Minecraft: Java Edition Snapshot 21w38a continues work on 'Caves and Cliffs Update' Part Two

Minecraft Caves and Cliffs Update Image
Minecraft Caves and Cliffs Update Image (Image credit: Mojang Studios)

What you need to know

  • For weeks, Mojang Studios was testing upcoming Minecraft features and changes through "experimental snapshots" on PC.
  • Now, these features from the upcoming 1.18 'Caves and Cliffs Update' Part Two are finally appearing in regular snapshots.
  • Minecraft: Java Edition Snapshot 21w38a is now available, and includes plenty of tweaks and fixes over the last snapshot.
  • There's also a new way for Mojang to collect telemetry data, to aid in developing for better performance in future snapshots.

As was foretold during the release of Minecraft: Bedrock Edition Beta yesterday, Mojang Studios is now releasing the second official snapshot for 1.18 Caves and Cliffs Update Part Two, which includes a new range of fixes and tweaks, the return of strongholds, and a new way for Mojang to collect vital performance-related telemetry data (more on that below).

Minecraft: Java Edition Snapshot 21w38a follows its predecessor with fewer new features and changes, but it does include some important alterations nonetheless. Besides the return of strongholds, which were removed in the previous snapshot, copper is now more common in this snapshot. Lots of bugs were also squashed, which should hopefully provide a more stable experience.

For many, though, the most interesting aspects of this build will be on the technical side of things. This snapshot allows world generation to take advantage of a greater number of background CPU threads, which will be particularly useful for those playing on higher-end gaming rigs. Mojang is also improving telemetry collection in this snapshot, and will collect vital information during world loading that, ideally, will help the studio improve performance for world generation and loading in future snapshots.

If you're interested in the information that will be collected by Mojang when playing this snapshot, be sure to check below.

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The full changelog for Minecraft: Java Edition Snapshot 21w38a includes:

Known issues

  • This snapshot is still not compatible with older worlds — to play in this snapshot, you will have to create a new world
  • World saving can sometimes take a very long time — if you see the world stop moving for a time, this is why


  • Sprinting is no longer reduced to walking when gently brushing your sleeve against a wall
  • Fixes for the distribution of ores


  • Previously, any collision with a block would reduce sprinting to walking. Now, for very minor angles of collision with a block, players retain their sprint. For larger angles of collision, the old behavior is retained

Ore distribution

  • Copper ore generates up to y 95 (as originally designed)
  • Increased the amount of copper
  • Lapis lazuli generates in blobs, like other ores (instead of spread out)

Technical updates

  • A new setting has been added: "Simulation distance"
  • Maximum amount of background threads increased
  • Added telemetry for world loading

Simulation distance

  • Entities will not be updated outside of simulation distance
  • Allows higher render distance with less CPU load
  • A new slider in video settings on the client
  • A new simulation-distance property in dedicated server properties
  • We will continue working on it with a goal of simulation distance also affecting block and fluid ticking

Background threads

  • Various background tasks including worldgen are executed on a background thread pool. Its size equals the amount of available CPU threads minus one, but there was an upper limit of 7. Now this upper limit is 255. This should help higher-end machines with world-gen performance
  • The upper limit can be overridden by max.bg.threads Java system property, for example when running multiple servers on a single machine


  • In this release, we are re-introducing diagnostic tracking, which was part of Minecraft: Java Edition until 2018. We are bringing it back to understand our players better and to improve their experience. Specifically, we hope to ensure stronger performance for the extremely heavy world generation in the second part of the Caves & Cliffs update later this year
  • In practice, this can mean identifying technical pain points in the game, like how less powerful hardware performs in high-complexity terrain. This information will also help us prioritize various aspects of development, balance game features (by learning, for instance, that the goats are pushing every single player off the cliffs) and lend new Minecrafters a helping hand (after realizing that many of them get stuck at the same points). All data is treated according to GDPR and CCPA best practices and is used to develop a better-performing and generally more enjoyable Minecraft: Java Edition
  • World load event
    • Sent when loading singleplayer world or connecting to multiplayer server
    • Contains following information:
    • launcher identifier
    • user identitifer (XUID)
    • client session id (changes on restart)
    • world session id (changes per world load, to be reused for later events)
    • game version
    • operating system name and version
    • Java runtime version
    • if client or server is modded (same information as on crash logs)
    • server type (single player, Realms or other)
    • game mode

Fixed bugs

  • Game fails to switch the audio output from one device to another after the game has started (from title screen)
  • No sound for using shears on weeping, twisting, cave vines or kelp
  • Strongholds don't generate in 21w37a
  • Liquids sometimes do not propagate
  • Random light level of 0 in sky
  • Crash when changing render distance / java.lang.ArrayIndexOutOfBoundsException: Index 28084 out of bounds for length 26136
  • Memory leak in 21w37a
  • Big Spruce Trees don't generate in Groves
  • Starting and stopping JFR profiling multiple times within the same minute overwrites previous JFR results saved during that minute
  • Dripstone caves generate above the ground
  • Cannot start with alternate JVM due to JFR requirement
  • Problems with JFR event metadata
  • Too many aquifers on biome surfaces at Y=63 and below
  • Aquifers often generate with harsh straight borders
Zachary Boddy
Staff Writer

Zachary Boddy (They / Them) is a Staff Writer for Windows Central, primarily focused on covering the latest news in tech and gaming, the best Xbox and PC games, and the most interesting Windows and Xbox hardware. They have been gaming and writing for most of their life starting with the original Xbox, and started out as a freelancer for Windows Central and its sister sites in 2019. Now a full-fledged Staff Writer, Zachary has expanded from only writing about all things Minecraft to covering practically everything on which Windows Central is an expert, especially when it comes to Microsoft. You can find Zachary on Twitter @BoddyZachary.