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Deduplication

Xet-enabled repositories utilize content-defined chunking (CDC) to deduplicate on the level of bytes (~64KB of data, also referred to as a “chunk”). Each chunk is identified by a rolling hash that determines chunk boundaries based on the actual file contents, making it resilient to insertions or deletions anywhere in the file. When a file is uploaded to a Xet-backed repository using a Xet-aware client, its contents are broken down into these variable-sized chunks. Only new chunks not already present in Xet storage are kept after chunking, everything else is discarded.

To avoid the overhead of communicating and managing at the level of chunks, new chunks are grouped together in 64MB blocks and uploaded. Each block is stored once in a content-addressed store (CAS), keyed by its hash.

The Hub’s current recommendation is to limit files to 20GB. At a 64KB chunk size, a 20GB file has 312,500 chunks, many of which go unchanged from version to version. Git LFS is designed to notice only that a file has changed and store the entirety of that revision. By deduplicating at the level of chunks, the Xet backend enables storing only the modified content in a file (which might only be a few KB or MB) and securely deduplicates shared blocks across repositories. For the large binary files found in Model and Dataset repositories, this provides significant improvements to file transfer times.

For more details, refer to the From Files to Chunks and From Chunks to Blocks blog posts, or the Git is for Data paper by Low et al. that served as the launch point for XetHub prior to being acquired by Hugging Face.

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