This page describes the compression options supported in AssetBundles and how this impacts the built-in AssetBundle caching support.
AssetBundle files are an archive format that comprises of a small header data structure, followed by a content section containing its virtual files. The header section is never compressed and the content section can optionally be compressed. By default Unity compresses the content section with full-file compression (LZMA) and caches AssetBundles with chunk-based compression (LZ4).
When LZMA compression is used the entire content section of the AssetBundle file is compressed as a single stream. This full content approach results in smaller file sizes than those with chunk-based compression. This is the preferred format for AssetBundles downloaded from a Content Delivery Network (CDN). The downside is that you must decompress the entire file into RAM in order to read an Asset from these archives. This is best used when an AssetBundle contains assets such that to use one asset from the bundle would mean all assets are going to be loaded. Packaging all assets for a character or scene are some examples of bundles that might use this. This is the format used when calling BuildPipeline.BuildAssetBundles with no specific compression specified (e.g. BuildAssetBundleOptions.None).
AssetBundles can also be built in such a way that the data is completely uncompressed. The downside to being uncompressed is the larger file download size because some types of content can be highly compressible inside a AssetBundle. However, the load times once downloaded can be much faster because no decompression needs to happen. This is particularly helpful when only a few objects are loaded out of a larger AssetBundle. Uncompressed AssetBundles are 16-byte aligned. This format is available by specifying the flag BuildAssetBundleOptions.UncompressedAssetBundle when calling BuildPipeline.BuildAssetBundles.
LZ4 uses a chunk based algorithm which allows the AssetBundle to be decompressed in pieces or “chunks”. While writing the AssetBundle each 128KB chunk of the content is compressed prior to saving it. Because each chunk is compressed individually the overall file size is larger than AssetBundles compressed with LZMA. But this approach makes it possible to selectively retrieve and load just the chunks needed for a requested object, rather than decompressing the entire AssetBundle. LZ4 has comparable loading times to uncompressed bundles with the added benefit of reduced size on disk. This is the format preferred by the AssetBundle cache, which is described below, and it can also be a good choice for AssetBundles that you distribute as part of your installation or in other cases where size is not of paramount importance. This format is available by specifying the flag BuildAssetBundleOptions.ChunkBasedCompression when calling BuildPipeline.BuildAssetBundles.
Because different data will compress with different degrees of size savings it can make sense to experiment by rebuilding you project with each supported option and measuring the actual size difference. The results can help guide a decision about what format to use.
If you download and store data with a custom caching solution you can use AssetBundle.RecompressAssetBundleAsync to change compression, for example to convert LZMA format AssetBundles to uncompressed or LZ4 format after download.
Note: The Web platform doesn’t support LZMA compression for AssetBundles. Use LZ4 compression with AssetBundles for the Web platform. For more information, see Reduce load times with AssetBundles.
When AssetBundles are being downloaded from a web service you need to consider caching, so that a device does not have to download the same content each time your player runs. Because AssetBundles may be rebuilt it is also important to have a mechanism to replace a locally cached AssetBundle with a newer version.
Unity provides a built-in disk-based cache to store AssetBundles that are downloaded through UnityWebRequestAssetBundle. To enable caching you must specify the version integer or version hash parameter when calling UnityWebRequestAssetBundle.GetAssetBundle. By default any AssetBundle added to the Disk Cache will be converted to LZ4 compression. Hence it takes longer to initially download and load LZMA AssetBundles as the recompression happens, but subsequent loads use the cached version and run quickly. If Caching.compressionEnabled is false, Unity writes AssetBundles into the Disk Cache in uncompressed format.
When downloading AssetBundles over the internet it is important to take steps to make sure that corrupted or tampered file content is not accepted into the cache. You should specify the expected CRC when calling UnityWebRequestAssetBundle.GetAssetBundle so that Unity can compare this value against the downloaded content while it adds the file to the cache. The CRC check can be performed at low cost during the conversion of LZMA AssetBundles to LZ4. The CRC check does not need to be repeated again once the validated file reaches the cache. See also AssetBundle Download Integrity and Security.
The Caching class can be used to manage the built-in AssetBundle cache, for example to clear its content or to check if an AssetBundle is already cached.
For performance purposes Unity holds some uncompressed data in memory while a chunk-based or uncompressed AssetBundle is loaded. But this caching has fixed size regardless of how large the underlying AssetBundle file is.
To load an AssetBundle, Unity requires random access to its content, either through a file on disk, a file in Memory or a C# FileStream. It also requires that the AssetBundle is uncompressed or uses chunk-based compression (LZ4). In order to establish a loadable AssetBundle, Unity will sometimes need to create a temporary in-memory AssetBundle. This is not always a bad thing, as AssetBundle content can load quickly once it is in memory. But in many cases it is better to try to have the file represented on disk, as a local AssetBundle or cached download, so that RAM usage is minimized and on-the-fly compression conversion does not slow down load times.
Temporary in-memory AssetBundles will be created in the following cases:
The memory used by the temporary file is released when all reads have completed and AssetBundle.Unload is called.
Note: On platforms that support disk-based AssetBundle caching, the Caching.compressionEnabled setting will influence the format that is used for temporary in-memory AssetBundles. By default it is true and in-memory AssetBundles use LZ4. When Caching.compressionEnabled is false these in-memory files will be uncompressed and hence potentially take substantially more RAM. On platforms that do not support caching the in-memory format is always LZ4. If the input is a different format then an on-the-fly conversion is performed, which can add to load times.
Note: doing a CRC check when calling AssetBundle.LoadFromFile, AssetBundle.LoadFromFileAsync, AssetBundle.LoadFromStream or AssetBundle.LoadFromStreamAsync for a chunk-based file will force a full read and decompression of each chunk of the file. This calculation happens chunk by chunk, rather than loading the full file into RAM, so it is not a memory concern, but it can slow down load times. For a LZMA format AssetBundle there is no significant extra cost to perform the CRC check, because loading it always does a read and decompression of all the content.
Memory profiler tools, such as the Memory Profiler package, can be useful to check how much memory your loaded AssetBundles are using.