.. include:: global.rst.inc .. _internals: Internals ========= This page documents the internal data structures and storage mechanisms of |project_name|. It is partly based on `mailing list discussion about internals`_ and also on static code analysis. It may not be exactly up to date with the current source code. Repository and Archives ----------------------- |project_name| stores its data in a `Repository`. Each repository can hold multiple `Archives`, which represent individual backups that contain a full archive of the files specified when the backup was performed. Deduplication is performed across multiple backups, both on data and metadata, using `Chunks` created by the chunker using the Buzhash_ algorithm. Each repository has the following file structure: README simple text file telling that this is a |project_name| repository config repository configuration data/ directory where the actual data is stored hints.%d hints for repository compaction index.%d repository index lock.roster and lock.exclusive/* used by the locking system to manage shared and exclusive locks Config file ----------- Each repository has a ``config`` file which which is a ``INI``-style file and looks like this:: [repository] version = 1 segments_per_dir = 10000 max_segment_size = 5242880 id = 57d6c1d52ce76a836b532b0e42e677dec6af9fca3673db511279358828a21ed6 This is where the ``repository.id`` is stored. It is a unique identifier for repositories. It will not change if you move the repository around so you can make a local transfer then decide to move the repository to another (even remote) location at a later time. Keys ---- The key to address the key/value store is usually computed like this: key = id = id_hash(unencrypted_data) The id_hash function is: * sha256 (no encryption keys available) * hmac-sha256 (encryption keys available) Segments and archives --------------------- A |project_name| repository is a filesystem based transactional key/value store. It makes extensive use of msgpack_ to store data and, unless otherwise noted, data is stored in msgpack_ encoded files. Objects referenced by a key are stored inline in files (`segments`) of approx. 5MB size in numbered subdirectories of ``repo/data``. They contain: * header size * crc * size * tag * key * data Segments are built locally, and then uploaded. Those files are strictly append-only and modified only once. Tag is either ``PUT``, ``DELETE``, or ``COMMIT``. A segment file is basically a transaction log where each repository operation is appended to the file. So if an object is written to the repository a ``PUT`` tag is written to the file followed by the object id and data. If an object is deleted a ``DELETE`` tag is appended followed by the object id. A ``COMMIT`` tag is written when a repository transaction is committed. When a repository is opened any ``PUT`` or ``DELETE`` operations not followed by a ``COMMIT`` tag are discarded since they are part of a partial/uncommitted transaction. The manifest ------------ The manifest is an object with an all-zero key that references all the archives. It contains: * version * list of archive infos * timestamp * config Each archive info contains: * name * id * time It is the last object stored, in the last segment, and is replaced each time. The archive metadata does not contain the file items directly. Only references to other objects that contain that data. An archive is an object that contains: * version * name * list of chunks containing item metadata * cmdline * hostname * username * time Each item represents a file, directory or other fs item and is stored as an ``item`` dictionary that contains: * path * list of data chunks * user * group * uid * gid * mode (item type + permissions) * source (for links) * rdev (for devices) * mtime * xattrs * acl * bsdfiles ``ctime`` (change time) is not stored because there is no API to set it and it is reset every time an inode's metadata is changed. All items are serialized using msgpack and the resulting byte stream is fed into the same chunker used for regular file data and turned into deduplicated chunks. The reference to these chunks is then added to the archive metadata. A chunk is stored as an object as well, of course. Chunks ------ The |project_name| chunker uses a rolling hash computed by the Buzhash_ algorithm. It triggers (chunks) when the last HASH_MASK_BITS bits of the hash are zero, producing chunks of 2^HASH_MASK_BITS Bytes on average. create --chunker-params CHUNK_MIN_EXP,CHUNK_MAX_EXP,HASH_MASK_BITS,HASH_WINDOW_SIZE can be used to tune the chunker parameters, the default is: - CHUNK_MIN_EXP = 10 (minimum chunk size = 2^10 B = 1 kiB) - CHUNK_MAX_EXP = 23 (maximum chunk size = 2^23 B = 8 MiB) - HASH_MASK_BITS = 16 (statistical medium chunk size ~= 2^16 B = 64 kiB) - HASH_WINDOW_SIZE = 4095 [B] (`0xFFF`) The default parameters are OK for relatively small backup data volumes and repository sizes and a lot of available memory (RAM) and disk space for the chunk index. If that does not apply, you are advised to tune these parameters to keep the chunk count lower than with the defaults. The buzhash table is altered by XORing it with a seed randomly generated once for the archive, and stored encrypted in the keyfile. This is to prevent chunk size based fingerprinting attacks on your encrypted repo contents (to guess what files you have based on a specific set of chunk sizes). Indexes / Caches ---------------- The files cache is stored in ``cache/files`` and is indexed on the ``file path hash``. At backup time, it is used to quickly determine whether we need to chunk a given file (or whether it is unchanged and we already have all its pieces). It contains: * age * file inode number * file size * file mtime_ns * file content chunk hashes The inode number is stored to make sure we distinguish between different files, as a single path may not be unique across different archives in different setups. The files cache is stored as a python associative array storing python objects, which generates a lot of overhead. The chunks cache is stored in ``cache/chunks`` and is indexed on the ``chunk id_hash``. It is used to determine whether we already have a specific chunk, to count references to it and also for statistics. It contains: * reference count * size * encrypted/compressed size The repository index is stored in ``repo/index.