Using rosbag2_storage_mcap: Integrating MCAP for Efficient ROS 2 Data Storage

yoshimura |July 3, 2025 | ROS

* This blog post is an English translation of an article originally published in Japanese on January 23, 2023.

Introduction

With https://github.com/ros2/rosbag2/pull/1163, the rosbag2_storage_mcap package was added, allowing MCAP to be selected as the storage format for rosbag2. This article introduces how to use rosbag2_storage_mcap.

Test Environment

The environment used for operational verification is as follows:

Ubuntu 22.04
ROS2 Humble
rosbag2_storage_mcap 0.6.0

What is MCAP?

MCAP (https://mcap.dev/index.html) is a data format for recording various types of timestamped data. It allows multiple streams of structured and unstructured data (e.g., ROS2, Protobuf, JSON Schema) to be recorded in a single file. For details on the MCAP file format, please refer to https://mcap.dev/specification/index.html. Furthermore, it has been announced at https://foxglove.dev/blog/mcap-as-the-ros2-default-bag-format that MCAP is planned to become the default storage for rosbag2 starting from ROS 2 Iron Irwini.

Benefits of MCAP

The benefits of MCAP, extracted from the “Benefits” section of https://mcap.dev/index.html, are listed below:

Good write performance
Easy recovery if data is corrupted during recording
Data can be extracted without scanning the entire file
No extra package dependencies for decoding

rosbag2_storage_mcap Installation

Execute the following command to install the rosbag2_storage_mcap package.

$ sudo apt install ros-humble-rosbag2-storage-mcap

Recording rosbag2 in MCAP Format

Execute the following command to save all topics in MCAP format. The -s option specifies the storage type. For detailed options, please check ros2 bag -h.

$ ros2 bag record -s mcap --all

Comparison of rosbag2 File Sizes (SQLite3 vs MCAP)

For rosbag2_storage_mcap, as described in https://github.com/ros2/rosbag2/blob/humble/rosbag2_storage_mcap/README.md#writer-configuration, the following can be specified for compression:

compression
- Can specify None, Lz4, or Zstd.
compressionLevel
- Can specify Fastest, Fast, Default, Slow, or Slowest.

Here, we checked the rosbag2 file sizes under the following conditions:

SQLite3 (ROS2 Humble default): No compression
SQLite3 (ROS2 Humble default): Zstd compression
MCAP: Zstd(fast)
MCAP: Zstd(default)
MCAP: Zstd(slow)
MCAP: Lz4(fast)

The file sizes of rosbag2 generated under each condition are shown in the table below. However, the MCAP+Lz4(default) and MCAP+Lz4(slow) cases were excluded from measurement because an error occurred when saving the rosbag2 file.

RGB-D Data

The results when using RGB-D data (see Appendix for details) are shown in the table below. It can be seen that MCAP+Zstd(slow) results in a file size comparable to SQLite3+Zstd.

storage type	compression	size(MB)
SQLite3	No compression	1359
SQLite3	Zstd	408
MCAP	Zstd(fast)	500
MCAP	Zstd(default)	472
MCAP	Zstd(slow)	393
MCAP	Lz4(fast)	633

Autoware sample rosbag2

The results when using the Autoware sample rosbag2 (see Appendix for details) are shown in the table below. It can be seen that MCAP+Zstd(slow) results in a file size comparable to SQLite3+Zstd.

storage type	compression	size(GB)
SQLite3	No compression	7.9
SQLite3	Zstd	2.8
MCAP	Zstd(fast)	3.1
MCAP	Zstd(default)	2.7
MCAP	Zstd(slow)	2.8
MCAP	Lz4(fast)	3.6

MCAP CLI Tools

We introduce CLI tools for handling MCAP. Download from https://github.com/foxglove/mcap/blob/releases/cpp/v0.6.0/go/cli/mcap/README.md. The execution options are as follows:

Usage:
  mcap [command]

Available Commands:
  add         Add records to an existing MCAP file
  cat         Cat the messages in an MCAP file to stdout
  completion  Generate the autocompletion script for the specified shell
  compress    Compress data in an MCAP file
  convert     Convert a bag file to an MCAP file
  doctor      Check an MCAP file structure
  filter      Copy some filtered MCAP data to a new file
  get         Get a record from an MCAP file
  help        Help about any command
  info        Report statistics about an MCAP file
  list        List records of an MCAP file
  merge       Merge a selection of MCAP files by record timestamp
  recover     Recover data from a potentially corrupt MCAP file
  version     Output version information

Flags:
      --config string   Config file (default is $HOME/.mcap.yaml)
  -h, --help            help for mcap

Use "mcap [command] --help" for more information about a command.

