# dwarfs-format(5) -- DwarFS File System Format v2.5 ## DESCRIPTION This document describes the DwarFS file system format, version 2.5. ## FILE STRUCTURE A DwarFS file system image is just a sequence of blocks, optionally prefixed by a "header", which is typically some sort of shell script or other executable that intends to use the "bundled" DwarFS image. Each block in the DwarFS image 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 can still read all file systems with a smaller minor version number, although very old versions may at some point no longer be supported. ### Header Detection 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. ### Section Types Currently, the following different section types are defined: - `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. The file data can only be interpreted using the metadata blocks. The metadata contains a list of chunks for each file, each of which references a small part of the data in a single `BLOCK`. - `METADATA_V2_SCHEMA` (7): The [schema](https://github.com/facebook/fbthrift/blob/main/thrift/lib/thrift/frozen.thrift) used to layout the `METADATA_V2` block contents. This is stored in "compact" thrift encoding. The metadata cannot be read without the schema, as it defines the exact bit widths used to store each metadata field. - `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 in `METADATA_V2_SCHEMA`. The metadata format is defined in [metadata.thrift](../thrift/metadata.thrift) and the binary format that derives from that definition uses [Frozen2](https://github.com/facebook/fbthrift/blob/main/thrift/lib/cpp2/frozen/Frozen.h). Frozen2 is not only extremely space efficient, it also allows accessing huge data structures directly through memory-mapping. - `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 the `SECTION_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 defined `thrift/history.thrift`. This is stored in "compact" thrift encoding. Zero or more history sections are supported. This section type is purely informational and not needed to read the DwarFS image. ### Compression Algorithms DwarFS supports a wide range of block compression algorithms, some of which require additional metadata. The full list of supported algorithms is defined in [`dwarfs/compression.h`](../include/dwarfs/compression.h). For compression algorithms with metadata, the metadata is defined in [`thrift/compression.thrift`](../thrift/compression.thrift). The metadata is stored in "compact" thrift encoding at the beginning of the block, just after the header. ## METADATA FORMAT 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 │ │ | self_entry: 0 | ┌───┤ name_index: 2 │ │ atime_offset: 0 │ │ ├──────────────────┤ ┌────┼───┤ inode_num: 5 ├───────┐ │ mtime_offset: 417 │ │ │ parent_entry: 0 │ │ │ ├────────────────┤ │ │ ctime_offset: 0 │ │ │ first_entry: 11 │ │ ┌──┼───┤ name_index: 3 │ │ ├───────────────────┤ │ | self_entry: 1 | │ │ │ │ inode_num: 9 ├────┐ │ │ ... │ │ ├──────────────────┤ │ │ │ ├────────────────┤ │ │ S_IFLNK ──►├───────────────────┤ │ │ parent_entry: 5 │ │ │ │ │ │ │ │ │ mode_index: 2 │ │ │ first_entry: 12 │ │ │ │ │ ... │ │ └────────────►│ owner_index: 2 │ │ | self_entry: 7 | │ │ │ │ │ │ │ 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. ### Determining Inode Offsets 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. ### Unique and Shared File Inodes 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. ### Traversing the Metadata 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] ## OPTIONALLY PACKED STRUCTURES 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. ### Shared Files Table Packing 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. ### Directories Packing 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. ### Chunk Table Packing 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. ### Names and Symlinks String Table Packing 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](https://github.com/cwida/fsst) 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. ### Binary Metadata Format Details The binary metadata is stored using [Frozen2](https://github.com/facebook/fbthrift/blob/main/thrift/lib/cpp2/frozen/Frozen.h). This format is, unfortunately, not really documented. Also, as of now, there is only a C++ implementation to read or write this format. To interpret the binary data in the `METADATA_V2` block, both the thrift definitions in [`metadata.thrift`](../thrift/metadata.thrift) and the [schema](https://github.com/facebook/fbthrift/blob/main/thrift/lib/thrift/frozen.thrift) from the `METADATA_V2_SCHEMA` block are needed. You can inspect the schema using `dwarfsck` in two different ways. First, as a "raw" schema dump: ``` $ dwarfsck image.dwarfs -d schema_raw_dump Schema { 4: fileVersion (i32) = 1, 1: relaxTypeChecks (bool) = true, 2: layouts (map) = map[44] { 0 -> Layout { 1: size (i32) = 0, 2: bits (i16) = 6, 3: fields (map) = map[0] { }, 4: typeName (string) = "", }, 1 -> Layout { 1: size (i32) = 0, 2: bits (i16) = 5, 3: fields (map) = map[0] { }, 4: typeName (string) = "", }, 2 -> Layout { 1: size (i32) = 0, 2: bits (i16) = 12, 3: fields (map) = map[0] { }, 4: typeName (string) = "", }, 3 -> Layout { 1: size (i32) = 0, 2: bits (i16) = 11, 3: fields (map) = map[0] { }, 4: typeName (string) = "", }, 4 -> Layout { 1: size (i32) = 0, 2: bits (i16) = 23, 3: fields (map) = map[2] { 2 -> Field { 1: layoutId (i16) = 2, 2: offset (i16) = 0, }, 3 -> Field { 1: layoutId (i16) = 3, 2: offset (i16) = -12, }, }, 4: typeName (string) = "", }, 5 -> Layout { 1: size (i32) = 0, 2: bits (i16) = 11, 3: fields (map) = map[3] { 1 -> Field { 1: layoutId (i16) = 0, 2: offset (i16) = -5, }, 2 -> Field { 1: layoutId (i16) = 1, 2: offset (i16) = 0, }, 3 -> Field { 1: layoutId (i16) = 4, 2: offset (i16) = 0, }, }, 4: typeName (string) = "", }, [...] 