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btrfs-fuse

About

This is a read-only btrfs implementation using FUSE (Filesystem in Userspace).

Although btrfs is already in mainline Linux kernel, there are still use-cases for such read-only btrfs implementation:

  • Educational purpose

    Let new developers get a quick way to learn how a complex and modern filesystem works.

  • For certain bootloaders

    Certain bootloaders need code base compatible with their license.

  • As a last resort method for subpage/multipage support

    Currently (v5.16-rc) Linux kernel can only support sectorsize == pagesize , and 4K sectorsize with 64K page size.

    Thus this project can act as a last resort method to read data from filesystem with unsupported sectorsize.

Build

This project uses meson build system.

$ cd btrfs-fuse
$ meson setup build
$ cd build
$ ninja

This project has the following dependency:

  • uuid

    For uuid parsing

  • libb2

    For BLAKE2 checksum support

  • libcrypto

    For SHA256 checksum support

  • libxxhash

    For XXHASH checksum support

  • zlib

    For zlib decompression support

  • lzo2

    For lzo decompression support

  • libzstd

    For zstd decompression support

  • fuse3

    For FUSE interface.

There are some extra dependency for self-test tools:

  • xfsprogs
  • btrfs-progs
  • aio (optional)
  • liburing (optional)

Above dependencies are all for fsstress program.

Limitation

Currently btrfs-fuse has the following btrfs features missing:

  • xattr/fattr support

Above features are still under active development.

When such missing features is hit, btrfs-fuse would return -EOPNOTSUPP.

While there are still some other FUSE related feature missing:

  • Proper subvolume inode address space

    This is due to FUSE limitation, that one FUSE must has the same stat::st_dev. In kernel btrfs returns different stat::st_dev for different subvolumes, but in FUSE we don't have the ability do the same thing.

Usage

$ btrfs-fuse [<fuse options>] <device> [<extra devs> ...] <mnt>

Please note that, if multiple devices are passed into btrfs-fuse and contains different file systems, btrfs-fuse will use the last device to initialize the mount.

That's to say, for the following example:

$ mkfs.btrfs -f /dev/test/scratch1
$ mkfs.btrfs -f /dev/test/scratch2
$ btrfs-fuse /dev/test/scratch1 /dev/test/scratch2 /tmp/mount

Then only btrfs on /dev/test/scratch2 will be mounted onto /tmp/mount.

Selftest

btrfs-fuse has a self-test script in tests/test.sh.

Usage:

# ./tests/test.sh <builddir>

The test case will utilize fsstress to generate a complex enough fs with btrfs kernel module, and then use fssum to generate a summary of the whole fs. Thus it needs root privilege.

Then mount using btrfs-fuse, and compare the filesystem content against the summary.

Also for profiles with duplication, the test case will also try to remove device(s) and manually corrupt device(s) and make sure the content is still correct.

As regular dm/dm-raid can handle missing devices, but since they don't have checksum, they are not able to handle corrupted devices.

License

All files at the root directory is under MIT license.

Files under libs and tests directories are under their own licenses. Mostly GPL-2.0+ or GPL-2.0-only.

Those external libs include:

  • crc32c.[ch]

    For CRC32C checksum support.

    Cross-ported from btrfs-progs, which is cross-ported from older kernel, which is still under GPL-2.0+ license.

  • list.h

    For kernel style list implementation.

    Cross-ported from btrfs-progs, which is cross-ported from kernel, and under GPL-2.0-only license.

  • rbtree.[ch] and rbtree_augmented.h

    For kernel style rb-tree implementation.

    Cross-ported from btrfs-progs, which is cross-ported from kernel, and under GPL-2.0+ license.

  • raid56.[ch] and tables.c

    For RAID56 rebuild.

    Cross-ported from btrfs-progs, which is cross-ported from kernel, and under GPL-2.0-only license.

  • fsstress.c

    For populating the test mount point.

    Cross-ported from fstests, which is cross-ported from LTP, and under GPL-2.0-only license.

  • fssum.c and md5.[ch]

    For verifying the content of the test filesystem.

    Cross-ported from fstests, under GPL-2.0-only license.

For projects which want to have btrfs read-only support, and already has a FUSE-like interface (like GRUB), those files should not be cross-ported to the project as above licenses are not compatible with the target project.

Instead either use wrappers around the interfaces provided by the target project, or start from scratch and follow the license of the target project.