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. If you are unsure, please stick to the default level of 7. This is intended to provide some sensible defaults and will depend on which compression libraries were available at build time. The default level has been chosen to provide you with the best possible compression while still keeping the file system very fast to access. Levels 8 and 9 will switch to LZMA compression (when available), which will likely reduce the file system image size, but will make it about an order of magnitude slower to access, so reserve these levels for cases where you only need to access the data infrequently. This -l option is meant to be the "easy" interface to configure mkdwarfs, and it will actually pick defaults for six distinct options: --block-size-bits, --compression, --schema-compression, --metadata-compression, --window-size and --order. 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. zstd will give you the best compression while still keeping decompression very fast. lzma will compress even better, but decompression will be around ten times slower.

• --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 7, has the benefit that it can be mapped to memory and used directly. This improves mount time for large file systems compared to e.g. an lzma compressed metadata block. If you don't care about mount time, you can safely choose lzma compression here, as the data will only have to be decompressed once when mounting the image.

• --recompress[=all|block|metadata|none]: Take an existing DwarFS file system and recompress it using different compression algorithms. If no argument or all is given, all sections in the file system image will be recompressed. Note that only the compression algorithms, i.e. the --compression, --schema-compression and --metadata-compression options, have an impact on how the new file system is written. Other options, e.g. --block-size-bits or --order, have no impact. If none is given as an argument, none of the sections will be recompressed, but the file system is still rewritten in the latest file system format. This is an easy way of upgrading an old file system image to a new format. If block or metadata is given, only the block sections (i.e. the actual file data) or the metadata sections are recompressed. This can be useful if you want to switch from compressed metadata to uncompressed metadata without having to rebuild or recompress all the other data.

• --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.

• --time-resolution=sec|sec|min|hour|day: Specify the resolution with which time stamps are stored. By default, time stamps are stored with second resolution. You can specify "odd" resolutions as well, e.g. something like 15 second resolution is entirely possible. Moving from second to minute resolution, for example, will save roughly 6 bits per file system entry in the metadata block.

• --keep-all-times: As of release 0.3.0, by default, mkdwarfs will only save the contents of the mtime field in order to save metadata space. If you want to save atime and ctime as well, use this option.

• --order=none|path|similarity|nilsimsa[:limit[:depth[:mindepth]]]|script: The order in which inodes will be written to the file system. Choosing none, the inodes will be stored in the order in which they are discovered. With path, they will be sorted asciibetically by path name of the first file representing this inode. With similarity, they will be ordered using a simple, yet fast and efficient, similarity hash function. nilsimsa ordering uses a more sophisticated similarity function that is typically better than similarity, but is significantly slower to compute. However, computation can happen in the background while already building the file system. nilsimsa ordering can be further tweaked by specifying a limit and depth. The limit determines how soon an inode is considered similar enough for adding. A limit of 255 means "essentially identical", whereas a limit of 0 means "not similar at all". The depth determines up to how many inodes can be checked at most while searching for a similar one. To avoid nilsimsa ordering to become a bottleneck when ordering lots of small files, the depth is adjusted dynamically to keep the input queue to the segmentation/compression stages adequately filled. You can specify how much the depth can be adjusted by also specifying mindepth. The default if you omit these values is a limit of 255, a depth of 20000 and a mindepth of 1000. Note that if you want reproducible results, you need to set depth and mindepth to the same value. Last but not least, if scripting support is built into mkdwarfs, you can choose script to let the script determine the order.

• -W, --window-size=value: Window size of cyclic hash used for segmenting. This is again an exponent to a base of two. Cyclic hashes are used by mkdwarfs for finding identical segments 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 much smaller, as this removes redundany that cannot be exploited by the block compression any longer. But it shouldn't make the resulting filesystem any bigger. This option is used along with --window-step to determine how extensive 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 and metadata size will grow. Passing -W0 will completely disable duplicate segment search.

• --window-step=value: This option specifies how often cyclic hash values are stored for lookup. It is specified relative to the window size, as a base-2 exponent that divides the window size. To give a concrete example, if --window-size=16 and --window-step=1, then a cyclic hash across 65536 bytes will be stored at every 32768 bytes of input data. If --window-step=2, then a hash value will be stored at every 16384 bytes. This means that not every possible 65536-byte duplicate segment will be detected, but it is guaranteed that all duplicate segments of (window_size + window_step) bytes or more will be detected (unless they span across block boundaries, of course). If you use a larger value for this option, the increments become smaller, and mkdwarfs will be slower and use more memory.

• -B, --max-lookback-blocks=value: Specify how many of the most recent blocks to scan for duplicate segments. By default, only the current block will be scanned. The larger this number, the more duplicate segments will likely be found, which may further improve compression. However, it can also slow down compression and could cause the resulting filesystem to be less efficient to use, as single small files can now potentially span multiple filesystem blocks. Passing -B0 will completely disable duplicate segment search.

• --remove-empty-dirs: Removes all empty directories from the output file system, recursively. This is particularly useful when using scripts that filter out a lot of file system entries.

• --with-devices: Include character and block devices in the output file system. These are not included by default, and due to security measures in FUSE, they will never work in the mounted file system. However, they can still be copied out of the mounted file system, for example using rsync.

• --with-specials: Include named fifos and sockets in the output file system. These are not included by default.

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

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

• --progress=none|simple|ascii|unicode: Choosing none is equivalent to specifying --no-progress. simple will print a single line of progress information whenever the progress has significantly changed, but at most once every 2 seconds. This is also the default when the output is not a tty. unicode is the default behaviour, which shows a nice progress bar and lots of additional information. If your terminal cannot deal with unicode characters, you can switch to ascii, which is like unicode, but looks less fancy.

• --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: A potentially large number of 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 compressed by default, but it doesn't have to be. Actually, if you drop the compression level from 7 (the default) to 6, the only difference is that the metadata is left uncompressed. This can be useful if mounting speed of the file system is important, as the uncompressed metadata part of the file can then simply be mapped into memory.

AUTHOR

Written by Marcus Holland-Moritz.

COPYRIGHT

Copyright (C) Marcus Holland-Moritz.

SEE ALSO

dwarfs(1), dwarfsextract(1)