.. 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. |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 `Segments` chunked with the Buzhash_ algorithm. Each repository has the following file structure: README simple text file describing the repository config description of the repository, includes the unique identifier. also acts as a lock file data/ directory where the actual data (`segments`) is stored hints.%d undocumented index.%d cache of the file indexes. those files can be regenerated with ``check --repair`` Indexes and memory usage ------------------------ Repository index 40 bytes x N ~ 200MB (If a remote repository is used this will be allocated on the remote side) Chunk lookup index 44 bytes x N ~ 220MB File chunk cache probably 80-100 bytes x N ~ 400MB The chunk lookup index (chunk hash -> reference count, size, ciphered size ; in file cache/chunk) and the repository index (chunk hash -> segment, offset ; in file ``repo/index.%d``) are stored in a sort of hash table, directly mapped in memory from the file content, 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 table is full at 90% its size is doubled, when it's empty at 25% its size is halfed. So operations on it have a variable complexity between constant and linear with low factor, and memory overhead varies between 10% and 300%. The file chunk cache (file path hash -> age, inode number, size, mtime_ns, chunks hashes ; in file cache/files) is stored as a python associative array storing python objects, which generate a lot of overhead. This takes around 240 bytes per file without the chunk list, to be compared to at most 64 bytes of real data (depending on data alignment), and around 80 bytes per chunk hash (vs 32), with a minimum of ~250 bytes even if only one chunck hash. The inode number is stored to make sure we distinguish between different files, as a single path may not be unique accross different archives in different setups. The ``index.%d`` files are random access but those files can be recreated if damaged or lost using ``check --repair``. Repository structure -------------------- |project_name| is a "filesystem based transactional key value store". Objects referenced by a key (256bits id/hash) are stored in line in files (segments) of size approx 5MB in ``repo/data``. They contain : header size, crc, size, tag, key, data. Tag is either ``PUT``, ``DELETE``, or ``COMMIT``. Segments are built locally, and then uploaded. Those files are strictly append-only and modified only once. 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. And 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 is an object with an id of only zeros (32 bytes), that references all the archives. It contains : version, list of archives, timestamp, config. Each archive 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 contain metadata : version, name, items list, cmdline, hostname, username, time. Each item represents a file or directory or symlink is stored as a ``item`` dictionnary that contains: path, list of 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 archvive metadata. This allows the archive to store many files, beyond the ``MAX_OBJECT_SIZE`` barrier of 20MB. A chunk is an object as well, of course, and its id is the hash of its (unencrypted and uncompressed) content. Hints are stored in a file (repo/hints) and contain: version, list of segments, compact. Chunks ------ |project_name| uses a rolling checksum with Buzhash_ algorithm, with window size of 4095 bytes, with a minimum of 1024, and triggers when the last 16 bits of the checksum are null, producing chunks of 64kB on average. All these parameters are fixed. The buzhash table is altered by XORing it with a seed randomly generated once for the archive, and stored encrypted in the keyfile. Repository config file ---------------------- Each repository has a ``config`` file which which is a ``INI`` formatted file which 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 in another (even remote) location at a later time. |project_name| will do a POSIX read lock on that file when operating on the repository. Encryption ---------- AES_ is used with CTR mode of operation (so no need for padding). A 64 bits initialization vector is used, a SHA256_ based HMAC_ is computed on the encrypted chunk with a random 64 bits 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 Attic 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 a passphrase, passed through the ``ATTIC_PASSPHRASE`` environment or prompted on the commandline, or stored in automatically generated key files. Key files --------- When initialized with the ``init -e keyfile`` command, |project_name| needs an associated file in ``$HOME/.attic/keys`` to read and write the repository. As with most crypto code in |project_name|, the format of those files is defined in `attic/key.py`_. The format is based on msgpack_, base64 encoding and PBKDF2_ SHA256 encryption, 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 resulting AES-encrypted data (256 bits) id_key the key used to HMAC the above chunks, the resulting hash is stored out of band (256 bits) chunk_seed the seed for the buzhash chunking table (signed 32 bit integer) Those fields are encoded using msgpack_. The utf-8-encoded phassphrase is encrypted with a PBKDF2_ and SHA256_ using 100000 iterations and a random 256 bits 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 encrypt the passphrase iterations number of iterations used to encrypt the passphrase (currently 100000) algorithm the hashing algorithm used to encrypt the passphrase and do the HMAC checksum (currently the string ``sha256``) hash the HMAC checksum of the encrypted derived key data the derived key, encrypted with AES over a PBKDF2_ SHA256 hash described above The resulting msgpack_ is then encoded using base64 and written to the key file, wrapped using the textwrap_ module with a header. The header is a single line with the string ``ATTIC_KEY``, a space and a hexadecimal representation of the repository id.