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mkcomposefs - Man Page

create a composefs filesystem image


mkcomposefs SOURCE IMAGE


The composefs project uses EROFS image file to store metadata, and one or more separate directories containing content-addressed backing data for regular files.

mkcomposefs constructs the mountable "composefs image" using the source as input. It can also create the backing store directory. Typically the source is a directory, but with --from-file it can also be a file.


The provided SOURCEDIR argument must be a directory and its entire contents will be read recursively.  The provided IMAGE argument will be a mountable composefs image.

mkcomposefs accepts the following options:


This path will become a composefs "object store". Regular files larger than 64 bytes in the SOURCEDIR will be copied (reflinked if possible) into this target directory, named after their fsverity digest. Small files will be inlined into the image metadata. If possible, the added files will have fs-verity enabled.This directory should be passed to the basedir option when you mount the image.


Print the fsverity digest of the composefs metadata file.


Print the fsverity digest of the composefs metadata file, but don't write the image. If this is passed, the IMAGE argument should be left out.


Use a zero time (unix epoch) as the modification time for all files.


Don't add device nodes to the image.


Don't add xattrs to files in the image.


Only add xattrs with the "user." prefix to files in the image.


The source is a file in the composefs-dump(5) format. If the specified file is "-", the data is read from stdin.


The base version to use for the image format.


If this specifies a version higher than --version, then the actual image format version used will be adjusted upwards if that is beneficial for the image, up to the max version.


Number of threads to be used to calculate the file digests and copy. Default thread count is the number of processors when --threads is not specified.

Format Versioning

Composefs images are binary reproduceable, meaning that for a given input the result is always the same, giving the same digest of the image. This is important as the digest is used to validate the image, even if the image was re-created rather than transferred as is. However, sometimes the format needs to be changed, such as for example when a new type of file is introduced or a bug is fixed. This is handled by introducing a format version.

Specifying the version is done with two options, the base version (--version) and the max version (--max-version). When building an image, mkcomposefs tries to keep the image format as low as possible, but if some particular requested feature is not available with the base feature, but is accessible in the max version then the version used will be increased. This allows us to introduce new features and fix bugs in a later version and migrate to that using max versions, but still keeping the digests identical for unaffected images.

If you need 100% binary reproducibility over time, specify the same version and a max version each time.

Format version history:

The default if no version arguments are specified is version 0 and max version 1.

See Also

composefs-info(1), mount.composefs(1), composefs-dump(5)

composefs upstream ⟨https://github.com/containers/composefs

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