dwarfs/doc/mkdwarfs.md

mkdwarfs(1) -- create highly compressed read-only file systems

SYNOPSIS

mkdwarfs -i path -o file [options...]
mkdwarfs -i file -o file --recompress [options...]

DESCRIPTION

mkdwarfs allows you to create highly compressed, read-only file systems in the DwarFS format. DwarFS is similar to file systems like SquashFS, cramfs or CromFS, but it has some distinct features. For more detail, see dwarfs(1).

In its simplest usage form, you can create a file system containing the full contents of /path/dir with:

mkdwarfs -i /path/dir -o image.dwarfs

After that, you can mount it with dwarfs(1):

dwarfs image.dwarfs /path/to/mountpoint

OPTIONS

There two mandatory options for specifying the input and output:

• -i, --input=path|file: Path to the root directory containing the files from which you want to build a filesystem. If the --recompress option is given, this argument is the source filesystem.

• -o, --output=file: File name of the output filesystem.

Most other options are concerned with compression tuning:

• -l, --compress-level=value: Compression level to use for the filesystem. This is intended to provide some sensible defaults and will depend on which compression libraries were available at build time. This is meant to be the "easy" interface to configure compression, and it will actually pick the defaults for five distinct options: --block-size-bits, --compression, --schema-compression, --metadata-compression and --blockhash-window-sizes. See the output of mkdwarfs --help for a table listing the exact defaults used for each compression level.

• -S, --block-size-bits=value: The block size used for the compressed filesystem. The actual block size is two to the power of this value. The valid range of this option is from 12 to 28, i.e. block sizes between 4kiB and 256MiB. Larger block sizes will offer better compression, but will be slower and consume more memory when actually using the filesystem, as blocks will have to be fully or at least partially decompressed into memory. Value between 20 and 24, i.e. between 1MiB and 16MiB, are usually a good compromise.

• -N, --num-workers=value: Number of worker threads used for building the filesystem. This defaults to the number of processors available on your system. Use this option if you want to limit the resources used by mkdwarfs.

• -M, --max-scanner-workers=value: Maximum number of worker threads used for building the filesystem. This defaults to the number of processors available on your system, but the number of active workers will be automatically adjusted based on load. With fast SSDs, scanning multiple files is probably fine, but with older spinning disks, having less concurrency can improve overall speed.

• -L, --memory-limit=value: Approximately how much memory you want mkdwarfs to use during filesystem creation. Note that currently this will only affect the block manager component, i.e. the number of filesystem blocks that are in flight but haven't been compressed and written to the output file yet. So the memory used by mkdwarfs can certainly be larger than this limit, but it's a good option when building large filesystems with expensive compression algorithms.

• -C, --compression=algorithm[:algopt[=value]]...: The compression algorithm and configuration used for file system data. The value for this option is a colon-separated list. The first item is the compression algorithm, the remaining item are its options. Options can be either boolean or have a value. For details on which algorithms and options are available, see the output of mkdwarfs --help.

• --schema-compression=algorithm[:algopt[=value]]...: The compression algorithm and configuration used for the metadata schema. Takes the same arguments as --compression above. The schema is very small, in the hundreds of bytes, so this is only relevant for extremely small file systems. The default (zstd) has shown to give considerably better results than any other algorithms.

• --metadata-compression=algorithm[:algopt[=value]]...: The compression algorithm and configuration used for the metadata. Takes the same arguments as --compression above. The metadata has been optimized for very little redundancy and leaving it uncompressed, the default for all levels below 8, has the benefit that it can be mapped to memory and used directly. This significantly improves mount time for large file systems compared to e.g. an lzma compressed metadata block.

• --recompress: Take an existing DwarFS filesystem and recompress it using a different compression algorithm. Note that only the compression algorithm, i.e. the --compression option, has an impact on the new filesystem. Other options, e.g. --block-size-bits, have no impact.

• --set-owner=uid: Set the owner for all entities in the file system. This can reduce the size of the file system. If the input only has a single owner already, setting this won't make any difference.

