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

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Message-ID: <CAPjX3FcLkgk4CbtwfFqNN+J=c5kTC0S=LYyeyryVKuSk_KF6Gw@mail.gmail.com>
Date: Fri, 4 Jul 2025 18:09:20 +0200
From: Daniel Vacek <neelx@...e.com>
To: dsterba@...e.cz
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, 23 Jun 2025 at 16:25, David Sterba <dsterba@...e.cz> wrote:
>
> 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.

I've posted the v3 just now.

https://lore.kernel.org/linux-btrfs/20250704160704.3353446-1-neelx@suse.com/