I need to migrate from ClearCase to Git. What is the best approach to do this while preserving file histories?

Grok 4.20

Answer: The best approach is to use a custom migration script with cleartool exports and git fast-import to systematically replay ClearCase versions as Git commits while mapping branches, labels, and metadata.

ClearCase's centralized, version-object model differs significantly from Git's distributed DAG of commits, making automated conversion necessary to preserve history rather than a simple file copy. Start by analyzing your VOBs to decide on branch mapping (e.g., ClearCase branches become Git branches, baselines become tags) and handle any UCM components or symbolic links carefully. Develop or adapt an open-source script (such as those based on Python with the cleartool command) that iterates through file versions in chronological order, extracts metadata like check-in comments, authors, and timestamps, then feeds this into git fast-import for efficient bulk loading without intermediate working-tree checkouts. This method scales better for large repositories than naive per-version commits and minimizes data loss, though some advanced ClearCase features like dynamic views or merge arrows may require manual reconciliation. Test the migration on a non-production subset first, verify history with git log and blame, and plan for post-migration cleanup like .gitattributes adjustments and user training on Git workflows.

Gemini 3.1 Pro

Answer: The best approach is to use an automated history-replay tool, such as `cc2git` or a commercial migration service, to chronologically extract ClearCase VOB metadata and translate it into Git commits, carefully mapping file-level branches to Git's snapshot-based branches.

Migrating from ClearCase to Git is inherently complex because they use fundamentally different architectures. ClearCase tracks history and branches at the individual file level, whereas Git operates on repository-wide snapshots. Because of this mismatch, you cannot simply copy files over if you want to preserve history. To preserve history, the most effective approach is the 'history replay' method. This requires using a specialized migration script or tool (like the open-source `cc2git`, `git-cc`, or robust commercial tools from companies like Tasktop or OpsHub). These tools work by querying the ClearCase VOB for its chronological history, incrementally checking out files into a dynamic or snapshot view, and then translating those changes into sequential Git commits. Before running the migration, extensive preparation is crucial. You must define clear mapping rules to translate ClearCase branches, labels, and UCM (Unified Change Management) baselines into their equivalent Git branches and tags. It is highly recommended to clean up your VOBs first—removing obsolete branches, derived objects, and unused labels—to prevent migrating decades of unnecessary clutter into your new Git repository. Finally, weigh the necessity of a full history migration. Because full history replays are extremely slow and often result in a tangled Git history due to ClearCase's file-based branching, many organizations opt for a 'tip migration' or 'shallow history' approach. In this hybrid model, you migrate only the active branches and recent baselines into Git, while preserving the legacy ClearCase server in a read-only state for historical audits.

GPT-5.5

Answer: The best approach is to perform a structured, tool-assisted migration that exports ClearCase history into Git incrementally, validates the result thoroughly, and preserves enough branch, tag, author, timestamp, and file-version history to support future development.

Start by deciding what history you actually need to preserve. ClearCase and Git have very different models: ClearCase tracks versions of individual files through VOBs, branches, labels, and config specs, while Git tracks repository-wide snapshots through commits. A perfect one-to-one conversion is rarely possible, so the goal should be a faithful and usable Git history rather than a literal replica of every ClearCase internal concept. Inventory your ClearCase environment first: VOBs, components, branches, labels, baselines, UCM projects, views, config specs, binary files, symlinks, generated files, and access-control assumptions. Decide whether to migrate everything into one Git repository, split by component, or create multiple repositories. In many cases, splitting by product/component produces cleaner Git repositories than blindly importing an entire ClearCase universe. Use a migration tool rather than a manual copy if file history matters. Options include commercial tools such as Polarion SVN Importer workflows, GitClearCase bridges, or custom scripts using ClearCase commands such as `cleartool lshistory`, `cleartool find`, and `cleartool get`, then feeding commits into Git via `git fast-import`. For complex ClearCase/UCM environments, a purpose-built migration tool or a carefully tested custom `git fast-import` pipeline is usually the most reliable approach. Map ClearCase concepts deliberately. Labels should usually become Git tags. ClearCase branches can become Git branches, but only if they are still meaningful; obsolete or short-lived branches may be better archived rather than migrated as active Git branches. UCM baselines often map well to annotated tags. Authors should be normalized into Git `Name <email>` identities, and original timestamps should be preserved where possible. Before the final migration, run at least one pilot migration on a representative subset of the repository. Compare file contents at important labels/baselines, verify commit counts and dates, test branch checkouts, and ask developers to validate whether the resulting Git history is understandable. This step is essential because many migration issues only appear after people try to use the imported repository. Consider pruning or archiving very old, irrelevant, or generated content. If the full ClearCase history is enormous, you may choose to migrate recent and meaningful development history into Git while keeping the original ClearCase VOBs read-only for legal or archaeological reference. That is often more practical than creating a huge, slow Git repository with decades of low-value history. For the cutover, freeze ClearCase or establish a short synchronization window, run the final migration, verify checksums/content at key baselines, and then switch users to Git. Document the new Git workflow, including branching, pull requests, release tagging, and permissions, because the migration is not just a repository conversion but also a process change. In short: do not simply export the latest ClearCase view and commit it to Git if preserving history is important. Use a repeatable migration process, map ClearCase metadata to Git concepts, validate against known baselines, and keep ClearCase read-only for anything that cannot be cleanly represented in Git.