• --set-group=gid: Set the group for all entities in the file system. This can reduce the size of the file system. If the input only has a single group already, setting this won't make any difference.

• --set-time=time|now: Set the time stamps for all entities to this value. This can significantly reduce the size of the file system. You can pass either a unix time stamp or now.

• --order=none|path|similarity|nilsimsa|script: The order in which files will be written to the filesystem. Choosing none, the files will be stored in the order in which they are discovered. With path, they will be sorted asciibetically by path name. With similarity, they will be ordered using a simple, yet fast and efficient, similarity hash function. This is the default, as it will cause similar files to be located close to each other, which means compression will be better. nilsimsa ordering uses a different similarity function that is likely even better than similarity, but is comparatively slow to compute. It can be very slow, though it shouldn't be for typical inputs. YMMV. Last but not least, if scripting support is built into mkdwarfs, you can choose script to let the script determine the order.

• --blockhash-window-sizes=value[,value]...: Window sizes used for block hashing. These sizes, separated by commas, are again exponents to a base of two. These block hashes are used by mkdwarfs for finding identical segments in across multiple files. This is done on top of duplicate file detection, If a reasonable amount of duplicate segments is found, this means less blocks will be used in the filesystem and potentially less memory will be used when accessing the filesystem. It doesn't necessarily mean that the filesystem will be smaller, as this is remove redundany that cannot be exploited by the block compression anymore. But it shouldn't make the resulting filesystem any bigger. This option is used along with --window-increment-shift to determine how extensively this segment search will be. The smaller the window sizes, the more segments will obviously be found. However, this also means files will become more fragmented and thus the filesystem can be slower to use. If multiple window sizes are specified, larger segments will be searched first, meaning less fragmentation and better efficiency. However, multiple window sizes will also make mkdwarfs slower. It's all a tradeoff, so YMMV.

• --window-increment-shift=value: This option specifies how many hash values are kept for lookup. It is specified in number of right shifts of the window size. To give an example, if --block-hash-window-sizes=16 and --window-increment-shift=1, then a block hash across 65536 bytes will be stored at every 32768 bytes of input data. This means that not every 65536-byte duplicate segment will be detected, but duplicate segments of 98304 bytes or more will be detected. If you use a larger value for this option, the increments become smaller, and mkdwarfs will be slower and use more memory.

• --log-level=name: Specifiy a logging level.

• --no-progress: Don't show progress output while building filesystem.

• --help: Show program help, including defaults, compression level detail and supported compression algorithms.

If experimental Python support was compiled into mkdwarfs, you can use the following option to enable customizations via the scripting interface:

• --script=file[:class[(arguments...)]]: Specify the Python script to load. The class name is optional if there's a class named mkdwarfs in the script. It is also possible to pass arguments to the constuctor.

TIPS & TRICKS

Compression Ratio vs Decompression Speed

If high compression ratio is your primary goal, definitely go for lzma compression. However, I've found that it's only about 10% better than zstd at the highest level. The big advantage of zstd over lzma is that its decompression speed is about an order of magnitude faster. So if you're extensively using the compressed file system, you'll probably find that it's much faster with zstd.

Block, Schema and Metadata Compression

DwarFS filesystems consist of three distinct parts of data. Many blocks, which store actual file data and are decompressed on demand, as well as one schema and one metadata section. The schema is tiny, typically less than 1000 bytes, and holds the details for how to interpret the metadata. The schema needs to be read into memory once and is subsequently never accessed again. The metadata itself is usually not compressed, although it can be if you want to squeeze a few more kilobytes out of the file system. If it is compressed, it will be fully decompressed into memory. Otherwise, the metadata part of the file will simply be mapped into memory. The main difference is that compressed metadata, which being smaller, will potentially consume more memory and it will definitely take longer to mount the filesystem initially.

AUTHOR

Written by Marcus Holland-Moritz.

COPYRIGHT

Copyright (C) Marcus Holland-Moritz.

SEE ALSO

dwarfs(1)