README.md

rosbag2_storage_mcap

This package provides a storage plugin for rosbag2 which extends it with support for the MCAP file format.

⚠️ Move from ros-tooling organization

This package was recently merged into https://github.com/ros2/rosbag2 from https://github.com/ros-tooling/rosbag2_storage_mcap. To view historical pull requests and git history of this package, check here.

Installation

rosbag2_storage_mcap is available as part of the current ROS 2 distributions. On Ubuntu, after following the ROS 2 installation instructions, you can use:

# Replace "rolling" with your ROS distro (`echo $ROS_DISTRO`)
$ sudo apt install ros-rolling-rosbag2-storage-mcap

Usage

Use MCAP files with regular ros2 bag commands by adding the --storage mcap option (abbreviated as -s mcap):

$ ros2 bag record -s mcap /topic1 /topic2 ...

$ ros2 bag play -s mcap path/to/your_recording.mcap

$ ros2 bag info -s mcap path/to/your_recording.mcap

Writer Configuration

To configure details of the MCAP writer for ros2 bag record, use the --storage-config-file options to provide a YAML file describing mcap::McapWriterOptions. Field descriptions below copied from McapWriterOptions declaration

Field	Type / Values	Description
noChunkCRC	bool	Disable CRC calculation for Chunks. Ignored if noChunking=true.
noAttachmentCRC	bool	Disable CRC calculation for Attachments.
enableDataCRC	bool	Enables CRC calculation for the entire Data section. Useful when noChunking=True.
noSummaryCRC	bool	Disable CRC calculation for the Summary section.
noChunking	bool	Do not write Chunks to the file, instead writing Schema, Channel, and Message records directly into the Data section.
noMessageIndex	bool	Do not write Message Index records to the file. If noSummary=true and noChunkIndex=false, Chunk Index records will still be written to the Summary section, providing a coarse message index.
noSummary	bool	Do not write Summary or Summary Offset sections to the file, placing the Footer record immediately after DataEnd. This can provide some speed boost to file writing and produce smaller files, at the expense of requiring a conversion process later if fast summarization or indexed access is desired.
chunkSize	unsigned int	Target uncompressed Chunk payload size in bytes. Once a Chunk's uncompressed data meets or exceeds this size, the Chunk will be compressed (if compression is enabled) and written to disk. Note that smaller Chunks may be written, such as the last Chunk in the Data section. This option is ignored if noChunking=true.
compression	"None", "Lz4", "Zstd"	Compression algorithm to use when writing Chunks. This option is ignored if noChunking=true.
compressionLevel	"Fastest", "Fast", "Default", "Slow", "Slowest"	Compression level to use when writing Chunks. Slower generally produces smaller files, at the expense of more CPU time. These levels map to different internal settings for each compression algorithm.
forceCompression	bool	By default, Chunks that do not benefit from compression will be written uncompressed. This option can be used to force compression on all Chunks. This option is ignored if noChunking=true.
noRepeatedSchemas	bool	Advanced option.
noRepeatedChannels	bool	Advanced option.
noMetadataIndex	bool	Advanced option.
noChunkIndex	bool	Advanced option.
noStatistics	bool	Advanced option.
noSummaryOffsets	bool	Advanced option.

Example:

# mcap_writer_options.yml
noChunkCRC: false
noChunking: false
noMessageIndex: false
noSummary: false
chunkSize: 786432
compression: "Zstd"
compressionLevel: "Fast"
forceCompression: false

$ ros2 bag record -s mcap -o my_bag --all --storage-config-file mcap_writer_options.yml

Storage Preset Profiles

You can also use one of the preset profiles described below, for example:

$ ros2 bag record -s mcap -o my_bag --all --storage-preset-profile fastwrite

fastwrite

Configures the MCAP writer for the highest possible write throughput and lowest resource utilization. This preset does not calculate CRCs for integrity checking, and does not write a message index. This preset profile is useful for resource-constrained robots.

Equivalent to this storage configuration:

noChunking: true
noSummaryCRC: true

Using MCAPs written with fastwrite as a long-term storage format is not recommended. Some features will not work when reading MCAP files without a message index, such as reading messages from a subset of topics or seeking. When recording MCAPs on your robot with fastwrite, it is a good idea to post-process these files afterwards, to restore the message index and also save storage space:

# Using the MCAP CLI https://github.com/foxglove/mcap/tree/main/go/cli/mcap
$ mcap compress fast.mcap -o compressed.mcap
# Using `ros2 bag convert`
$ cat << EOF > convert.yaml
output_bags:
  - uri: compressed
    storage_id: mcap
    storage_preset_profile: zstd_small
EOF
$ ros2 bag convert -i fast.mcap -o convert.yaml

Equivalent to this storage configuration:

noChunking: true
noSummaryCRC: true

zstd_fast

Configures the MCAP writer to use chunk compression with zstd. Chunk compression yields file sizes comparable to bags compressed with file-level compression, but allows tools to efficiently read messages without decompressing the entire bag. This preset uses the lowest compression ratio and disables CRC calculation, to achieve high throughput while conserving disk space.

Equivalent to this storage configuration:

compression: "Zstd"
compressionLevel: "Fastest"
noChunkCRC: true

zstd_small

Configures the MCAP writer to write 4MB chunks, compressed with zstd using its highest compression ratio. This produces very small bags, but can be resource-intensive to write. This preset also calculates chunk CRCs, which allow a reader to determine if a chunk is corrupted. This preset is useful when using ros2 bag convert as a post-processing step.

Equivalent to this storage configuration:

compression: "Zstd"
compressionLevel: "Slowest"
chunkSize: 4194304 # 4 * 1024 * 1024

ROS 2 Distro maintenance

Whenever a ROS 2 distribution reaches EOL, search for comments marked COMPATIBILITY - which may no longer be needed when no new releases will be made for that distro.