dwarfs/doc/dwarfs.md

dwarfs(1) -- mount highly compressed read-only file system

SYNOPSIS

dwarfs image mountpoint [options...]

DESCRIPTION

dwarfs is the FUSE driver for DwarFS, a highly compressed, read-only file system. As such, it's similar to file systems like SquashFS, cramfs or CromFS, but it has some distinct features.

Other than that, it's pretty straightforward to use. Once you've created a file system image using mkdwarfs(1), you can mount it with:

dwarfs image.dwarfs /path/to/mountpoint

OPTIONS

In addition to the regular FUSE options, dwarfs supports the following options:

• -o cachesize=value: Size of the block cache, in bytes. You can append suffixes (k, m, g) to specify the size in KiB, MiB and GiB, respectively. Note that this is not the upper memory limit of the process, as there may be blocks in flight that are not stored in the cache. Also, each block that hasn't been fully decompressed yet will carry decompressor state along with it, which can use a significant amount of additional memory. For more details, see mkdwarfs(1).

• -o workers=value: Number of worker threads to use for decompressing blocks. If you have a lot of CPUs, increasing this number can help speed up access to files in the filesystem.

• -o decratio=value: The ratio over which a block is fully decompressed. Blocks are only decompressed partially, so each block has to carry the decompressor state with it until it is fully decompressed. However, if a certain fraction of the block has already been decompressed, it may be beneficial to just decompress the rest and free the decompressor state. This value determines the ratio at which we fully decompress the block rather than keeping a partially decompressed block. A value of 0.8 means that as long as we've decompressed less than 80% of the block, we keep the partially decompressed block, but if we've decompressed more then 80%, we'll fully decompress it.

• -o mlock=none|try|must Set this to try or must instead of the default none to try or require mlock()ing of the file system metadata into memory.

• -o debuglevel=name: Use this for different levels of verbosity along with either the -f or -d FUSE options. This can give you some insight over what the file system driver is doing internally, but it's mainly meant for debugging and the debug and trace levels in particular will slow down the driver.

There's two particular FUSE options that you'll likely need at some point, e.g. when trying to set up an overlayfs mount on top of a DwarFS image:

• -o allow_root and -o allow_other These will ensure that the mounted file system can be read by either root or any other user in addition to the user that started the fuse driver. So if you're running dwarfs as a non-privileged user, you want to -o allow_root in case root needs access, for example when you're trying to use overlayfs along with dwarfs. If you're running dwarfs as root, you need allow_other.

TIPS & TRICKS

Setting up a writable file system on top of a DwarFS image

This will show you how to set up a read/write layer on top of a read-only DwarFS image, which can be incredibly handy if you want to be able to partially and/or temporarily modify/amend the contents of your read-only image.

My primary use case for this feature is keeping over 1000 Perl versions in the DwarFS image and then setting up a read/write layer to be able to install additional modules for each of these versions. When I didn't need the modules anymore, I could just completely wipe the read/write layer and get my pristine original set of Perl versions back.

Here's what you need to do:

• Create a set of directories. In my case, these are all located in /tmp/perl as this was the orginal install location.

  cd /tmp/perl
  mkdir install-ro
  mkdir install-rw
  mkdir install-work
  mkdir install
  

• Mount the DwarFS image. -o allow_root is needed to make sure overlayfs has access to the mounted file system. In order to use -o allow_root, you may have to uncomment or add user_allow_other in /etc/fuse.conf.

  dwarfs perl-install.dwarfs install-ro -o allow_root
  

• Now set up overlayfs.

  sudo mount -t overlay overlay -o lowerdir=install-ro,upperdir=install-rw,workdir=install-work install
  

• That's it. You should now be able to access a writeable version of your DwarFS image in install.

You can go even further than that. Say you have different sets of modules that you regularly want to layer on top of the base DwarFS image. In that case, you can simply build a new DwarFS image from the read-write directory after unmounting the overlayfs, and selectively add this by passing a colon-separated list to the lowerdir option when setting up the overlayfs mount:

sudo mount -t overlay overlay -o lowerdir=install-ro:install-modules install

If you want this merged overlay to be writable, just add in the upperdir and workdir options from before again.

AUTHOR

Written by Marcus Holland-Moritz.

COPYRIGHT

Copyright (C) Marcus Holland-Moritz.

SEE ALSO

mkdwarfs(1)