CRC32C and Installing crcmod
Cloud Storage provides a cyclic redundancy check (CRC) header that
allows clients to verify the integrity of object contents. For non-composite
objects Cloud Storage also provides an MD5 header to allow clients to
verify object integrity, but for composite objects only the CRC is available.
gsutil automatically performs integrity checks on all uploads and downloads.
Additionally, you can use the
gsutil hash command to calculate a CRC for
any local file.
The CRC variant used by Cloud Storage is called CRC32C (Castagnoli), which is not available in the standard Python distribution. The implementation of CRC32C used by gsutil is provided by a third-party Python module called crcmod.
The crcmod module contains a pure-Python implementation of CRC32C, but using it results in very poor performance. A Python C extension is also provided by crcmod, which requires compiling into a binary module for use. gsutil ships with a precompiled crcmod C extension for macOS; for other platforms, see the installation instructions below.
At the end of each copy operation, the
gsutil cp and
commands validate that the checksum of the source file/object matches the
checksum of the destination file/object. If the checksums do not match,
gsutil will delete the invalid copy and print a warning message. This very
rarely happens, but if it does, please contact email@example.com.
To determine if the compiled version of crcmod is available in your Python
environment, you can inspect the output of the
gsutil version command for
the "compiled crcmod" entry:
$ gsutil version -l ... compiled crcmod: True ...
If your crcmod library is compiled to a native binary, this value will be True. If using the pure-Python version, the value will be False.
To control gsutil's behavior in response to crcmod's status, you can set the
"check_hashes" configuration variable. For details on this variable, see the
surrounding comments in your boto configuration file. If "check_hashes"
is not present in your configuration file, rerun
gsutil config to
regenerate the file.
These installation instructions assume that:
- You have
pipinstalled. Consult the pip installation instructions for details on how to install
- Your installation of
pipcan be found in your
PATHenvironment variable. If it cannot, you may need to replace
pip3in the commands below with the full path to the executable.
- You are installing the crcmod package for use with your system installation
of Python, and thus use the
sudocommand. If installing crcmod for a different Python environment (e.g. in a virtualenv), you should omit
sudofrom the commands below.
- You are using a Python 3 version with gsutil. You can determine which
Python version gsutil is using by running
gsutil version -land looking for the
python version: 2.x.xor
python version: 3.x.xline.
CentOS, RHEL, and Fedora
To compile and install crcmod:
yum install gcc python3-devel python3-setuptools redhat-rpm-config sudo pip3 uninstall crcmod sudo pip3 install --no-cache-dir -U crcmod
Debian and Ubuntu
To compile and install crcmod:
sudo apt-get install gcc python3-dev python3-setuptools sudo pip3 uninstall crcmod sudo pip3 install --no-cache-dir -U crcmod
To compile and install crcmod when using Enterprise SUSE for SAP 12:
sudo zypper install gcc python-devel sudo pip uninstall crcmod sudo pip install --no-cache-dir -U crcmod
To compile and install crcmod when using Enterprise SUSE for SAP 15:
sudo zypper install gcc python3-devel sudo pip uninstall crcmod sudo pip install --no-cache-dir -U crcmod
gsutil distributes a pre-compiled version of crcmod for macOS, so you shouldn't
need to compile and install it yourself. If for some reason the pre-compiled
version is not being detected, please let the Cloud Storage team know
gsutil help support).
To compile manually on macOS, you will first need to install XCode and then run:
pip3 install -U crcmod
An installer is available for the compiled version of crcmod from the Python Package Index (PyPi) at the following URL:
In some cases the installer will incorrectly install to <python_dir>\Lib\site-packages\crcmod
Manually copying the crcmod directory to the correct location should resolve the issue.