Here, we will introduce the following commands:

info
cat
doctor

info command

Displays a summary of the information recorded in the MCAP file. An execution example is as follows:

$ mcap info zstd_default_0.mcap

library: libmcap 0.7.0
profile: ros2
messages: 5326
duration: 47.452457991s
start: 2023-01-05T16:28:57.516754852+09:00 (1672903737.516754852)
end: 2023-01-05T16:29:44.969212843+09:00 (1672903784.969212843)
compression:
        zstd: [1194/1194 chunks] [1356 MB/470 MB (65.29%)] [9.90 MB/sec]
channels:
        (1) /rosout                                      35 msgs (0.74 Hz)    : rcl_interfaces/msg/Log [ros2msg]
        (2) /events/write_split                           0 msgs (0.00 Hz)    : rosbag2_interfaces/msg/WriteSplitEvent [ros2msg]
        (3) /events/read_split                            0 msgs (0.00 Hz)    : rosbag2_interfaces/msg/ReadSplitEvent [ros2msg]
        (4) /parameter_events                             0 msgs (0.00 Hz)    : rcl_interfaces/msg/ParameterEvent [ros2msg]
        (5) /camera/depth/image_rect_raw                661 msgs (13.93 Hz)   : sensor_msgs/msg/Image [ros2msg]
        (6) /camera/aligned_depth_to_color/camera_info  661 msgs (13.93 Hz)   : sensor_msgs/msg/CameraInfo [ros2msg]
        (7) /camera/depth/metadata                      661 msgs (13.93 Hz)   : realsense2_camera_msgs/msg/Metadata [ros2msg]
        (8) /camera/extrinsics/depth_to_depth             1 msgs (0.02 Hz)    : realsense2_camera_msgs/msg/Extrinsics [ros2msg]
        (9) /camera/aligned_depth_to_color/image_raw    661 msgs (13.93 Hz)   : sensor_msgs/msg/Image [ros2msg]
        (10) /camera/color/camera_info                  661 msgs (13.93 Hz)   : sensor_msgs/msg/CameraInfo [ros2msg]
        (11) /camera/color/image_raw                    661 msgs (13.93 Hz)   : sensor_msgs/msg/Image [ros2msg]
        (12) /camera/color/metadata                     661 msgs (13.93 Hz)   : realsense2_camera_msgs/msg/Metadata [ros2msg]
        (13) /camera/depth/camera_info                  661 msgs (13.93 Hz)   : sensor_msgs/msg/CameraInfo [ros2msg]
        (14) /tf_static                                   1 msgs (0.02 Hz)    : tf2_msgs/msg/TFMessage [ros2msg]
        (15) /camera/extrinsics/depth_to_color            1 msgs (0.02 Hz)    : realsense2_camera_msgs/msg/Extrinsics [ros2msg]
        (16) /camera/imu                                  0 msgs (0.00 Hz)    : sensor_msgs/msg/Imu [ros2msg]
attachments: 0
metadata: 0

cat command

Displays a summary of the recorded topic contents to standard output. An execution example is as follows:

$ mcap cat zstd_default_0.mcap

1672903737516754852 /rosout [rcl_interfaces/msg/Log] [0 1 0 0 57 124 182 99 88 237]...
1672903737516870652 /rosout [rcl_interfaces/msg/Log] [0 1 0 0 57 124 182 99 108 241]...
≪Omitted≫
1672903741204468335 /camera/aligned_depth_to_color/camera_info [sensor_msgs/msg/CameraInfo] [0 1 0 0 61 109 182 99 0 8]...
1672903741204537835 /camera/depth/metadata [realsense2_camera_msgs/msg/Metadata] [0 1 0 0 61 109 182 99 0 8]...
1672903741204545735 /tf_static [tf2_msgs/msg/TFMessage] [0 1 0 0 4 0 0 0 28 109]...
1672903741204576835 /camera/extrinsics/depth_to_color [realsense2_camera_msgs/msg/Extrinsics] [0 1 0 0 0 0 0 192 247 255]...
≪Omitted≫

doctor command

Checks if the rosbag2 file is corrupted. An execution example is as follows. The data used this time was not corrupted, so no error messages were displayed.

$ mcap doctor zstd_default_0.mcap

Examining zstd_default_0.mcap

Caveats

As of this writing, rqt_bag does not seem to support MCAP, and attempting to open a rosbag2 (MCAP) file with rqt_bag results in the following error message:

sqlite3.DatabaseError: file is not a database

Measures have been taken for Rolling, so it may be backported to Humble in the future.

Warning message: ‘No storage plugin found with id mcap’

This has been reported at https://github.com/ros2/rosbag2/issues/1195. This message has been removed for Rolling, but it seems that how to address it for Humble is under consideration.

Future Plans

Benchmarking
- Run benchmarks with reference to https://mcap.dev/performance-comparison/rosbag2-plugin-comparison.html
MCAP API
- Read and write MCAP from Python, C++, etc.
Visualization of MCAP data

Appendix

Input rosbag2 files used for file size verification

The input rosbag2 files (SQLite3, uncompressed) used for file size verification are as follows.

