Re: [PATCH] fetch, clone: add fetch.blobSizeLimit config

[Jeff King <peff@peff.net>]

On Mon, Mar 02, 2026 at 12:53:32PM +0100, Patrick Steinhardt wrote:

> On Sun, Mar 01, 2026 at 04:44:59PM +0000, Alan Braithwaite via GitGitGadget wrote:
> > From: Alan Braithwaite <alan@braithwaite.dev>
> > 
> > External tools like git-lfs and git-fat use the filter clean/smudge
> > mechanism to manage large binary objects, but this requires pointer
> > files, a separate storage backend, and careful coordination. Git's
> > partial clone infrastructure provides a more native approach: large
> > blobs can be excluded at the protocol level during fetch and lazily
> > retrieved on demand. However, enabling this requires passing
> > `--filter=blob:limit=<size>` on every clone, which is not
> > discoverable and cannot be set as a global default.
> 
> I'm not sure that we should make blob size limiting the default. The
> problem with specifying a limit is that this is comparatively expensive
> to compute on the server side: we have to look up each blob so that we
> can determine its size. Unfortunately, such requests cannot (currently)
> be optimized via for example bitmaps, or any other cache that we have.

We actually can do blob:limit filters with bitmaps. See 84243da129
(pack-bitmap: implement BLOB_LIMIT filtering, 2020-02-14). It's more
expensive than blob:none, but not much. Once we have the list of blobs
we can get their sizes directly from the packfile. It's stuff like
path-limiting that is truly expensive, because it requires a traversal.

All that said, I'd be wary of turning on partial clones like this by
default. I feel like there are still a lot of performance gotchas
lurking (and possibly some correctness ones, too).

-Peff