The 45-Gigabyte Misplacement: A Case Study in Path Resolution Bugs During SNARK Proving Infrastructure Deployment

Introduction

In the course of building a high-performance pipelined SNARK proving engine for Filecoin, a single command execution — FIL_PROOFS_PARAMETER_CACHE=/data/zk/params curio fetch-params 32GiB — produced over 200 lines of log output that encapsulate a perfect microcosm of the challenges inherent in deploying real-world zero-knowledge proof infrastructure. This message, submitted by a user to an AI assistant during a pair-programming session, documents not just a parameter download but a cascading failure rooted in a subtle path resolution bug in the curio tool's fetch-params subcommand. The resulting error cascade — files downloaded to the wrong directory, sanity checks failing on nonexistent paths, and a final error message listing dozens of failed file removals — provides a rich case study in how assumptions about working directories, absolute versus relative paths, and tool behavior can derail an otherwise well-architected system.

This article examines that single message in depth: the reasoning that led to its execution, the assumptions embedded in the command, the bug it revealed, the diagnostic knowledge it produced, and the broader lessons it offers for anyone building or operating cryptographic proof infrastructure. The message sits at a critical juncture in a larger project — the implementation of Phase 0 of the cuzk pipelined SNARK proving daemon — where the software architecture had been validated end-to-end, but the real-world environmental dependencies (specifically, ~47 GiB of Groth16 parameters) were still missing. The user's command was the final bridge between a working scaffold and a fully operational proof pipeline.

Context: The cuzk Proving Engine and Its Parameter Dependency

To understand the significance of this message, one must first understand the broader project. The cuzk (pronounced "cuzk" — a pipelined SNARK proving daemon) was being built to address a critical bottleneck in Filecoin storage proving. Filecoin miners must periodically generate Groth16 zero-knowledge proofs to demonstrate they are still storing their pledged data. These proofs — particularly the "C2" (Commit Phase 2) step of Proof-of-Replication (PoRep) — are computationally intensive, requiring GPU-accelerated Number Theoretic Transforms (NTTs), Multi-Scalar Multiplications (MSMs), and other operations on elliptic curve groups over the BLS12-381 curve. The existing proof generation pipeline, embedded in the supraseal-c2 library and orchestrated by the Curio mining framework, suffered from a ~200 GiB peak memory footprint and high per-proof latency, largely because each proof invocation loaded and discarded the Structured Reference String (SRS) parameters — the ~45 GiB Groth16 proving key — from disk.

The cuzk project aimed to solve this by architecting a persistent proving daemon that keeps the SRS parameters resident in GPU memory across proof invocations, eliminating the repeated I/O overhead. Phase 0 — the scaffold — had just been implemented: a Rust workspace with six crates, a gRPC API for submitting proof jobs, a priority scheduler, and a prover module wired to the real filecoin-proofs-api calls. The assistant and user had successfully validated the entire gRPC pipeline: the daemon started, accepted a 51 MB C1 proof input, deserialized it, dispatched it to a worker, called seal_commit_phase2(), and returned an error — all without the actual parameters being present. The error was expected: seal_commit_phase2 failed because the 32 GiB Groth16 parameters weren't available in the parameter cache directory.

The next step was obvious: fetch the parameters. The user ran the command that produced the subject message.

The Command and Its Assumptions

The command was straightforward:

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

This command sets an environment variable FIL_PROOFS_PARAMETER_CACHE pointing to /data/zk/params and then invokes curio fetch-params 32GiB to download the parameters for 32 GiB sectors. The assumptions embedded in this command are worth examining:

Assumption 1: The environment variable controls the download target. The user assumed that FIL_PROOFS_PARAMETER_CACHE would be read by curio fetch-params to determine where to place the downloaded files. This is a reasonable assumption — the same environment variable is used by the Filecoin proof libraries (filecoin-proofs, bellperson, etc.) to locate parameters at runtime. However, as the output reveals, the fetch-params subcommand may resolve the path relative to the current working directory rather than treating it as an absolute path, or it may have a different mechanism for determining the download location.

Assumption 2: The directory /data/zk/params exists and is writable. The user had previously checked that /data/zk/params was the intended parameter cache location. The assistant had recommended this path in earlier messages. However, the directory structure may not have been fully set up, or the permissions may not have been correct.

