Jun 2025 at 14:57, David Sterba <dsterba@...e.cz> wrote:
> >
> > On Thu, Jun 12, 2025 at 10:47:23AM +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.
> > >
> > > Note: This works even with unaligned tree blocks as we can still get unique
> > >       index by doing eb->start >> nodesize_shift.
> >
> > Can we have at least some numbers? As we've talked about it and you
> > showed me the number of radix nodes or other internal xarray structures
> > before/after.
> 
> The numbers are in this email thread. Do you mean to put them directly
> into the commit message?

Yes, it's relevant information and part of why the patch is done. Mails
are for discussion, for permanent stoarge we want it in changelog in a
presentable form.