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From: Kent Overstreet <kent.overstreet@linux.dev>
To: Viacheslav Dubeyko <slava@dubeyko.com>
Cc: lsf-pc@lists.linux-foundation.org, linux-bcachefs@vger.kernel.org, 
	linux-fsdevel@vger.kernel.org
Subject: Re: [LSF/MM/BPF TOPIC] bcachefs
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On Tue, Jan 02, 2024 at 11:02:59AM +0300, Viacheslav Dubeyko wrote:
> 
> 
> > On Jan 2, 2024, at 1:56 AM, Kent Overstreet <kent.overstreet@linux.dev> wrote:
> > 
> > LSF topic: bcachefs status & roadmap
> > 
> 
> <skipped>
> 
> > 
> > A delayed allocation for btree nodes mode is coming, which is the main
> > piece needed for ZNS support
> > 
> 
> I could miss some emails. But have you shared the vision of ZNS support
> architecture for the case of bcachefs already? It will be interesting to hear
> the high-level concept.

There's not a whole lot to it. bcache/bcachefs allocation is already
bucket based, where the model is that we allocate a bucket, then write
to it sequentially and never overwrite until the whole bucket is reused.

The main exception has been btree nodes, which are log structured and
typically smaller than a bucket; that doesn't break the "no overwrites"
property ZNS wants, but it does mean writes within a bucket aren't
happening sequentially.

So I'm adding a mode where every time we do a btree node write we write
out the whole node to a new location, instead of appending at an
existing location. It won't be as efficient for random updates across a
large working set, but in practice that doesn't happen too much; average
btree write size has always been quite high on any filesystem I've
looked at.

Aside from that, it's mostly just plumbing and integration; bcachefs on
ZNS will work pretty much just the same as bcachefs on regular block devices.