Assumption 3: The command was being run from a neutral working directory. The user was in ~/scrot when executing the command. This turns out to be the critical factor — the fetch-params tool appears to resolve the parameter cache path relative to the current working directory when the path is specified as an absolute path via environment variable, or it may construct download paths by concatenating the cache directory with the CWD.

Assumption 4: The download would complete successfully and the parameters would be available for the next daemon run. This was the operational goal — to enable a full end-to-end proof generation test.

The Output: A Chronicle of Failure

The output begins innocuously enough, with INFO-level log messages showing the tool fetching various parameter and verification key files from IPFS gateways:

2026-02-17T16:00:31.045+0100	INFO	paramfetch	fastparamfetch/paramfetch.go:258	Fetching /data/zk/params/v28-proof-of-spacetime-fallback-merkletree-poseidon_hasher-8-8-0-0377ded656c6f524f1618760bffe4e0a1c51d5a70c4509eedae8a27555733edc.vk from https://proofs.filecoin.io/ipfs/

The naming convention reveals the Filecoin proof parameter taxonomy: v28 indicates the parameter version, stacked-proof-of-replication or proof-of-spacetime-fallback indicates the proof type, merkletree-poseidon_hasher and sha256_hasher indicate the hash functions used in the Merkle tree commitments, and the numeric triple 8-8-0 or 8-8-2 encodes the sector size class (8-8-0 = 32 GiB V1.1, 8-8-2 = 32 GiB V1.2). The long hexadecimal suffix is a content hash for integrity verification.

The download proceeds with multiple concurrent IPFS downloads, as indicated by the [NOTICE] Downloading 1 item(s) messages from what appears to be an IPFS/aria2 download backend. Then the first sign of trouble appears:

02/17 16:00:31 [NOTICE] GID#293fc1a69ee6ad8c - Download has already completed: /home/theuser/scrot//data/zk/params/v28-empty-sector-update-merkletree-poseidon_hasher-8-0-0-fb9e095bebdd77511c0269b967b4d87ba8b8a525edaa0e165de23ba454510194.vk

Notice the path: /home/theuser/scrot//data/zk/params/.... The double slash (//) is a telltale sign of string concatenation gone wrong. The files are being written to a path that prepends the current working directory (/home/theuser/scrot) to the parameter cache path (/data/zk/params), resulting in /home/theuser/scrot//data/zk/params/.... The double slash occurs because the concatenation likely joins the CWD (with trailing slash) with the absolute path (with leading slash), producing a technically valid but semantically incorrect path.

This is the root cause of the failure. The fetch-params tool, when given a parameter cache path via environment variable, is not treating it as an absolute path for download purposes. Instead, it appears to be resolving the path relative to the current working directory, or it's using the environment variable as a suffix to the CWD. The exact mechanism depends on the implementation in fastparamfetch/paramfetch.go, but the effect is clear: files land in the wrong directory.

The output then shows a series of "Download complete" messages, each confirming that files were written to the ~/scrot//data/zk/params/ path. These are followed by a cascade of ERROR messages:

2026-02-17T16:00:31.146+0100	ERROR	paramfetch	fastparamfetch/paramfetch.go:161	sanity checking fetched file failed, removing and retrying: %!w(*fs.PathError=&{open /data/zk/params/v28-fil-inner-product-v1.srs 2})

The tool is trying to sanity-check the downloaded files by opening them at the intended path (/data/zk/params/...), but the files were actually written to ~/scrot//data/zk/params/.... The open call fails with "no such file or directory" because the directory /data/zk/params/ either doesn't exist or is empty. The tool then attempts to remove the file (presumably the one it thought it was checking) and retry the download. But the remove also fails because the file isn't at that path either.

This error repeats for every single file that was downloaded — 20+ ERROR messages, each with the same pattern: "sanity checking fetched file failed, removing and retrying" followed by a path error. The tool enters a futile loop: download to the wrong path, fail to verify at the right path, attempt to remove at the right path (which fails because the file isn't there), and presumably retry the download to the same wrong path.

