Hot-Swapping a GPU Proving Binary: A Critical Deployment in the Debugging Trenches

In the middle of a high-stakes debugging session targeting production failures in a zero-knowledge proof system, message [msg 1967] captures a moment of decisive operational action. The assistant executes a single SSH command that hot-swaps the cuzk GPU proving binary on a remote production instance, replacing a suspected faulty binary with one known to pass benchmarks. The command is terse, surgical, and carries significant operational risk—yet it represents a logical next step in a systematic investigation that had already ruled out data differences, configuration mismatches, and code path variations.

The Message

The assistant runs the following command over SSH:

ssh -p [PORT] root@[HOST_IP] 'mv /usr/local/bin/cuzk /usr/local/bin/cuzk-bad && mv /tmp/cuzk-container /usr/local/bin/cuzk && chmod +x /usr/local/bin/cuzk && md5sum /usr/local/bin/cuzk && ls -la /usr/local/bin/cuzk*'

The output confirms success:

e6b3ed802a5f9b0cba5487ca1a2f80d2  /usr/local/bin/cuzk
-rwxr-xr-x 1 root root 27905520 Mar 13 09:00 /usr/local/bin/cuzk
-rwxr-xr-x 1 root root 27905520 Mar 13 07:26 /usr/local/bin/cuzk-bad
-rwxr-xr-x 1 root root  5765768 Mar 12 10:39 /usr/local/bin/cuzk-bench
-rwxr-xr-x 1 root root 27895032 Mar 13 07:26 /usr/local/bin/cuzk-old

The new binary has hash e6b3ed802a5f9b0cba5487ca1a2f80d2—identical to the container-built binary that had passed all benchmarks. The old binary is preserved as cuzk-bad for rollback. Two other binaries remain: cuzk-bench (the benchmarking tool) and cuzk-old (an even earlier version).

The Reasoning: Why This Message Was Written

To understand why the assistant issued this command, we must trace the debugging journey that preceded it. The user had reported a baffling production failure: all ten PoRep (Proof of Replication) partitions were producing invalid proofs when the cuzk daemon ran under the production run.sh script with Curio. Yet the exact same benchmark, run inside a Docker container with the same configuration (partition_workers=16) and the same sector data (miner ID 1000, sector 1), passed without issue.

This contradiction—benchmark passes, production fails—presented a classic systems debugging puzzle. The assistant methodically worked through the possibilities:

1. Data differences: The user confirmed ([msg 1961]) that the benchmark sector was "similar, maybe even the same" as the production data. Ruled out.
2. Configuration differences: Both benchmark.sh and run.sh used partition_workers=16. Ruled out.
3. Code path differences: The assistant examined the engine.rs Phase 7 partitioned proving flow ([msg 1960]) and found no structural divergence between benchmark and production paths.
4. Binary differences: This was the remaining hypothesis. The assistant checked the host binary hash (34295ce6... in [msg 1962]) against the container binary hash (e6b3ed80... in [msg 1963]). They were different. The container's binary—freshly built from the full Dockerfile.cuzk—worked. The host's binary—built earlier from the minimal Dockerfile.cuzk-rebuild—did not. The binary mismatch hypothesis was compelling. If the container build and the host build produced different binaries, perhaps a compiler flag, a dependency version, or a caching issue had introduced a subtle bug in the host binary that caused systematic proof failures. The logical next step was to deploy the known-good container binary to the host.

Operational Decisions Embedded in the Command

The command reveals several deliberate operational choices:

Atomic replacement via mv: The assistant uses mv (move) rather than cp (copy) to place the new binary. On Linux, a mv within the same filesystem is atomic—the file's directory entry is updated instantaneously. If the cuzk daemon were restarted during the replacement, it would either see the old binary or the new one, never a partially-written file. This is a production-grade deployment technique.

Backup before replacement: The old binary is moved to cuzk-bad before the new one is installed. This enables instant rollback: if the new binary crashes or behaves unexpectedly, the operator can restore the old one with a single mv command. The assistant learned this lesson the hard way—earlier in the session ([msg 1962]), a failed chained kill + mv command had left the system in an inconsistent state.

Verification via md5sum: The command concludes with a hash check and directory listing, providing immediate confirmation that the deployment succeeded and the hash matches expectations. This is not cosmetic—it is the critical feedback loop that closes the loop on the hypothesis. Without this verification, the assistant would not know whether the binary was actually replaced.

