This document describes the DwarFS file system format, version 2.5.
A DwarFS file system image is just a sequence of blocks, optionally prefixed by a "header", which is typically some sort of shell script. Each block has the following format:
┌───┬───┬───┬───┬───┬───┬───┬───┐
0x00 │'D'│'W'│'A'│'R'│'F'│'S'│MAJ│MIN│ MAJ=0x02, MIN=0x05 for v2.5
├───┴───┴───┴───┴───┴───┴───┴───┤
0x08 │ │ Used for full (slow) integrity
├─ SHA-512/256 integrity hash ─┤ check with `dwarfsck`.
0x10 │ over the remainder of the │
├─ block data, starting at ─┤
0x18 │ offset 0x28. │
├─ ─┤
0x20 │ │
├───────────────────────────────┤
0x28 │ XXH3-64 hash over remainder │ Used for fast integrity check.
├───────────────┬───────┬───────┤
0x30 │Section Number │SecType│CompAlg│ All integer fields are in LE
├───────────────┴───────┴───────┤ byte order.
0x38 │ Length of remaining data │
├───────────────────────────────┤
0x40 │ │
│ Section data compressed using │
│ CompAlg algorithm. │
│ │
│ │
│ │
└───────────────────────────────┘
A couple of notes:
-
No padding is added between blocks.
-
The list of blocks can easily be traversed by using the length field to skip to the start of the next section.
-
Corruption can easily be detected using the XXH3-64 hash. Computation of this hash is so fast that it is in fact checked every single time a file system block is loaded.
-
Integrity can furthermore be checked using the SHA-512/256 hash. This is much slower, but should rarely be needed.
-
All header fields, except for the magic and version number, are protected by the hashes.
-
In case of corruption, sections can easily be retrieved by scanning for the magic. The version number can be recovered by looking at all sections and choosing the majority. The explicit section number helps to recover data if multiple sections are missing.
-
A major version number change will render the format incompatible.
-
A minor version number change will be backwards compatible, i.e. an old program will refuse to read a file system with a minor version larger than the one it supports. However, a new program will still read all file systems with a smaller minor version number.
In order to access the file system data when it is prefixed by a header,
the size of the header must be known. It can either be given to the
tools or the FUSE driver explicitly (using e.g. the --image-offset or
-o offset options), or it can be determined automatically (by passing
auto as the argument to the aforementioned options).
Automatic detection works by scanning the file for the section header
magic (DWARFS) and validating the match by looking up the second
section header using the length of the first section and also checking
its magic. It is rather unlikely that a file is created accidentally
that would pass this check, although one could be crafted manually
without any problems.
There are currently 4 different section types.
-
BLOCK(0): A block of data. This is where all file data is stored. There can be an arbitrary number of blocks of this type. -
METADATA_V2_SCHEMA(7): The schema used to layout theMETADATA_V2block contents. This is stored in "compact" thrift encoding. -
METADATA_V2(8): This section contains the bulk of the metadata. It's essentially just a collection of bit-packed arrays and structures. The exact layout of each list and structure depends on the actual data and is stored separately inMETADATA_V2_SCHEMA. -
SECTION_INDEX(9): The section index is, well, an index of all sections in the file system. If present (creation of the index can be suppressed with--no-section-index), this is required to be the last section. Each entry in the section index is a 64-bit value with the upper 16 bits being the section type and the lower 48 bits being the offset relative to the first section. That is, the section index is independent of whether or not a header is present before the first section. The whole point of the section index is to avoid having to build an index by visiting all section headers. In order to find the start of the section index, you only have to read the last 64-bit value from the file, check if the upper 16 bits match theSECTION_INDEX, then add the image offset (header size) to the lower 48 bits. At that position in the file, you should find a valid section header for the section index. -
HISTORY(10): File system history information as definedthrift/history.thrift. This is stored in "compact" thrift encoding. Zero or more history sections are supported.
