When 45 Gigabytes Go Missing: Diagnosing a Path Resolution Bug in Filecoin's Parameter Fetching

Introduction

In the course of building a pipelined SNARK proving daemon called cuzk for Filecoin's Proof-of-Replication (PoRep) system, a team had just completed Phase 0: the entire gRPC scaffold was implemented, compiled, and validated end-to-end. The daemon could receive a 51 MB C1 proof request, deserialize it, dispatch it through a priority scheduler, invoke the real seal_commit_phase2 function from filecoin-proofs-api, and return results with timing breakdowns. Prometheus metrics tracked successes and failures. The software architecture was sound.

But there was one problem: the proof itself failed. The error was not a bug in the Rust code, nor a misconfiguration in the gRPC service, nor a scheduler deadlock. It was something far more mundane and far more frustrating: the 32 GiB Groth16 parameters—a set of cryptographic files totaling roughly 45 GiB for the critical stacked-proof-of-replication parameter alone—were not on the machine.

Message 185 of the conversation captures the moment when the assistant pivots from celebrating a working software scaffold to confronting the messy reality of production infrastructure. This is the story of that message: a diagnostic deep-dive into why curio fetch-params 32GiB failed, what it reveals about the assumptions embedded in tooling, and how the team bridged the gap between a working prototype and a runnable system.

The Context: A Working Daemon with No Parameters

To understand message 185, we must first understand what came before it. The preceding messages (180–184) document the successful end-to-end validation of the Phase 0 cuzk daemon. The assistant had built a six-crate Rust workspace (cuzk-proto, cuzk-core, cuzk-server, cuzk-daemon, cuzk-bench, cuzk-ffi) that implemented a gRPC-based proving service. The daemon could accept a SealCommitPhase1Output (the C1 result from PoRep phase 1), deserialize it from a base64-encoded JSON wrapper, and invoke the real seal_commit_phase2 function to produce a Groth16 proof.

The test was run with a pre-generated C1 input from a 32 GiB sector benchmark. The daemon processed it correctly—the error was caught, propagated back to the client, and recorded in Prometheus metrics as proofs failed: 1. But the failure was expected: the 32 GiB Groth16 parameters had never been downloaded to this machine.

The user then ran the command that would trigger the crisis:

FIL_PROOFS_PARAMETER_CACHE=/data/zk/params curio fetch-params 32GiB

The output (captured in message 184) was a firehose of log lines. The tool reported fetching dozens of files from IPFS gateways, with download paths like /data/zk/params/v28-stacked-proof-of-replication-merkletree-poseidon_hasher-8-8-0-sha256_hasher-82a357d2f2ca81dc61bb45f4a762807aedee1b0a53fd6c4e77b46a01bfef7820.params. But then came the errors: every single file failed a "sanity check" with open /data/zk/params/...: no such file or directory. The tool then tried to remove the files it thought it had downloaded, failed again, and exited with a wall of error messages.

To anyone reading this output, the situation was confusing. The tool claimed to have downloaded files to /data/zk/params/, but then claimed it couldn't find them there. What was happening?

The Diagnostic Message: Message 185

The assistant's response in message 185 is a model of systematic debugging. It begins with a concise summary of the problem:

The fetch-params failed because /data/zk/params/ doesn't exist (or has wrong permissions). The downloads went to ~/scrot/ prefix path instead. The error is open /data/zk/params/...: no such file or directory — it downloaded to a relative path under the CWD.

This single paragraph contains three distinct insights:

  1. The root cause: /data/zk/params/ doesn't exist or has wrong permissions.
  2. The actual behavior: Downloads went to ~/scrot/ prefix path instead.
  3. The mechanism: The tool interpreted the absolute path as relative to the current working directory. The assistant then runs two ls commands to verify the state of the filesystem. The first checks /data/zk/:
total 4
drwxr-xr-x  3 theuser theuser   28 Feb 17 15:19 .
drwxr-xr-x 40 theuser theuser 4096 Feb 17 15:19 ..
drwxr-xr-x  2 theuser theuser   10 Feb 17 16:00 params

This output is revealing. The /data/zk/ directory exists, and it contains a params subdirectory. But the size of 10 for the params directory indicates it is empty—a newly created directory with no files. The timestamp of 16:00 matches the time the fetch-params command was run, suggesting the tool created the directory (or it was created by a previous attempt) but never populated it.

The second command checks /data/ more broadly:

total 1018561584
drwxr-xr-x    40 theuser theuser         4096 Feb 17 15:19 .
drwxr-xr-x     1 root    root             202 Jan 31 00:29 ..
-rw-r--r--     1 theuser theuser  32949454816 Sep  1 10:46 0f68dc134d3d7cb7990f66d9ed82a3af_baga6ea4seaqnmlexpxpw6wvpp2rihlm3frvmdhvsdkwtwvcme5qowjj6dhavciy.car

This shows a 33 GiB .car file (a Filecoin/IPFS archive) sitting in /data/, likely from a previous operation. The total block size of 1018561584 bytes (about 1 GB in block-level allocation) confirms this is a large-data volume.

