Storage and Access

NCore V4 components (see Data Formats) can be persisted in two storage formats and accessed from local or remote storage backends. Each group of component is represented as a zarr group, which can be stored as a directory-based zarr stores or as single-file indexed tar archives.

Indexed Tar Archive Format (.itar)

NCore defines a custom .itar (indexed tar) container format specifically tailored to dataset and use-case characteristics in robotics and autonomous vehicle applications. The .itar format packages zarr chunks as sequential tar members in a single file and appends a compressed index at the end of a regular tar archive, combining the streaming efficiency of tar with random-access capability.

Comparison of regular tar files with it’s 512 byte blocks (as used by, e.g., WebDataset, supporting linear streaming but no random access) with NCore’s indexed tar format, which appends a compressed index enabling O(1) key lookups and direct seeks to any chunk.

The .itar store implements the abstract zarr Store interface, so it can be used as a drop-in replacement for directory stores in all NCore APIs. Via UPath, .itar containers can also be accessed transparently from cloud storage backends (e.g., S3, GCS) without requiring a local copy.

Tradeoffs:

• .itar (container file) – efficient for distribution, cloud storage, and atomic transfers; supports both sequential streaming and random access via the appended index

• directory store – individual chunk files on disk; simpler for debugging and incremental updates

Both formats are accessed through the same SequenceComponentGroupsReader and SequenceComponentGroupsWriter APIs.

Read Performance

The chart below compares .itar read throughput against four alternative storage formats on a synthetic dataset of the same 1k JPEG images (2k and 4k resolutions, ~4.5 GB on local SSD) with associated per-image meta-data (poses, timestamps, etc.). Throughput is measured with init cost excluded (formats pre-opened); init cost is reported separately as time-to-first-read.

Read throughput across five storage formats. Full bar = sequential; inner bar = random access.

Format	Seq (MB/s)	Rand (MB/s)	Seq latency	Rand latency	Time-to-first-read
.itar	9847	9492	0.46 ms	0.48 ms	2.1 ms (decompress index)
tarfile (pre-parsed)	8470	7402	0.54 ms	0.62 ms	82.6 ms (scan all file headers)
tarfile (linear scan)	—	119	—	38.2 ms	per-access (no index)
WebDataset	1 557	—	2.91 ms	—	3.2 ms (pipeline build)
Parquet	1552	1542	2.92 ms	2.98 ms	2 929 ms (materialise table)
HDF5	754	1027	6.01 ms	4.48 ms	1.0 ms (B-tree open)

Even with tarfile’s headers pre-parsed, .itar is 16% faster sequential and 28% faster random. The gap comes from tarfile’s extractfile() wrapping data in three Python layers (ExFileObject → _FileInFile → BufferedReader), while .itar does a single seek + read.

.itar’s time-to-first-read is 2.1 ms (decompressing a ~14 KB CBOR/LZMA trailer index) vs tarfile’s 82.6 ms (sequential scan of all tar headers). The tarfile linear-scan scenario (119 MB/s) shows the 80 × penalty of random access without any index.

Note

All reads are from OS page cache (local SSD). On cold disk or network I/O the differences narrow.

Loading V4 Data

V4 sequences are loaded by specifying one or more component store paths:

from ncore.data.v4 import SequenceComponentGroupsReader
from pathlib import Path

# Load sequence from multiple component stores
reader = SequenceComponentGroupsReader([
    Path("ncore4.zarr.itar"),           # default components
    Path("ncore4-calibv2.zarr.itar"),   # alternative calibration
])

# Access specific components
poses_readers = reader.open_component_readers(PosesComponent.Reader)
camera_readers = reader.open_component_readers(CameraSensorComponent.Reader)

Cloud and Remote Storage Access

NCore accesses all data paths through UPath (universal_pathlib), a drop-in pathlib.Path replacement built on top of fsspec. This means component stores can be read transparently from cloud storage backends – the same SequenceComponentGroupsReader API works for local files and remote URLs alike.

Supported URL Schemes

Any protocol that fsspec supports can be used as a component store path. Common examples:

Protocol	Example URL
S3	s3://my-bucket/sequences/seq01/ncore4.zarr.itar
GCS	gs://my-bucket/sequences/seq01/ncore4.zarr.itar
Azure Blob	az://my-container/sequences/seq01/ncore4.zarr.itar
HTTP(S)	https://example.com/data/ncore4.zarr.itar
Local	/data/sequences/seq01/ncore4.zarr.itar

Required Dependencies

nvidia-ncore ships with universal_pathlib (and its transitive dependency fsspec), which is sufficient for local paths. To access remote storage you need to install the corresponding fsspec filesystem implementation:

Protocol	Extra package	Credentials / configuration
S3	s3fs	AWS credentials (~/.aws/credentials, env vars, or IAM role)
GCS	gcsfs	GOOGLE_APPLICATION_CREDENTIALS or gcloud auth
Azure Blob	adlfs	AZURE_STORAGE_CONNECTION_STRING or az login
HTTP(S)	(built-in)	n/a

Install the extra package for the protocol you need, for example:

pip install nvidia-ncore s3fs          # for S3
pip install nvidia-ncore gcsfs         # for GCS
pip install nvidia-ncore adlfs         # for Azure Blob

Loading Remote Component Stores

Pass remote URLs directly to SequenceComponentGroupsReader:

from ncore.data.v4 import SequenceComponentGroupsReader
from upath import UPath

reader = SequenceComponentGroupsReader([
    UPath("s3://my-bucket/sequences/seq01/ncore4.zarr.itar"),
    UPath("s3://my-bucket/sequences/seq01/ncore4-labels.zarr.itar"),
])

For S3-compatible endpoints or when a specific AWS profile is needed, pass additional keyword arguments through UPath:

# Use a named AWS profile
store_path = UPath(
    "s3://my-bucket/sequences/seq01/ncore4.zarr.itar",
    profile="my-aws-profile",
)

# Tune download performance
store_path = UPath(
    "s3://my-bucket/sequences/seq01/ncore4.zarr.itar",
    profile="my-aws-profile",
    default_block_size=50 * 1024 * 1024,   # 50 MB download chunks
    default_cache_type="readahead",          # fsspec file-descriptor caching strategy
)

# Point to an S3-compatible endpoint (e.g. MinIO)
store_path = UPath(
    "s3://my-bucket/sequences/seq01/ncore4.zarr.itar",
    client_kwargs={"endpoint_url": "https://minio.example.com"},
)

reader = SequenceComponentGroupsReader([store_path])

All keyword arguments accepted by the underlying S3FileSystem (or the respective fsspec filesystem class for other protocols) can be forwarded this way.

Performance Recommendations

• Use the .itar format for cloud-stored data. The indexed tar archive enables random access with a single file, avoiding the large number of small HTTP requests that directory-based zarr stores would incur.

• Enable consolidated metadata (the default). The open_consolidated parameter on SequenceComponentGroupsReader is True by default, which pre-loads all zarr metadata in a single read. This is especially important for remote stores where each metadata lookup would otherwise be a separate round-trip.

• Increase the block size for high-bandwidth connections. The default_block_size parameter on UPath controls how much data is fetched per request. Larger values (e.g. 50–100 MB) reduce the number of requests at the cost of higher per-request latency.

• Consider local caching for repeated access to the same data. fsspec supports transparent caching via the simplecache or filecache protocols:

  # Cache remote files locally on first access
  store_path = UPath(
      "simplecache::s3://my-bucket/sequences/seq01/ncore4.zarr.itar",
      s3={"profile": "my-aws-profile"},
      simplecache={"cache_storage": "/tmp/ncore_cache"},
  )