The final output is a massive aggregated error message:

ERROR: fetching proof parameters: remove file /data/zk/params/v28-fil-inner-product-v1.srs failed: remove /data/zk/params/v28-fil-inner-product-v1.srs: no such file or directory; remove file /data/zk/params/v28-empty-sector-update-merkletree-poseidon_hasher-8-0-0-fb9e095bebdd77511c0269b967b4d87ba8b8a525edaa0e165de23ba454510194.vk failed: remove /data/zk/params/v28-empty-sector-update-merkletree-poseidon_hasher-8-0-0-fb9e095bebdd77511c0269b967b4d87ba8b8a525edaa0e165de23ba454510194.vk: no such file or directory; ...

This is a laundry list of every file that the tool tried and failed to remove, each with the same "no such file or directory" error. The message is truncated in the display (ending with "investigate"), but the pattern is clear: the tool considers the entire operation failed and reports every failed removal.

The Diagnostic Value: What This Message Reveals

Despite being a failure, this message is extraordinarily informative. It reveals:

1. The exact path resolution bug. The download paths contain ~/scrot//data/zk/params/ while the sanity check paths use /data/zk/params/. This discrepancy immediately identifies the root cause: the tool is constructing download paths by joining the CWD with the parameter cache path, rather than using the parameter cache path directly.

2. The complete set of parameters required for 32 GiB proving. The output lists every VK and params file that the tool attempted to fetch. This is valuable documentation of the parameter dependency graph. The key files include:

The Reasoning Behind the Message

Why did the user run this command at this specific moment? The reasoning chain is traceable through the conversation history:

  1. The gRPC pipeline was validated. In the immediately preceding messages (msg 157-180), the assistant had started the cuzk daemon, submitted a 51 MB PoRep C1 proof via the cuzk-bench tool, and confirmed that the full pipeline worked — the daemon received the request, deserialized the C1 wrapper, decoded the base64-encoded SealCommitPhase1Output, dispatched it to the GPU worker, and called seal_commit_phase2(). The proof failed with seal_commit_phase2 failed because the 32 GiB parameters weren't available.
  2. The parameter location was identified. The assistant had checked /data/zk/params/ and found it empty (msg 159). The default location /var/tmp/filecoin-proof-parameters/ contained only small-sector (2 KiB) parameters, not the 32 GiB ones needed for the test.
  3. The fix was prescribed. In msg 183, the assistant explicitly stated: "The 32 GiB Groth16 parameters (~47 GiB .params file) need to be fetched: FIL_PROOFS_PARAMETER_CACHE=/data/zk/params curio fetch-params 32GiB"
  4. The user executed the prescribed command. The user ran the exact command from the assistant's recommendation, from their shell in the ~/scrot directory. The user's motivation was to clear the final obstacle to a fully successful proof generation test. The software architecture was proven; only the environmental dependency remained. This command was the logical next step.

Mistakes and Incorrect Assumptions

Several mistakes and incorrect assumptions are visible in this message:

Mistake 1: Running fetch-params from a non-root working directory. The user was in ~/scrot when executing the command. While this shouldn't matter for a well-behaved tool that respects absolute paths, the curio fetch-params implementation clearly has a path resolution bug that makes the CWD matter. The fix would be to run the command from / or from a directory where the relative path resolution produces the correct result.

Mistake 2: Assuming FIL_PROOFS_PARAMETER_CACHE controls download destination. The environment variable is used by the proof libraries at runtime to locate parameters, but the fetch-params tool may use a different mechanism or may resolve paths differently. The tool's behavior suggests it concatenates the CWD with the parameter cache path rather than using the path directly.

Mistake 3: Not verifying the directory existed before running. The user (and assistant) assumed /data/zk/params was ready. Subsequent investigation (msg 186) showed the directory existed but was empty — it had been created by a previous operation but never populated. The fetch-params tool may have attempted to create it but failed due to permissions, or the path resolution bug caused it to create the directory under ~/scrot/ instead.

Mistake 4: The tool's error handling is self-defeating. The sanity check opens files at the intended path, fails, then tries to remove them at that path (which also fails), and presumably retries the download to the same wrong path. This creates an infinite loop of download → verify-fail → remove-fail → retry. The tool eventually gives up with a composite error, but not before generating dozens of error messages.

Assumption about parameter versions. The command requests 32GiB parameters, but the output shows both 8-8-0 and 8-8-2 variants being fetched. The 8-8-0 variant corresponds to V1.1 sectors (the older format), while 8-8-2 corresponds to V1.2 (the newer format). The test data at /data/32gbench/ uses StackedDrg32GiBV1_1 (the V1.1 proof type), so the 8-8-0 params are the correct ones. But the tool fetches both, which is either thorough or wasteful depending on perspective.