The Reasoning Process: Tracing the Path Bug

The assistant's diagnostic reasoning can be reconstructed as follows. The curio fetch-params tool uses the FIL_PROOFS_PARAMETER_CACHE environment variable to determine where to store downloaded parameters. The user set this to /data/zk/params, an absolute path. However, the tool internally delegates the actual download to an external downloader (likely aria2c, based on the [NOTICE] log format showing GID identifiers and download status lines). This downloader receives the target path and interprets it relative to the current working directory.

The user was running the command from ~/scrot/ (as shown in the shell prompt: theuser@biryani ~/scrot). So the downloader wrote files to ~/scrot//data/zk/params/—note the double slash in the log output, which is a classic artifact of concatenating a relative path with an absolute path. The tool's "sanity check" then tried to verify the files at the intended absolute path /data/zk/params/, which was empty, and failed.

This is a subtle but important bug. The tool's log messages show the intended path (/data/zk/params/...) in the "Fetching" lines, but the actual download destination (shown in the "Download Results" section) reveals the true path: /home/theuser/scrot//data/zk/params/.... The assistant correctly identified this discrepancy by carefully reading the log output.

Assumptions and Their Consequences

Message 185 reveals several assumptions that were baked into the tooling and the debugging process:

Assumption 1: Absolute paths are absolute. The curio fetch-params tool assumed that passing an absolute path like /data/zk/params/ to the downloader would result in files being placed at that absolute location. This assumption was wrong—the downloader treated the path as relative to CWD.

Assumption 2: The directory exists. The tool assumed that /data/zk/params/ existed and was writable. While the directory was created (possibly by a previous run or by the tool itself), it was empty because the files went elsewhere.

Assumption 3: The sanity check is reliable. The tool's post-download sanity check opened files at the intended absolute path and failed, causing it to delete and retry. But the files it deleted were at the wrong path (the relative one), so the retry also failed. This created an infinite loop of failure.

Assumption 4: The error message tells the full story. The user and assistant initially assumed that the error open /data/zk/params/...: no such file or directory meant the directory didn't exist. The ls output showed it did exist but was empty, refining the diagnosis.

Input Knowledge Required

To fully understand message 185, a reader needs knowledge of several domains:

Filecoin proof architecture: Understanding that Groth16 proofs for 32 GiB sectors require large structured reference strings (SRS) and circuit parameters, totaling tens of gigabytes. These are fetched once and cached.

The FIL_PROOFS_PARAMETER_CACHE convention: This environment variable is the standard mechanism for telling Filecoin proof libraries where to find pre-downloaded parameters. It is used by filecoin-proofs, bellperson, and related tools.

Path resolution semantics: Understanding the difference between absolute paths (starting with /) and relative paths, and how tools like aria2c handle path arguments.

The curio toolchain: Curio is a Filecoin storage mining implementation that includes parameter fetching as a subcommand. The fetch-params command downloads the necessary proving parameters from IPFS gateways.

The cuzk project context: Message 185 is part of a larger effort to build a pipelined SNARK proving daemon. The parameter issue was the final blocker before the first successful proof execution.

Output Knowledge Created

Message 185 produced several valuable pieces of knowledge:

  1. The root cause diagnosis: The fetch-params tool has a path resolution bug when run from a non-root directory with an absolute FIL_PROOFS_PARAMETER_CACHE path.
  2. The actual file locations: Files were confirmed to be at ~/scrot//data/zk/params/ (relative to CWD), not at /data/zk/params/.
  3. The state of the filesystem: /data/zk/params/ existed but was empty; /data/ contained a 33 GiB .car file from prior operations.
  4. A path forward: The diagnosis enabled the next steps—copying the files from the relative path to the absolute path, or re-running the fetch from the correct directory.

The Broader Significance

Message 185 is, on its surface, a simple debugging message. An assistant runs two ls commands and explains why a parameter download failed. But in the context of the larger project, it represents a critical transition point.

The team had just finished building a sophisticated distributed proving system with gRPC, priority scheduling, Prometheus metrics, and multi-crate workspace organization. All of that engineering work was validated and working. But the system could not produce a single proof because of a path resolution bug in a download tool. This is the classic tension between architecture and infrastructure: no matter how elegant the software design, it is useless if the environment is not properly configured.

The message also illustrates a key skill in systems engineering: the ability to read error messages carefully and trace through the chain of assumptions. The fetch-params output was overwhelming—dozens of "Fetching" lines, followed by dozens of "ERROR" lines, followed by a wall of "remove file failed" messages. Most readers would see the error and assume the tool simply couldn't download. The assistant instead noticed the discrepancy between the "Fetching" paths (absolute) and the "Download Results" paths (relative to CWD), and used that to pinpoint the bug.

Conclusion

Message 185 is a masterclass in diagnostic reasoning. It takes a confusing, multi-megabyte log output and distills it to a single root cause: a path resolution bug in the parameter fetching tool. The assistant's analysis—that the downloader interpreted an absolute path as relative to the current working directory—was correct, as confirmed by subsequent messages where the files were found at ~/scrot/data/zk/params/ and copied to the correct location.

This message also serves as a reminder that in complex systems, the most frustrating failures are often the simplest ones. Not a GPU kernel bug, not a memory corruption, not a protocol mismatch—just a tool that didn't handle absolute paths correctly. The ability to recognize and articulate such problems is what separates effective debugging from guesswork.