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Reference Migration modes

TheMeinerLP edited this page Aug 24, 2026 · 1 revision

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 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IN_MEMORY on every read no paid again on every load of every outdated chunk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

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. 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 · Explanation How world migration works · Reference Supported versions

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