Claude Opus 4.7

Answer: The best approach is to use git-cc or the IBM-provided clearcase2git/cc2git tools to convert your VOB history incrementally, but for most real-world migrations a pragmatic hybrid — performing a tool-assisted history conversion of recent/active history while archiving deep ClearCase history read-only — yields the best balance of fidelity, time, and risk.

ClearCase-to-Git migrations are notoriously painful because ClearCase's data model (versioned directories, branches as first-class objects, config specs, labels, hyperlinks, triggers, derived objects, and especially UCM activities) does not map cleanly to Git's snapshot-of-files model. There is no tool that gives a 100% lossless conversion, so the question is really which trade-offs you accept. Recommended approach, in order: 1) Inventory and scope first. For each VOB/component decide: (a) is full history required for legal/audit reasons, (b) is only recent history needed for active development, or (c) is a tip-only import sufficient? Most teams discover that 80% of the value lives in the last 1–3 years of history on a few branches. Time-box the rest as a read-only archive. 2) Clean up in ClearCase before migrating. Remove obsolete branches, lock dead VOBs, normalize line endings, and identify the small set of branches/labels you actually want represented in Git. The migration is dramatically faster and cleaner if you do this first. 3) Choose a conversion tool based on whether you use Base ClearCase or UCM: - git-cc (Charles O'Farrell) — works well for UCM, imports streams as Git branches and activities as commits. Good default for UCM shops. - clearcase2git / cc2git (community Python scripts) — walks lshistory output and replays into Git; flexible and scriptable. - IBM Rational's own export utilities or Polarion/Plastic SCM bridges — useful if you already have those licenses. - As a last resort, a custom script driving cleartool find -all -version and git fast-import; this is what large migrations end up doing because every VOB has quirks. Avoid trying to do this with a simple snapshot view + git add loop except for tip-only migrations. 4) Run the conversion on a dedicated machine with a dynamic view (or efficient snapshot) and expect it to take hours to days per VOB. Map ClearCase users to Git author identities via a users file. Convert labels to Git tags and branches to Git branches; preserve activity headlines and timestamps in commit messages. 5) Validate. Diff the tip of each migrated branch against the corresponding ClearCase view byte-for-byte. Spot-check historical versions for a handful of files. Verify tag contents match labeled baselines. 6) Cut over with a freeze. Lock the VOBs read-only on cutover day, run a final incremental import, push to your Git server (GitHub/GitLab/Bitbucket), and redirect developers. Keep the ClearCase server alive read-only for at least a year for audit lookups — this is what makes the hybrid approach safe and lets you skip converting truly ancient history. 7) Train the team. The tooling change is the easy part; the mental model shift (no checkouts/locks, rebasing, feature branches, pull requests) is what determines whether the migration succeeds. If I had to pick a single starting point: try git-cc against a representative VOB on a sandbox, measure fidelity and runtime, and use that data to decide between full history conversion and the hybrid archive strategy.

Final Summary

The AI Roundtable unanimously recommends a "history-replay" method, using tools like `git-cc` or custom scripts to translate ClearCase's file-based versioning into Git's snapshot-based commits. Claude Opus 4.7 and Gemini 3.1 Pro highlight the pragmatic choice of a hybrid approach—migrating only recent history to keep the new Git repository performant while retaining the original VOBs for audits. Grok 4.20 and GPT-5.5 emphasize using `git fast-import` for speed and rigorous validation to ensure that branches, labels, and author metadata are faithfully preserved.

All 4 models agreed