Caching Simulations to Disk the Right Way in Houdini

Caching a heavy simulation to disk sounds simple: enable Allow Caching To Disk, run the sim, and let Houdini handle the rest. The problem is that this option does not work the way many artists expect. Houdini still relies on memory first, and once a simulation grows beyond available cache memory, playback, storage, and recovery after a crash can become much more complicated. In this guide, I’ll explain how Houdini simulation caching actually works and show a more reliable workflow for saving large simulations to disk.

In a DOP simulation, Houdini caches simulation frames in memory first. Turning on Allow Caching to Disk does not make every frame write directly to storage. According to SideFX, Houdini starts writing .sim files to the temporary directory only when the simulation exceeds the amount of RAM allocated through Cache Memory (MB).

This distinction matters because enabling the option does not remove RAM from the caching process. Disk-cached frames are also slower to scrub than frames kept in memory, although they allow Houdini to retain a larger simulation instead of dropping older cached frames when memory fills up.

There is one more setting to check when running simulations in the background. SideFX states that disk caching is disabled for background simulations unless Cache to Disk in Non-Interactive Sessions is enabled.

One clear warning sign is that you can no longer scrub back through the entire simulation. SideFX explains that when Houdini runs out of memory for new cached frames, it drops older cached frames. Those earlier parts of the simulation then have to be cooked again if they were not saved to disk.

You can catch this early by watching the timeline. In Houdini, blue indicates frames cached in memory, while purple indicates DOP frames cached to .sim files. If earlier cached frames start disappearing instead of remaining available for playback, your memory cache may have reached its limit.

For SOP-based simulations, File Cache SOP gives you direct control over what is saved and where it is stored. Instead of relying on Houdini’s temporary simulation cache, you can write the result to a chosen location and load it from disk later without recooking the upstream network. 

For geometry, SideFX recommends .bgeo.sc as the general-purpose format. If the input consists entirely of VDB volumes, you can save it as .vdb. Use a clear folder structure and file naming convention so each cache is easy to identify and reuse. 

Set the frame range you actually need before using Save to Disk. Once the cache is complete, enable Load from Disk to read the saved files instead of cooking the input again. This also makes it easier to resume your workflow from an existing cache rather than rebuilding the same upstream result.

Once a simulation is cached to disk, storage capacity and read/write performance become part of the workflow. Large Houdini caches can generate substantial amounts of data, so a slow drive can become a bottleneck when writing cache files or loading them back for playback.

For heavy simulation work, a fast SSD, especially NVMe storage, can reduce the time spent reading and writing cache data compared with slower storage. Before starting a long cache, estimate the required storage and make sure the destination drive has enough free space for the full frame range.

Before committing to a long simulation, cache a short frame range and inspect the result first. This helps catch incorrect settings, unexpected behavior, or file path problems before spending hours on the full cache.

For longer jobs, cache in manageable frame ranges where your setup allows it, and use clear versioned folders so test caches do not overwrite approved results. Remove obsolete test caches when they are no longer needed to avoid filling the drive.

For DOP simulations, SideFX also provides Save Checkpoints, which periodically saves simulation state to disk. If the simulation is interrupted, Houdini can resume from a saved checkpoint instead of starting again from the beginning.

Some simulations simply exceed what a local workstation can handle comfortably, either because there is not enough RAM to run the simulation or not enough storage for the resulting cache. In that case, moving the workload to a machine with more memory and storage can be more practical than repeatedly reducing the simulation just to fit local hardware.

Disk caching can protect completed work, but it cannot solve a simulation that needs more RAM or storage than your local workstation can provide. For larger Houdini simulations, whether you are running heavy particle solvers or looking for a dedicated houdini render farm, iRender offers remote machines with 256GB RAM and NVMe storage, giving you more room to run the simulation and write large caches without reducing the workload simply to fit your local hardware.

More hardware does not make caching crash-proof. For long simulations, you should still use File Cache SOP, divide long frame ranges where appropriate, and verify a short cache before committing to the full run. Otherwise, a failure late in the simulation can still waste significant time.

Available configurations include:

• CPU: AMD Ryzen™ Threadripper™ PRO 3955WX (3.9-4.2GHz) and AMD Ryzen™ Threadripper™ PRO 5975WX (3.6-4.5GHz)
• GPU: 1/2/4/8 RTX 4090/5090

For new users, iRender currently offers a 100% bonus on the first deposit within 24 hrs of registration, giving you additional rendering credits to get started with larger projects.

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                                                    Source: Pugetsystems, SideFX Docs

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1. Does Allow Caching to Disk stop Houdini from running out of RAM?
Not completely. Houdini caches simulation data in memory first. With Allow Caching to Disk enabled, DOP simulation frames are written to disk when the simulation exceeds the allocated cache memory. For more direct control over SOP-based caches, you can use File Cache SOP.

2. What format should I cache Houdini simulations to?
For geometry, SideFX recommends .bgeo.sc as a general-purpose format. If the input consists entirely of VDB volumes, .vdb is also an option. Choose the format based on the data you need to store rather than using one format for every simulation.

3. Why can’t I scrub back through my whole simulation?
If older cached frames are no longer available, Houdini may have dropped them to make room for newer frames after reaching the memory cache limit. Saving the simulation to disk lets you retain frames that you need to revisit instead of relying only on the memory cache.