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Mosaic is a tool and library to configure hybrid storage to be used in VM/CT environments. A storage is supposed to be suitable for both -- keeping lots of small files for VM/CT meta-data as well as fast and scalable disks for VMs and CTs data.

Mosaic: a place where metadata and data are. Mosaic consists of one (for now) place for metadata -- FS, and zero or more places for VM/CT data -- volumes.

Volume: this is where VM/CT files are. It can be thought of as a disk for VM or FS subtree for CT.

Both moctl (the tool) and libmosaic work on a pre-created storage that is described by a config file (in yaml). The config states the storage type (btrfs, loop, ploop, dmthin, etc.) and data location -- a string that is interpreted in type-specific manner. Other config options are, well, optional and also type-dependent.

When working with mosaic one should load config and then do whatever he wants. The moctl tool loads config on every start since all of the operations are "stateless" from the library point of view.

type: btrfs
location: /foo/bar

Describes a btrfs mosaic located at /foo/bar. The metadata part is then in the /foo/bar, so are the volumes. Adding

layout:
    fs: subdir

will mean that the metadata is at /foo/bar/subdir. Volumes remain in the /foo/bar and thus do not intersect with metadata.

The detailed description of config file is in doc/config.txt

The moctl usage is

moctl <config_file> <action> <arguments>

The config file should exist, moctl loads one before doing actions. The actions are mount, umount, new, clone and drop.

The first two mount and umount metadata or volume, new and clone create new empty volume or clone one from the existing, drop removes the volume. Each action accepts its own set of arguments.

The detailed description of moctl is in doc/moctl.txt

The API is declared in include/uapi/mosaic.h. Typical workflow with it looks like

mosaic_t m;

m = mosaic_open(name, 0);
mosaic_make_vol(m, name, size_in_bytes, 0);
...
mosaic_close(m, 0);

The detailed description of the API is in doc/libmosaic.txt