Preservation of history: The directory listing shows cuzk-old (from a March 12 build) and cuzk-bench alongside the new files. The assistant is maintaining a version history on the production host, enabling forensic comparison if needed.

Assumptions and Their Risks

The assistant made several assumptions in issuing this command:

The container binary is fully compatible with the host environment. The container was built on nvidia/cuda:13.0.2-devel-ubuntu24.04, while the host runs a different Ubuntu version with potentially different CUDA runtime libraries. If the binary links against shared libraries not present on the host, it would fail at load time. The assistant implicitly assumed the CUDA runtime compatibility was sufficient.

The binary mismatch is the root cause. This was the strongest assumption—and, as the chunk summary reveals, it was incorrect. The real root cause was a job ID collision: concurrent proofshare challenges all targeted the same miner=1000, sector=1, producing identical job_id values that caused the cuzk engine's partition assembler to mix results from different proofs. The binary swap would not fix this.

No running process depends on the old binary. The command does not stop the cuzk daemon before replacing the binary. If the daemon were running, the old binary's file would remain accessible via the running process's file descriptor even after the directory entry was moved. A subsequent restart would pick up the new binary—but the assistant did not explicitly handle this transition.

Input Knowledge Required

To understand this message, a reader must grasp several layers of context:

• The cuzk architecture: cuzk is a GPU-accelerated proving daemon for Filecoin proofs. It uses a partitioned proving pipeline (partition_workers) where multiple GPU workers process proof partitions in parallel. The ProofAssembler collects and verifies partition results.
• The Docker build workflow: Two Dockerfiles exist—Dockerfile.cuzk (full build) and Dockerfile.cuzk-rebuild (minimal rebuild). The assistant had used the minimal rebuild earlier to deploy the self-check fix, but the resulting binary had a different hash from the full build.
• The benchmark vs. production distinction: benchmark.sh runs inside the Docker container using the container's binary, while run.sh runs on the host using the host's binary. This distinction was the key insight that led to the binary mismatch hypothesis.
• SSH and remote file management: The command uses SSH with a non-standard port to connect to a remote vast.ai instance and perform file operations.

Output Knowledge Created

The message produces several concrete outputs:

1. A deployed binary: The host now has /usr/local/bin/cuzk with hash e6b3ed802a5f9b0cba5487ca1a2f80d2, matching the container binary.
2. A preserved backup: The old binary is at /usr/local/bin/cuzk-bad with hash 34295ce6..., enabling rollback.
3. Verification evidence: The md5sum output and directory listing provide immediate confirmation that the deployment was successful.
4. A test of the binary mismatch hypothesis: The real output of this message is not the files on disk, but the knowledge gained when the system is tested next. If the binary swap fixes the proof failures, the hypothesis is confirmed. If not, the assistant must look deeper—which is exactly what happened.

The Broader Debugging Context

This message sits within a larger narrative of systematic debugging across multiple sessions. The assistant had already:

• Fixed a deadlock in TaskRequestProofs caused by HTTP 429 retries blocking the poll loop ([msg 1960] context)
• Added queue cleanup and dedup improvements
• Built and deployed a cuzk binary with self-check enforcement The proof failure investigation was the second major thread in this chunk. The binary swap was a reasonable step, but it turned out to be a red herring. The real fix—adding the harmony task ID to the cuzk RequestId to prevent job ID collisions—required a deeper understanding of how concurrent proofshare challenges interacted with the cuzk engine's partition assembler.

Conclusion

Message [msg 1967] is a snapshot of a developer in the trenches of a production debugging session, acting on the best available hypothesis with surgical precision. The command is technically competent—atomic file replacement, backup preservation, hash verification—but it ultimately addressed the wrong root cause. This is not a failure of the assistant's reasoning; it is the natural rhythm of debugging, where hypotheses are tested and discarded until the true cause emerges.

The message also reveals the operational complexity of GPU proving systems. Binaries must be built with the right CUDA version, deployed to remote instances, verified by hash, and tested against real production workloads. A simple mv command carries the weight of all this context. And when the hypothesis proves wrong, the assistant pivots—preserving the old binary for rollback, and continuing the investigation until the real culprit (the job ID collision) is found and fixed.