Here is a high-level overview of how all the bits and pieces relate to each other:
═════════════ ┌─────────────────────────────────────────────────────────────────────────┐
DwarFS v2.5 │ │
═════════════ │ ┌───────────────────────────────────────────┐ │
│ │ │ │
dir_entries[] ▼ │ inodes[] │ directories[] │
╔════╗ ┌────────────────┐ │ S_IFDIR ──►┌───────────────────┐ │ ┌────────────────┴─┐
║root╟──►│ name_index: 0 │ │ │ mode_index: 0 ├──────┐ └─►│ parent_entry: 0 │
╚════╝ │ inode_num: 0 ├───────┴────────────►│ owner_index: 0 │ │ │ first_entry: 1 │
├────────────────┤ │ group_index: 0 │ │ ├──────────────────┤
┌───┤ name_index: 2 │ │ atime_offset: 0 │ │ │ parent_entry: 0 │
┌────┼───┤ inode_num: 5 ├───────┐ │ mtime_offset: 417 │ │ │ first_entry: 11 │
│ │ ├────────────────┤ │ │ ctime_offset: 0 │ │ ├──────────────────┤
│ ┌──┼───┤ name_index: 3 │ │ ├───────────────────┤ │ │ parent_entry: 5 │
│ │ │ │ inode_num: 9 ├────┐ │ │ ... │ │ │ first_entry: 12 │
│ │ │ ├────────────────┤ │ │ S_IFLNK ──►├───────────────────┤ │ ├──────────────────┤
│ │ │ │ │ │ │ │ mode_index: 2 │ │ │ │
│ │ │ │ ... │ │ └────────────►│ owner_index: 2 │ │ │ ... │
│ │ │ │ │ │ │ group_index: 0 │ │ │ │
│ │ │ └────────────────┘ │ │ atime_offset: 0 │ │ └──────────────────┘
│ │ │ │ │ mtime_offset: 298 │ │
│ │ │ │ │ ctime_offset: 0 │ │
│ │ │ names[] │ ├───────────────────┤ │ modes[]
│ │ │ ┌────────────┐ │ │ ... │ │ ┌─────────────┐
│ │ │ │ "usr" │ │ S_IFREG ──►├───────────────────┤ └────►│ 0040775 │
│ │ │ ├────────────┤ │ (unique) │ mode_index: 1 │ ├─────────────┤
│ │ │ │ "share" │ ├───────────────►│ owner_index: 0 ├──────┐ │ 0100644 │
│ │ │ ├────────────┤ │ │ group_index: 0 │ │ ├─────────────┤
│ │ └──►│ "words" │ │ │ atime_offset: 0 │ │ │ ... │
│ │ ├────────────┤ │ │ mtime_offset: 298 │ │ └─────────────┘
│ └─────►│ "lib" │ │ │ ctime_offset: 0 │ │
│ ├────────────┤ │ ├───────────────────┤ │ uids[]
│ │ "ls" │ │ │ ... │ │ ┌─────────────┐
│ ├────────────┤ │ S_IFREG ──►├───────────────────┤ └────►│ 0 │
│ │ ... │ │ ┌──(shared) │ mode_index: 4 │ ├─────────────┤
▼ └────────────┘ │ │ │ owner_index: 2 │ │ 1000 │
(inode-off) │ │ │ group_index: 1 ├──────┐ ├─────────────┤
│ │ │ │ atime_offset: 0 │ │ │ ... │
│ symlink_table[] │ │ │ mtime_offset: 298 │ │ └─────────────┘
│ ┌────────────┐ │ │ │ ctime_offset: 0 │ │
│ │ 1 ├───┐ │ │ ├───────────────────┤ │ gids[]
│ ├────────────┤ │ │ │ │ ... │ │ ┌─────────────┐
└───────►│ 0 │ │ │ │ S_IFBLK ──►├───────────────────┤ │ │ 0 │
├────────────┤ │ │ │ S_IFCHR │ │ │ ├─────────────┤
│ ... │ │ ┌─┼──┼─────────────┤ ... │ └────►│ 100 │
└────────────┘ │ │ │ │ │ │ ├─────────────┤
│ │ │ │ S_IFSOCK ──►├───────────────────┤ │ ... │
│ │ │ │ S_IFIFO │ │ └─────────────┘
symlinks[] │ │ │ │ │ ... │
┌────────────┐ │ │ │ │ │ │
│ "../foo" │ │ │ │ │ └───────────────────┘ chunks[]
├────────────┤ │ │ │ │ ┌──────────────┐
│ "foo/bar" │◄──┘ │ │ │ ┌────►│ block: 0 │
├────────────┤ │ └──┼──────────►(inode-off) │ │ offset: 1698 │
│ ... │ │ │ │ chunk_table[] │ │ size: 1012 │
└────────────┘ ▼ ▼ │ ┌─────────────┐ │ ├──────────────┤
(inode-off) (inode-off) └──────────►│ 0 ├─┘ ┌──►│ block: 0 │
│ │ ├─────────────┤ │ │ offset: 1604 │
devices[] │ │ shared_files_table[] │ 1 ├───┘ │ size: 94 │
┌────────────┐ │ │ ┌───────────┐ ├─────────────┤ ├──────────────┤
│ 0x0107 │ │ └────►│ 0 ├───┬─────►│ 2 ├───┬──►│ block: 0 │
├────────────┤ │ ├───────────┤ │ ├─────────────┤ │ │ offset: 0 │
│ 0x0502 │◄─────┘ │ 0 ├───┘ │ 2 ├───┘ │ size: 1517 │
├────────────┤ ├───────────┤ ├─────────────┤ ├──────────────┤
│ ... │ │ ... │ │ ... │ │ ... │
└────────────┘ └───────────┘ └─────────────┘ └──────────────┘
Thanks to the bit-packing, fields that are unused or only contain a
single (zero) value, e.g. a group_index that's always zero because
all files belong to the same group, does not occupy any space in the
metadata block.
