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Wed, 7 Oct 2026 01:50:25 -0400 (EDT) Received: by mail (OpenSMTPD) with ESMTPSA id 87110095 (TLSv1.3:TLS_AES_256_GCM_SHA384:256:NO); Wed, 7 Oct 2026 05:50:24 +0000 (UTC) Date: Wed, 7 Oct 2026 07:50:21 +0200 From: Patrick Steinhardt To: Karthik Nayak Cc: git@vger.kernel.org Subject: Re: [PATCH 00/13] odb/source-files: move alternates into the backend Message-ID: References: <20261002-pks-odb-move-alternates-v1-0-8a63507b88c4@pks.im> Precedence: bulk X-Mailing-List: git@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Tue, Oct 06, 2026 at 01:53:24PM -0700, Karthik Nayak wrote: > Patrick Steinhardt writes: > > > Hi, > > > > Originally, when designing pluggable object databases the goal was that > > the object database can have multiple sources, and every source attached > > to it could use a different backend. This would have allowed for quite a > > lot of flexibility, as you could trivially mix and match different kinds > > of object storages in whatever way you like. > > > > But while well-intentioned, this design led to a bunch of conceptual > > problems: > > > > - We're now trying to read objects in source order, whereas we > > previously tried to read objects via packfiles before trying to read > > them via loose objects. This led to a performance regression when > > using alternates or when using a quarantine directory. > > > > - Some data structures are supposed to only ever exist once, like for > > example bitmaps and commit graphs. At the same time, those data > > structures also span across the union of all objects, so they may > > cross sources. > > > > - It is unclear how we can extend GIT_OBJECT_DIRECTORY or > > GIT_ALTERNATE_OBJECT_DIRECTORIES to become backend-agnostic in a > > backwards-compatible way. In general, introducing an object storage > > extension into the current status quo where alternates may have to > > be extended to become generic was proving to be painful. > > > > - Some mechanisms of alternates assume way too much about how exactly > > their backends work. Alternate refs for example assume that the > > alternate is backed by a filesystem path, and that this filesystem > > path may also allow us to read references. This is not a given > > though, as backends may not even have local data at all. > > > > In short, there are a bunch of conceptual mismatches when we have > > alternates and pluggable object databases coexist. So while the original > > idea was nice, it does not result in a system that is easy to reason > > about. > > > > This patch series corrects course by moving alternates into the "files" > > backend itself so that they become another implementation detail. It's > > unfortunately on the bigger side, and I'm sorry about that, but I > > couldn't really find a way to split it up further in a sensible way. > > I went through the series, took attention split over two days. The > changes look good to me, but would definitely like to see another review :) Thanks for your review! Patrick