On Tue, Jan 02, 2024 at 11:02:59AM +0300, Viacheslav Dubeyko wrote: > > > > On Jan 2, 2024, at 1:56 AM, Kent Overstreet <[email protected]> > > 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.