43 -> Layout { 1: size (i32) = 36, 2: bits (i16) = 282, 3: fields (map) = map[19] { 1 -> Field { 1: layoutId (i16) = 5, 2: offset (i16) = 0, }, 2 -> Field { 1: layoutId (i16) = 8, 2: offset (i16) = -11, }, 3 -> Field { 1: layoutId (i16) = 12, 2: offset (i16) = -23, }, [...] }, 4: typeName (string) = "", }, }, 3: rootLayout (i16) = 43, } ``` To make *any* sense of this, you need to look at the [`metadata.thrift`](../thrift/metadata.thrift) with the explicit knowledge that the `rootLayout` in the schema refers to the `struct metadata` in the thrift IDL. With that in mind, you can now see that the `struct metadata` itself uses 36 bytes (or 282 bits) of storage. By definition, these bytes are located at the start of the `METADATA_V2` block data. Note that these sizes are *solely* defined by the schema; another DwarFS image may store the `struct metadata` in fewer or more bits. You can also line up the `fields` map in the `Layout` of `struct metadata` with the fields from the thrift IDL. While the *names* of the struct members can change, the numeric id *never* changes. So you can see that field `1` refers to the `chunks` member. You can also see that the layout for that field is `5`, which can be looked up again in the `layouts` map of the schema. The tricky bit is that layout `5` does *not* refer to the `struct chunk` in the IDL, but *actually* to the `list`. A `list` (or an `ArrayLayout` in Frozen2) is represented using 3 fields: `distance` (`1`), `count` (`2`) and `item` (`3`). `count` is just the actual length of the list/array/vector. `distance` is the offset at which the data for the list starts. And `item` finally refers to the layout for the `struct chunk`, in this case `4`. Layout `4` contains 2 out of the 3 members of `struct chunk`: `offset` (`2`) and `size` (`3`). The first member, `block`, is missing simply because there is only one block in the DwarFS image we're looking at. Thus, no bits are used to represent the `block` member in `struct chunk`. For `offset`, 12 bits are allocated per item and for `size`, 11 bits are allocated. Now, if we look at a hex dump of the `METADATA_V2` block, we have enough context to navigate the data: ``` v offset 0 91 ac 55 b6 3e 2b 1a b2 c8 24 69 92 |......U.>+...$i.| | | | `-- 0b10101100 | vvv ^^^ -> 0b100100 = distance = 36 `-- 0b10010001 ^^^^^ count = 17 be 82 f7 0b 00 00 73 fa c3 2e db 6e 4b 7e 17 3e |......s....nK~.>| v offset 36 6c 0d 77 b9 51 ef eb 02 a6 2a 00 4b 15 40 2d d0 |l.w.Q....*.K.@-.| | | | | | `- 0b00000000 | `---- 0b00101010 0b00000000010 = size = 2 `------- 0b10100110 0b101010100110 = offset = 2726 0f 53 05 80 aa 02 70 55 04 88 aa 00 3c 55 00 aa |.S....pU....`. We know that the `count` is represented using 5 bits starting at offset 0. Reading the actual bits, we find that there are 17 chunks stored in the metadata. Reading the 6 `distance` bits starting at an offset of 5 bits (negative offsets are "bits", while positive offsets are "bytes"), we find that the 17 chunks are stored starting at the 36th byte. If we move to that location and read 12 bits for the chunk `offset` and 11 bits of the chunk `size`, we find that the first chunk is 2 bytes from offset 2726 in block 0. Another option to look at the schema is via `frozen_layout`: ``` $ dwarfsck image.dwarfs -d frozen_layout 36 byte (with 282 bits) ::dwarfs::thrift::metadata::metadata chunks @ start 11 bit range of std::vector > distance @ bit 5 6 bit packed unsigned unsigned long count @ start 5 bit packed unsigned unsigned long item @ start 23 bit ::dwarfs::thrift::metadata::chunk block @ start empty packed unsigned unsigned int offset @ start 12 bit packed unsigned unsigned int size @ bit 12 11 bit packed unsigned unsigned int directories @ bit 11 12 bit range of std::vector > distance @ bit 5 7 bit packed unsigned unsigned long count @ start 5 bit packed unsigned unsigned long item @ start 12 bit ::dwarfs::thrift::metadata::directory parent_entry @ start 6 bit packed unsigned unsigned int first_entry @ bit 6 6 bit packed unsigned unsigned int self_entry @ start empty packed unsigned unsigned int [...] ``` This makes a lot more sense now that we've already looked at the raw schema dump. This representation already associates the types from the thrift IDL with the layouts in the schema. ## AUTHOR Written by Marcus Holland-Moritz. ## COPYRIGHT Copyright (C) Marcus Holland-Moritz. ## SEE ALSO mkdwarfs(1), dwarfs(1), dwarfsextract(1), dwarfsck(1)