> For the complete documentation index, see [llms.txt](https://docs.onelitefeather.net/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.onelitefeather.net/falco/reference/migration-modes.md).

# Migration modes

The `ChunkMigrationMode` constants, what each costs, and the settings around the backup.

> **This page is a generation target, currently written by hand.** Source of truth: [`ChunkMigrationMode`](https://github.com/OneLiteFeatherNET/Falco/blob/main/falco-anvil/src/main/java/net/onelitefeather/falco/anvil/ChunkMigrationMode.java) and the `falco-migration` engine.

## `ChunkMigrationMode`

| Mode              | Translates    | Writes to the world | Cost                                             |
| ----------------- | ------------- | ------------------- | ------------------------------------------------ |
| `OFF` *(default)* | nothing       | no                  | none; identical to before the option existed     |
| `IN_MEMORY`       | on every read | **no**              | paid again on every load of every outdated chunk |
| `ON_DISK`         | on every read | **yes**             | paid once per chunk, then never again            |

|                |                                                                                  |
| -------------- | -------------------------------------------------------------------------------- |
| Builder slot   | `migration(ChunkMigrationMode)`                                                  |
| Discovery      | selecting any mode other than `OFF` turns on `ChunkMigrator` discovery by itself |
| Missing engine | the loader fails to build                                                        |
| Order          | migration runs **before** `ChunkVersionPolicy`                                   |
| Startup        | a loader in either active mode logs the mode and its cost                        |

After `ON_DISK`, the world's chunks carry the running server's data version and the older server can no longer read them.

## Backup

|                 |                                                                             |
| --------------- | --------------------------------------------------------------------------- |
| Can be disabled | **no**                                                                      |
| Path slot       | `migrationBackup(Path)`                                                     |
| Default path    | `<worldRoot>/falco-migration-backup/<dimension>`                            |
| Granularity     | per region file, immediately before that file is first written              |
| Mechanism       | written under a `.partial` name, then moved atomically                      |
| Existing backup | never overwritten                                                           |
| Removal         | none; Falco never deletes a backup, so it stays until it is removed by hand |

A world whose chunks are all current is never copied at all.

## Floors

|                                         |                                         |
| --------------------------------------- | --------------------------------------- |
| Lowest `DataVersion` the engine accepts | `1519` (Minecraft 1.13, the flattening) |
| Below it                                | declined, not attempted                 |
| Covers                                  | blocks, biomes, block entities          |
| Does not cover                          | entities, Bedrock worlds, downgrades    |

## Rule versions

Rules live in [`BlockStateRules`](https://github.com/OneLiteFeatherNET/Falco/blob/main/falco-migration/src/main/java/net/onelitefeather/falco/migration/BlockStateRules.java). Each carries a `since()` — the `DataVersion` the change happened in — and applies exactly when `since() > sourceVersion`. **`since()` names the snapshot, not the release.** Every number in that file carries its source in a comment beside it.

Related: [How-to Migrate a world from an older version](/falco/how-to-guides/migrate-a-world-from-an-older-version.md) · [Explanation How world migration works](/falco/explanation/how-world-migration-works.md) · [Reference Supported versions](/falco/reference/supported-versions.md)


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