%d`` and is indexed on the ``chunk id_hash``. It is used to determine a chunk's location in the repository. It contains: * segment (that contains the chunk) * offset (where the chunk is located in the segment) The repository index file is random access. Hints are stored in a file (``repo/hints.%d``). It contains: * version * list of segments * compact hints and index can be recreated if damaged or lost using ``check --repair``. The chunks cache and the repository index are stored as hash tables, with only one slot per bucket, but that spreads the collisions to the following buckets. As a consequence the hash is just a start position for a linear search, and if the element is not in the table the index is linearly crossed until an empty bucket is found. When the hash table is almost full at 90%, its size is doubled. When it's almost empty at 25%, its size is halved. So operations on it have a variable complexity between constant and linear with low factor, and memory overhead varies between 10% and 300%. Indexes / Caches memory usage ----------------------------- Here is the estimated memory usage of |project_name|: chunk_count ~= total_file_size / 2 ^ HASH_MASK_BITS repo_index_usage = chunk_count * 40 chunks_cache_usage = chunk_count * 44 files_cache_usage = total_file_count * 240 + chunk_count * 80 mem_usage ~= repo_index_usage + chunks_cache_usage + files_cache_usage = chunk_count * 164 + total_file_count * 240 All units are Bytes. It is assuming every chunk is referenced exactly once (if you have a lot of duplicate chunks, you will have less chunks than estimated above). It is also assuming that typical chunk size is 2^HASH_MASK_BITS (if you have a lot of files smaller than this statistical medium chunk size, you will have more chunks than estimated above, because 1 file is at least 1 chunk). If a remote repository is used the repo index will be allocated on the remote side. E.g. backing up a total count of 1Mi files with a total size of 1TiB. a) with create --chunker-params 10,23,16,4095 (default): mem_usage = 2.8GiB b) with create --chunker-params 10,23,20,4095 (custom): mem_usage = 0.4GiB Note: there is also the --no-files-cache option to switch off the files cache. You'll save some memory, but it will need to read / chunk all the files then as it can not skip unmodified files then. Encryption ---------- AES_ is used in CTR mode (so no need for padding). A 64bit initialization vector is used, a `HMAC-SHA256`_ is computed on the encrypted chunk with a random 64bit nonce and both are stored in the chunk. The header of each chunk is : ``TYPE(1)`` + ``HMAC(32)`` + ``NONCE(8)`` + ``CIPHERTEXT``. Encryption and HMAC use two different keys. In AES CTR mode you can think of the IV as the start value for the counter. The counter itself is incremented by one after each 16 byte block. The IV/counter is not required to be random but it must NEVER be reused. So to accomplish this |project_name| initializes the encryption counter to be higher than any previously used counter value before encrypting new data. To reduce payload size, only 8 bytes of the 16 bytes nonce is saved in the payload, the first 8 bytes are always zeros. This does not affect security but limits the maximum repository capacity to only 295 exabytes (2**64 * 16 bytes). Encryption keys are either derived from a passphrase or kept in a key file. The passphrase is passed through the ``BORG_PASSPHRASE`` environment variable or prompted for interactive usage. Key files --------- When initialized with the ``init -e keyfile`` command, |project_name| needs an associated file in ``$HOME/.borg/keys`` to read and write the repository. The format is based on msgpack_, base64 encoding and PBKDF2_ SHA256 hashing, which is then encoded again in a msgpack_. The internal data structure is as follows: version currently always an integer, 1 repository_id the ``id`` field in the ``config`` ``INI`` file of the repository. enc_key the key used to encrypt data with AES (256 bits) enc_hmac_key the key used to HMAC the encrypted data (256 bits) id_key the key used to HMAC the plaintext chunk data to compute the chunk's id chunk_seed the seed for the buzhash chunking table (signed 32 bit integer) Those fields are processed using msgpack_. The utf-8 encoded passphrase is processed with PBKDF2_ (SHA256_, 100000 iterations, random 256 bit salt) to give us a derived key. The derived key is 256 bits long. A `HMAC-SHA256`_ checksum of the above fields is generated with the derived key, then the derived key is also used to encrypt the above pack of fields. Then the result is stored in a another msgpack_ formatted as follows: version currently always an integer, 1 salt random 256 bits salt used to process the passphrase iterations number of iterations used to process the passphrase (currently 100000) algorithm the hashing algorithm used to process the passphrase and do the HMAC checksum (currently the string ``sha256``) hash the HMAC of the encrypted derived key data the derived key, encrypted with AES over a PBKDF2_ SHA256 key described above The resulting msgpack_ is then encoded using base64 and written to the key file, wrapped using the standard ``textwrap`` module with a header. The header is a single line with a MAGIC string, a space and a hexadecimal representation of the repository id. Compression ----------- |project_name| currently always pipes all data through a zlib compressor which supports compression levels 0 (no compression, fast) to 9 (high compression, slow). See ``borg create --help`` about how to specify the compression level and its default. Note: zlib level 0 creates a little bit more output data than it gets as input, due to zlib protocol overhead.