Record disk I/O for each self-hosted CI job by ejc3 · Pull Request #924 · ejc3/fcvm

ejc3 · 2026-09-13T16:19:59Z

Why

The runners use metal types with local NVMe because /mnt/fcvm-btrfs is built from instance store. Types without it are materially cheaper for x86: c5.metal spot is about $0.70/h, against $1.10-1.19 for r5d.metal. Such a type would put btrfs on EBS instead, and gp3 tops out at 16,000 IOPS and 1,000 MB/s. Nothing records how much disk I/O the suite really does, so there is no way to tell whether EBS would slow it down.

Measured on this PR's first run (34768256944): replaying each job's samples against one maxed gp3 volume (1,000 MB/s, 16,000 IOPS) adds 6–13% to job time, so storeless types stay excluded. The per-job table is in ejc3/aws GITHUB-RUNNERS.md (ejc3/aws#130). The record stays so a later runner change can be measured the same way.

What changes

"Create test log directory": in host, host-root and container, this step now starts iostat -dxmty 10 over every disk (loop devices excluded).
- It is detached from the step's output, so the step doesn't wait on it; the runner reaps it with the job's other processes.
- It writes to $RUNNER_TEMP, not /tmp/fcvm-test-logs, which make clean-test-data recreates. The runner empties $RUNNER_TEMP for each job, so a job never saves an earlier job's record.
New "Save disk I/O record" step: runs always() just before "Upload test logs". It stops the sampler by the PID recorded in $RUNNER_TEMP, waits for it to exit, and copies the log into the artifact as iostat.log.
- analyze_ci_vms.py only counts *-base-* and *-clone-* logs, so it ignores the new file.

Checks

Rebased onto main after #923 merged. cargo test --test test_ci_workflow_coverage: 31/31 pass on 79b2abe. disk_io_record_is_this_jobs_and_complete, added for CodeRabbit's two findings, failed on the unfixed steps. actionlint runs in CI. Since #923, a pull request runs every arm64 job and one x64 job, so this run should upload an iostat.log in each of those jobs' test-logs-* artifacts.

Summary by CodeRabbit

Tests
- Improved disk I/O monitoring during host, host-root, and container test jobs.
- Test artifacts now capture complete, job-specific disk I/O records, including samples from the beginning and end of each test.
- Added validation to ensure monitoring data is safely collected and saved even when test jobs finish unexpectedly.

chatgpt-codex-connector · 2026-09-13T16:20:04Z

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coderabbitai · 2026-09-13T16:20:12Z

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📝 Walkthrough

Walkthrough

The CI workflow now uses per-job RUNNER_TEMP files and tracked iostat PIDs for disk I/O capture. Always-run steps validate, stop, and copy each sampler log. Tests verify complete job-specific records.

Changes

CI disk I/O logging

Layer / File(s)	Summary
Start `iostat` sampling `.github/workflows/ci.yml`	The `host`, `host-root`, and `container` jobs start detached `iostat` samplers. Each sampler writes to `$RUNNER_TEMP/fcvm-iostat.log` and records its PID in `$RUNNER_TEMP/fcvm-iostat.pid`.
Save and validate `iostat` records `.github/workflows/ci.yml`, `tests/test_ci_workflow_coverage.rs`	Always-run steps verify the PID belongs to `iostat`, stop the sampler, wait for exit, and copy the log to `/tmp/fcvm-test-logs/iostat.log`. The coverage test verifies that each job saves both initial and final samples and does not use `pkill` or `/tmp/fcvm-iostat`.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Feature

Merge Risk: ⚪ Minimal · up to 79b2a

Disk I/O records are captured and saved per CI job without the prior stale-log or incomplete-copy races. The change is ready to merge.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: recording disk I/O for each self-hosted CI job.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 1 files. (1 skipped: 1 …
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Commit unit tests in branch ci/capture-disk-io

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ejc3

NOT-A-DEFECT: neither undisposed body carries a finding. One is a Codex usage-limit notice; the other is CodeRabbit's "Review skipped" status, posted because auto review is off for non-default base branches (this PR was stacked on #923 at the time). This PR's own run covers the change: every self-hosted job planned for a pull_request ran and passed, including the new "Save disk I/O record" step.

ejc3 · 2026-09-14T00:29:59Z

@coderabbitai review

coderabbitai · 2026-09-14T00:30:06Z

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iostat samples every disk each 10 s from log-directory setup to the end of the job, and the log is saved as iostat.log in the job's test-logs artifact. The numbers show whether btrfs on EBS, instead of local NVMe, would slow the suite down.

ejc3 · 2026-09-14T02:05:59Z

@coderabbitai review

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Actionable comments posted: 2

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Inline comments:
In @.github/workflows/ci.yml:
- Around line 682-684: Update all three iostat save steps to retain the sampler
PID, terminate that specific process, and wait for it to exit before copying
/tmp/fcvm-iostat.log into the test logs directory. Preserve the existing
fallback behavior when no log is available.
- Around line 635-637: Scope the iostat sampler and log handling to the current
job in the workflow steps around the sampler lifecycle. Replace the global
/tmp/fcvm-iostat.log path with a ${RUNNER_TEMP}-scoped path, record the launched
process PID, and update cleanup and log-copy steps to use that PID and path
instead of pkill -x iostat; apply the same change to all referenced sampler
blocks.

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coderabbitai · 2026-09-14T02:15:54Z

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The save step signalled every iostat on the host and copied at once, racing the last write, and a job whose sampler never started copied the log an earlier job left in /tmp.

ejc3

RED-VERIFIED: disk_io_record_is_this_jobs_and_complete

Answers CodeRabbit's review of 8fa2a6e ("Actionable comments posted: 2") and its walkthrough, whose merge-risk note is the same truncated-record finding. Both findings are fixed in 79b2abe and answered on their threads. The test failed on 565c817, before the fix, and all 31 coverage tests pass with it.

ejc3 · 2026-09-14T03:08:01Z

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coderabbitai · 2026-09-14T03:08:15Z

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ejc3

NOT-A-DEFECT: CodeRabbit's walkthrough for 79b2abe reports no actionable comments and minimal merge risk, with every pre-merge check passed; its earlier truncated-record note is gone now that the fix is in. It carries no finding.

ejc3 force-pushed the ci/pr-single-x64 branch from c873f10 to 28581a8 Compare September 13, 2026 21:29

Base automatically changed from ci/pr-single-x64 to main September 14, 2026 00:22

ejc3 commented Sep 14, 2026

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ejc3 force-pushed the ci/capture-disk-io branch from 669696b to cd8b529 Compare September 14, 2026 00:30

Record disk I/O for each self-hosted CI job

8fa2a6e

iostat samples every disk each 10 s from log-directory setup to the end of the job, and the log is saved as iostat.log in the job's test-logs artifact. The numbers show whether btrfs on EBS, instead of local NVMe, would slow the suite down.

ejc3 force-pushed the ci/capture-disk-io branch from cd8b529 to 8fa2a6e Compare September 14, 2026 02:05

coderabbitai Bot reviewed Sep 14, 2026

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Comment thread .github/workflows/ci.yml Outdated

Comment thread .github/workflows/ci.yml Outdated

ejc3 added 2 commits September 14, 2026 02:31

Test that the disk I/O record is this job's own and complete

565c817

Stop the iostat sampler by its PID and keep its files in RUNNER_TEMP

79b2abe

The save step signalled every iostat on the host and copied at once, racing the last write, and a job whose sampler never started copied the log an earlier job left in /tmp.

ejc3 commented Sep 14, 2026

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