The default arrangement is the simple one: every node keeps its own copy.
So a source naming a remote file is now fetched before the engine is
launched, rather than left to fail as a missing path minutes into a load.
Eight shards of a tensor-parallel replica start at the same instant and
want the same file; they produce one download. An interrupted fetch resumes
instead of starting over, which matters at forty gigabytes, and the
download is written beside its target and renamed only when complete — a
partial file under the model's name would be found by the next scan,
reported as present, and handed to an engine that fails on it.
The exotic arrangements need no code, which is the point. Weights on a
Lustre mount or an image exported over the fabric are simply present, and
present means nothing is fetched. The escape hatch from a distribution
mechanism is a distribution mechanism that checks first.
Wiring it up caught a regression in my own design. A bare model id —
"Qwen/Qwen2.5-7B-Instruct" — is a documented source that vLLM and SGLang
resolve themselves, and treating every non-remote source as a local path
made every existing vLLM profile fail. Only an absolute or explicitly
relative path is now checked as a file; anything else is passed through
untouched. Three agent tests failed on the same change for a better reason:
they used "/models/x", and the new check is exactly the one that catches
that.
Whole repositories of shards are deliberately not fetched here. vLLM and
SGLang do that well already, and a second snapshot downloader would be work
spent arriving back where we started.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
