lerobot-lancedb

Convert a LeRobot dataset to a Lance layout once, then train from it - local disk or object storage - faster than upstream and without downloading the whole dataset first.

Three pieces:

• lerobot-lance-convert - turns a LeRobot v3.0 dataset (local dir or Hub id) into a three-table Lance layout.
• lerobot-lance-doctor - audits an upstream-format dataset for silent defects before you convert or train. Every large public dataset we converted failed at least one check.
• LanceDBDataset - the map-style training loader for the Lance layout, so the round trip works from this one package.

Why convert

The Lance layout is map-style random access over object storage: a training worker fetches the exact byte ranges one batch needs, so you get a true global shuffle straight from S3 without downloading the dataset and without holding a big reservoir buffer in RAM. Upstream's only remote mode is an iterable streamer (reservoir shuffle, one worker per shard, OOMs on large frames).

Batch 32, 8 workers, steady-state samples/s (global shuffle for lance; upstream's streamer only does its windowed reservoir shuffle):

The column that matters is lance S3 vs upstream Hub stream: same remote data, lance is 6-15x faster and simply runs where the streamer exhausts RAM or a decode worker crashes. Lance from local disk matches or beats upstream local too, so converting doesn't cost you anything on the machine you already have. A same-bucket, same-access apples-to-apples comparison (only the loader/format differs) and the full methodology are in the converter walkthrough.

Install

pip install --extra-index-url https://pypi.fury.io/lancedb/ lerobot-lancedb

The extra index is needed because lancedb>=0.37.1b0 (blob v2 + fetch_blob_ranges, what makes the remote reads fast) is a beta, published on fury.io, not PyPI yet.

lerobot also supplies the loader's heavy deps (torch, torchcodec, numpy), so they're pinned there.

Convert

# from the Hub (downloads if not cached):
lerobot-lance-convert --repo-id lerobot/pusht --out ./pusht-lance
# or a dataset already on disk (nothing downloaded):
lerobot-lance-convert --root /path/to/dataset --out ./my-lance

Then push ./pusht-lance/ to object storage (aws s3 cp --recursive, etc.) and point training at the URI.

Load it back

from lerobot_lancedb import LanceDBDataset, lance_mp_context
from torch.utils.data import DataLoader

ds = LanceDBDataset(root="./pusht-lance")              # or "s3://bucket/pusht-lance"
item = ds[0]                                           # same keys/tensors as LeRobotDataset

loader = DataLoader(ds, batch_size=32, num_workers=8,  # pair with EpisodeAwareSampler
                    multiprocessing_context=lance_mp_context())

It is a map-style torch.utils.data.Dataset returning items bit-exact with LeRobotDataset. root may be a local dir or an s3:// / gs:// / hf:// URI (ranged reads, nothing downloads up front).

Two things to know. (1) LanceDBDataset here is a vendored copy of lerobot's loader (the open reader PR), included so this package works end to end against released lerobot. Once the loader lands in lerobot core this re-exports lerobot.datasets.lancedb_dataset.LanceDBDataset and the copy is deleted; your import keeps working either way. (2) The lerobot-train CLI auto-detecting a Lance root is part of the lerobot reader PR (in make_dataset), so training straight off --dataset.root ...-lance needs that PR merged; until then, build the DataLoader yourself as above.

The layout

Three Lance tables next to a verbatim copy of the standard meta/ directory:

<out>/
  meta/             # byte-identical LeRobot v3.0 metadata
  frames.lance      # one row per frame: tabular features (dots -> underscores)
  videos.lance      # one row per source mp4: bytes in a blob v2 column + byte index
  meta.lance        # one row per meta/ file (path, bytes): metadata transport for remote roots

• frames.lance - every tabular feature, one row per frame, sorted by index. Row N is frame N, so a batch of indices is one point-read, no index structure needed. Numeric vectors become fixed-size lists; language columns (lerobot#3467) keep their nested list<struct> with extension types stripped.
• videos.lance - each mp4 verbatim in a blob v2 column, plus byte-index columns (file_size, moov_offset, moov_size, kf_indices, kf_positions). The loader turns a frame window into a keyframe-aligned byte range and fetches a whole batch's video bytes in one fetch_blob_ranges: an 8-frame window costs ~100 KB of transfer, not the whole file.
• meta.lance - the meta/ files as (path, bytes), so a remote root can materialize meta/ through the same Lance connection instead of a side channel (droid's per-episode stats alone are 566 MB).

Both large tables are written as a single streaming commit (bounded memory, no per-512 MB fragmentation), and scalar indexes (episode_index, task_index, video_key) are built for ad-hoc SQL - the training loader reads by row id and doesn't need them.

Dataset doctor

Silent defects in public datasets are common, and they surface as confusing failures at convert or train time (or worse, don't surface at all). lerobot-lance-doctor runs five read-only checks against an upstream-format dataset: metadata loads, episode ranges tile [0, total_frames), every referenced parquet exists with the right total row count and no orphans, every referenced video exists non-empty and readable with no orphans, and every video actually contains the frames the metadata implies it should (container metadata only, no decoding).

Real example: lerobot/berkeley_autolab_ur5 ships aggregated videos short a tail of frames relative to what its own metadata implies. The doctor catches it:

$ lerobot-lance-doctor --root ~/.cache/.../lerobot/berkeley_autolab_ur5 --repo-id lerobot/berkeley_autolab_ur5
[ ok ] META: 1000 episodes, 97939 frames, fps 5
[ ok ] BOUNDARIES: episode ranges tile [0, total_frames)
[ ok ] DATA: parquet files referenced, accounted, no orphans
[ ok ] VIDEOS: referenced, non-empty, readable, no orphans
[FAIL] SUPPLY: every video holds the frames meta implies
         videos/observation.images.image/chunk-000/file-000.mp4: container declares 30091 frames, episodes imply 30175 (short by 84)
         ... and 27 more

1 of 5 checks failed

The exit code is the number of failed checks, so it drops straight into CI. Other defects we've hit and now check for: droid 1.0.1 has 44% orphan parquet rows plus orphan videos (it loads correctly only by filename-sort luck), and agibot shipped zero-byte videos inside otherwise-complete episodes.

Note on the old plugin

Versions before 0.3.0 were a different thing: a standalone loader plugin (LeRobotLanceDataset, LeRobotLanceVideoDataset) with its own storage layouts, superseded by the native loader. If you depend on those classes, pin lerobot-lancedb<0.3 and plan to move to LanceDBDataset - datasets converted with the old plugin are not compatible with the native loader, so re-convert with lerobot-lance-convert.

License

Apache-2.0.