Before you can start traversing the metadata, you need to determine
the offsets for symlinks, regular files, devices etc. in the inodes
list. The index into this list is the inode_num from dir_entries,
but you can perform direct lookups based on the inode number as well.
The inodes list is strictly in the following order:
- directory inodes (
S_IFDIR) - symlink inodes (
S_IFLNK) - regular unique file inodes (
S_IREG) - regular shared file inodes (
S_IREG) - character/block device inodes (
S_IFCHR,S_IFBLK) - socket/pipe inodes (
S_IFSOCK,S_IFIFO)
The offsets can thus be found by using a binary search with a
predicate on the inode mode. The shared file offset can be found
by subtracting the length of shared_files_table from the total
number of regular files.
The difference between unique and shared file inodes is that
there is only one unique file inode that references a particular
index in the chunk_table, whereas there are multiple shared
file inodes that will reference the same index. This is how DwarFS
implements file-level de-duplication beyond hardlinks. Hardlinks
share the same inode. Duplicate files that are not hardlinked each
have a unique inode, but still reference the same content through
the chunk_table.
The shared_files_table provides the necessary indirection that
maps a shared file inode to a chunk_table index.
You typically start at the root directory which is at dir_entries[0],
inodes[0] and directories[0]. Note that the root directory
implicitly has no name, so that dir_entries[0].name_index
should not be used.
To determine the contents of a directory, we determine the range
of entries from directories[inode_num].first_entry to
directories[inode_num + 1].first_entry. If both values are equal,
the directory is empty. Otherwise, we can look up the entries in
dir_entries[].
So for directory inodes, you can directly index into directories
using the inode number.
For link inodes, you can index into symlink_table, but you have
to adjust the index for the link inode offset determined before:
link_index = symlink_table[inode_num - link_inode_offset]
With that, you can look up the contents of the symlink:
contents = symlinks[link_index]
For unique regular file inodes, you can index into chunk_table
after adjusting the index:
chunk_index = inode_num - file_inode_offset
For shared regular file inodes, you can index into the (unpacked)
shared_files_table:
shared_index = shared_files[inode_num - file_inode_offset - num_unique_files]
Then, you can index into chunk_table, but you need to adjust the
index once more:
chunk_index = shared_index + num_unique_files
The range of chunks that make up a regular file inode is
chunk_table[chunk_index] to chunk_table[chunk_index + 1]. If
these values are equal, the file is empty. Otherwise, you need
to look up the range of chunks in chunks.
Each chunk references a range of bytes in one file system BLOCK.
These need to be concatenated to produce the file contents.
Both chunk_table and directories have a sentinel entry at the
end to make sure you can perform range lookups for all indices.
Last but not least, to read the device id for a device inode, you
can index into devices:
device_id = devices[inode_num - device_inode_offset]
The overview above assumes metadata without any additional packing, which can be produced using:
mkdwarfs --pack-metadata=none,plain
However, this isn't the default, and parts of the metadata are likely stored in a packed format. These are mostly easy to unpack.
The shared_files_table can be stored in a packed format that
only encodes the number of shared links to a chunk_table index.
As the minimum number of links is always 2 (otherwise it wouldn't
be shared), the numbers in the packed format are additionally
offset by 2. So for example, a packed table like
[0, 3, 1, 0, 1]
would unpack to:
[0, 0, 1, 1, 1, 1, 1, 2, 2, 2, 3, 3, 4, 4, 4]
The packed format is used when options.packed_shared_files_table
is true.
The directories table, when stored in packed format, omits
all parent_entry fields and uses delta compression for the
first_entry fields.
In order to unpack all information, you first have to delta-
decompress the first_entry fields, then traverse the whole
directory tree once to fill in the parent_entry fields.
This sounds like a lot of work, but it's actually reasonably
fast. For example, for a file system with 15 million entries
in 90,000 directories, reconstructing the directories takes
only about 50 milliseconds.
The packed format is used when options.packed_directories
is true.
The chunk_table can also be stored delta-compressed and
must be unpacked accordingly.
The packed format is used when options.packed_chunk_table
is true.
Both the names and symlinks tables can be stored in a
packed format in compact_names and compact_symlinks.
There are two separate packing schemes which can be combined.
If none of these schemes is active, the difference between
e.g. names and compact_names is that the former is stored
as a "proper" list, whereas the latter is stored as a single
string plus an index of offsets. As lists of strings store
both offset and length for each element, this already saves
the storage for the length fields, which can easily be
determined from the offsets at run-time.
If the packed_index scheme is used in addition, the index
is stored delta-compressed.
Last but not least, the individual strings can be compressed
as well. The fsst library
allows for compression of short strings with random access
and is typically able to reduce the overall size of the
string tables by 50%, using a dictionary that is only a few
hundred bytes long. If a symtab is set for the string table,
this compression is used.
Written by Marcus Holland-Moritz.
Copyright (C) Marcus Holland-Moritz.
mkdwarfs(1), dwarfs(1), dwarfsextract(1), dwarfsck(1)