RGB-D Data

$ ros2 bag info rosbag2_2023_01_05-15_25_01/

Files:             rosbag2_2023_01_05-15_25_01_0.db3
Bag size:          1.3 GiB
Storage id:        sqlite3
Duration:          44.66s
Start:             Jan  5 2023 15:25:01.719 (1672899901.719)
End:               Jan  5 2023 15:25:45.786 (1672899945.786)
Messages:          5301
Topic information: Topic: /events/write_split | Type: rosbag2_interfaces/msg/WriteSplitEvent | Count: 0 | Serialization Format: cdr
                   Topic: /camera/imu | Type: sensor_msgs/msg/Imu | Count: 0 | Serialization Format: cdr
                   Topic: /camera/aligned_depth_to_color/camera_info | Type: sensor_msgs/msg/CameraInfo | Count: 661 | Serialization Format: cdr
                   Topic: /camera/depth/metadata | Type: realsense2_camera_msgs/msg/Metadata | Count: 661 | Serialization Format: cdr
                   Topic: /camera/extrinsics/depth_to_depth | Type: realsense2_camera_msgs/msg/Extrinsics | Count: 1 | Serialization Format: cdr
                   Topic: /camera/aligned_depth_to_color/image_raw | Type: sensor_msgs/msg/Image | Count: 661 | Serialization Format: cdr
                   Topic: /camera/color/camera_info | Type: sensor_msgs/msg/CameraInfo | Count: 661 | Serialization Format: cdr
                   Topic: /camera/color/image_raw | Type: sensor_msgs/msg/Image | Count: 661 | Serialization Format: cdr
                   Topic: /rosout | Type: rcl_interfaces/msg/Log | Count: 10 | Serialization Format: cdr
                   Topic: /camera/color/metadata | Type: realsense2_camera_msgs/msg/Metadata | Count: 661 | Serialization Format: cdr
                   Topic: /camera/depth/camera_info | Type: sensor_msgs/msg/CameraInfo | Count: 661 | Serialization Format: cdr
                   Topic: /camera/depth/image_rect_raw | Type: sensor_msgs/msg/Image | Count: 661 | Serialization Format: cdr
                   Topic: /parameter_events | Type: rcl_interfaces/msg/ParameterEvent | Count: 0 | Serialization Format: cdr
                   Topic: /tf_static | Type: tf2_msgs/msg/TFMessage | Count: 1 | Serialization Format: cdr

Autoware sample rosbag2

Used sample_moriyama_150324.bag2 from https://gitlab.com/autowarefoundation/autoware.ai/autoware/-/wikis/ROSBAG-Demo.

$ ros2 bag info sample_moriyama_150324.bag2/

Files:             sample_moriyama_150324.bag2/sample_moriyama_150324.bag2_0.db3
Bag size:          7.9 GiB
Storage id:        sqlite3
Duration:          479.172s
Start:             Feb 21 2020 02:04:31.772 (1582218271.772)
End:               Feb 21 2020 02:12:30.944 (1582218750.944)
Messages:          16768
Topic information: Topic: /clock | Type: rosgraph_msgs/msg/Clock | Count: 0 | Serialization Format: cdr
                   Topic: /nmea_sentence | Type: nmea_msgs/msg/Sentence | Count: 11980 | Serialization Format: cdr
                   Topic: /points_raw | Type: sensor_msgs/msg/PointCloud2 | Count: 4788 | Serialization Format: cdr

Reference URLs

https://github.com/ros2/rosbag2/pull/1160
- PR to make MCAP the default storage in Rolling
https://vimeo.com/showcase/9954564/video/767149733
- ROSCon 2022 video
http://download.ros.org/downloads/roscon/2022/MCAP%20A%20Next-Generation%20File%20Format%20for%20ROS%20Recording.pdf
- ROSCon 2022 slides
https://foxglove.dev/blog/announcing-the-mcap-storage-plugin-for-ros2
- Blog post about rosbag2_storage_mcap
https://foxglove.dev/blog/mcap-as-the-ros2-default-bag-format
- Announcement that MCAP will be the default storage for rosbag2 from ROS 2 Iron Irwini
https://foxglove.dev/blog/plotjuggler-adds-support-for-mcap
- Blog post about PlotJuggler adding MCAP support
https://mcap.dev/performance-comparison/rosbag2-plugin-comparison.html
- Benchmark results

About us

IR

Using rosbag2_storage_mcap: Integrating MCAP for Efficient ROS 2 Data Storage

Introduction

Test Environment

What is MCAP?

Benefits of MCAP

rosbag2_storage_mcap Installation

Recording rosbag2 in MCAP Format

Comparison of rosbag2 File Sizes (SQLite3 vs MCAP)

RGB-D Data

Autoware sample rosbag2

MCAP CLI Tools

info command

cat command

doctor command

Caveats

Warning message: ‘No storage plugin found with id mcap’

Future Plans

Appendix

Input rosbag2 files used for file size verification

RGB-D Data

Autoware sample rosbag2

Reference URLs

Author

yoshimura