Input Knowledge Required to Understand This Message

To fully understand this message, a reader needs:

1. Filecoin proof architecture knowledge. Understanding that Filecoin uses Groth16 proofs over BLS12-381, that proving requires large Structured Reference Strings (SRS) / proving keys, that different sector sizes (2 KiB, 32 GiB, 64 GiB) require different parameters, and that the parameter naming convention encodes version, proof type, and hash configuration.

2. The cuzk project context. Knowing that this is Phase 0 of a pipelined proving daemon, that the gRPC pipeline has been validated, that the only remaining blocker is the parameter availability, and that the command is the prescribed fix.

3. Understanding of IPFS and content-addressed storage. The download URLs point to IPFS gateways, and the file names include content hashes for integrity verification. The [NOTICE] messages with GIDs suggest an aria2 or similar download manager.

4. Path resolution semantics. Recognizing that /data/zk/params is an absolute path (starts with /), that ~/scrot expands to /home/theuser/scrot, and that the download paths ~/scrot//data/zk/params/... indicate a path concatenation bug where the CWD is prepended to the absolute path.

5. Go error formatting. The %!w(*fs.PathError=&{...}) syntax is Go's formatted error output for wrapped errors. The 2 at the end of the path error (open ... 2) is the Unix errno for ENOENT (No such file or directory).

6. The concept of sanity checking in paramfetch. The tool downloads files, then opens them to verify integrity before considering them complete. This is a defense against corrupted downloads, but in this case the verification path is wrong.

Output Knowledge Created by This Message

This message produces several forms of knowledge:

1. Identification of a path resolution bug in curio fetch-params. The primary output is the discovery that the tool resolves the parameter cache path relative to the CWD rather than using it as an absolute path. This is a bug that needs fixing in the fastparamfetch/paramfetch.go source file.

2. Documentation of the complete parameter set for 32 GiB proving. The log output serves as an inventory of every file needed for PoRep C2 proof generation on 32 GiB sectors. This includes the specific IPFS CIDs (content identifiers) for each file, which can be used for manual download or verification.

3. Evidence that the files were actually downloaded successfully. Despite the error, the files exist at ~/scrot//data/zk/params/. This is confirmed in subsequent messages (msg 188-190) where the assistant finds and copies them to the correct location. The download itself succeeded; only the path resolution and verification failed.

4. Confirmation of the parameter size. The subsequent investigation (msg 189-190) reveals the actual file sizes: the PoRep params file is 45 GiB, the empty-sector-update params are 33 GiB, and the PoSt params are 57 GiB. These sizes confirm the ~200 GiB peak memory footprint identified in earlier analysis — the parameters alone account for over 100 GiB of disk space, and the proving process loads multiple sets into memory simultaneously.

5. A diagnostic pattern for similar issues. The telltale sign of this bug is the double slash in download paths (~/scrot//data/zk/params/) combined with sanity check failures at the intended path (/data/zk/params/). Anyone seeing this pattern in their own deployment can immediately diagnose a CWD-dependent path resolution bug.

6. Operational knowledge about the test environment. The message reveals that the machine has:

The Thinking Process Visible in the Message

While the message is raw command output rather than a reasoning trace, the thinking process is visible in several ways:

The user's decision to run from ~/scrot. The shell prompt shows ~/scrot as the current directory. This suggests the user was working in that directory previously — perhaps examining downloaded files, running other commands, or organizing test data. They didn't think to cd / before running fetch-params, which is a natural oversight when the tool is expected to handle absolute paths correctly.

The tool's retry logic. The ERROR messages show the tool attempting to "remove and retry" after each sanity check failure. This reveals a design assumption: if a file fails sanity checking, it might be corrupted, so remove it and download again. However, the removal also fails (because the file isn't at the expected path), which means the retry will download to the same wrong path again. The tool doesn't check whether the removal succeeded before retrying.

The composite error construction. The final error message concatenates all individual removal failures into one massive error string. This is a common Go pattern for collecting multiple errors, but it produces an overwhelming message that buries the root cause in repetition. A better approach would be to report the first failure with context about the path mismatch.

The absence of a CWD check. The tool never logs the current working directory or the resolved download path. If it had printed "Downloading to /home/theuser/scrot//data/zk/params/..." the user might have noticed the anomaly immediately. The lack of this diagnostic output is a design flaw that made the bug harder to detect.

The Resolution and Its Implications

The subsequent conversation (msg 185-192) shows the assistant diagnosing the issue:

  1. The assistant checks /data/zk/params/ and finds it empty (msg 186).
  2. It realizes the files were downloaded to ~/scrot//data/zk/params/ (msg 187).
  3. It checks the default location and finds only small params (msg 187).
  4. It discovers the files at ~/scrot/data/zk/params/ (msg 188) — note the single slash this time, as the shell expands ~ to /home/theuser and the path becomes /home/theuser/scrot/data/zk/params/.
  5. It lists the actual .params files and finds the critical 45 GiB PoRep params file (msg 189-190).
  6. It copies the files to /data/zk/params/ using cp -n (msg 190). The resolution is straightforward once the bug is understood: copy the files from where they were actually downloaded to where they need to be. But the deeper fix — correcting the curio fetch-params path resolution — remains for a future code change. This episode illustrates a fundamental tension in infrastructure development: the software architecture (the cuzk daemon, the gRPC pipeline, the priority scheduler) can be perfectly designed and validated, but the environmental dependencies (parameter files, directory structures, environment variables, working directory assumptions) can still derail the operational deployment. The cuzk project's Phase 0 scaffold was proven correct; the fetch-params tool had a bug. The two are independent, but the proving pipeline depends on both.

Broader Lessons

This single message offers several broader lessons for systems engineering:

1. Absolute paths are not always absolute. A path starting with / should be treated as absolute by any well-behaved tool, but implementation bugs can cause even absolute paths to be resolved relative to the CWD. Always verify that tools handle paths correctly, especially when dealing with large file downloads.

2. Sanity checks must use the same path as the operation. The fetch-params tool's sanity check opened files at the intended path while the download wrote to a different path. This mismatch made the sanity check not just ineffective but actively harmful — it triggered unnecessary retries and produced confusing errors.

3. Error messages should report the root cause, not just the symptoms. The final error message listed dozens of failed file removals without ever saying "the files were downloaded to the wrong directory." A single diagnostic message — "expected files at /data/zk/params/ but found them at /home/theuser/scrot//data/zk/params/" — would have been infinitely more useful.

4. Environment variables are not a universal configuration mechanism. The FIL_PROOFS_PARAMETER_CACHE variable is used by multiple tools (filecoin-proofs, bellperson, curio fetch-params) but each may interpret it differently. The proving libraries use it as a lookup path; the fetch tool uses it as a download target with different path resolution semantics. This inconsistency is a source of bugs.

5. Large file downloads need robust verification. The parameters total over 100 GiB across all files. A corrupted download could waste hours of bandwidth and produce cryptic proof failures. The sanity check is a good idea in principle, but it must be implemented correctly.

6. Infrastructure debugging often involves following the data. The assistant's diagnostic process — check the expected location, find it empty, trace where the files actually went, find them in a different directory, copy them to the right place — is a textbook example of data-driven debugging. The log output provided all the clues needed; the skill was in interpreting them.

Conclusion

The message FIL_PROOFS_PARAMETER_CACHE=/data/zk/params curio fetch-params 32GiB and its voluminous output is far more than a failed command execution. It is a window into the real-world challenges of deploying cryptographic proof infrastructure, where a single path resolution bug in a parameter download tool can block an entire proving pipeline. The message documents the complete parameter dependency graph for Filecoin 32 GiB PoRep proving, reveals the internal workings of the fetch-params tool, and provides a diagnostic pattern for similar path resolution issues.

Most importantly, the message captures a moment of transition in the cuzk project: from software validation to environmental integration. The gRPC pipeline was proven; the parameters were the last missing piece. The bug in fetch-params was a temporary setback, but the files were ultimately recovered and copied to the correct location, enabling the first successful end-to-end proof generation test in the subsequent session.

For the technical reader, this message serves as a case study in the importance of path handling in infrastructure tools, the value of verbose logging for debugging, and the necessity of verifying assumptions about tool behavior. The 45 GiB PoRep parameter file was misplaced, but the knowledge gained from the misplacement — about the tool, the environment, and the parameter dependency graph — was invaluable.