improve internals docs

docs/internals.rst

@@ -6,38 +6,43 @@ 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.
+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 `Segments` chunked with the Buzhash_
-algorithm. Each repository has the following file structure:
+data and metadata, using `Chunks` created by the chunker using the Buzhash_
+algorithm.
+
+Each repository has the following file structure:
 
 README
-  simple text file describing the repository
+  simple text file telling that this is a |project_name| repository
 
 config
-  description of the repository, includes the unique identifier. also
-  acts as a lock file
+  repository configuration and lock file
 
 data/
-  directory where the actual data (`segments`) is stored
+  directory where the actual data is stored
 
 hints.%d
-  undocumented
+  hints for repository compaction
 
 index.%d
-  cache of the file indexes. those files can be regenerated with
-  ``check --repair``
+  repository index
+
 
 Config file
 -----------
 
-Each repository has a ``config`` file which which is a ``INI``
-formatted file which looks like this::
+Each repository has a ``config`` file which which is a ``INI``-style file
+and looks like this::
 
     [repository]
     version = 1
@@ -48,20 +53,35 @@ formatted file which looks like this::
 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.
+the repository to another (even remote) location at a later time.
 
-|project_name| will do a POSIX read lock on that file when operating
+|project_name| will do a POSIX read lock on the config file when operating
 on the repository.
 
+
+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
 ---------------------
 
-|project_name| is a "filesystem based transactional key value
-store". It makes extensive use of msgpack_ to store data and, unless
+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 (256bits id/hash) are stored inline in
-files (`segments`) of size approx 5MB in ``repo/data``. They contain:
+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
@@ -77,21 +97,26 @@ 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. And if an object is deleted a ``DELETE`` tag is appended
+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 is an object with an id of only zeros (32 bytes), that
-references all the archives. It contains:
+
+The manifest
+------------
+
+The manifest is an object with an all-zero key that references all the
+archives.
+It contains:
 
 * version
-* list of archives
+* list of archive infos
 * timestamp
 * config
 
-Each archive contains:
+Each archive info contains:
 
 * name
 * id
@@ -102,21 +127,21 @@ 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:
+object that contains:
 
 * version
 * name
-* items list
+* list of chunks containing item metadata
 * cmdline
 * hostname
 * username
 * time
 
-Each item represents a file or directory or
-symlink is stored as an ``item`` dictionary that contains:
+Each item represents a file, directory or other fs item and is stored as an
+``item`` dictionary that contains:
 
 * path
-* list of chunks
+* list of data chunks
 * user
 * group
 * uid
@@ -135,124 +160,136 @@ 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. This allows the archive to store many files,
-beyond the ``MAX_OBJECT_SIZE`` barrier of 20MB.
+to the archive metadata.
 
-A chunk is an object as well, of course. The chunk id is either 
-HMAC-SHA256_, when encryption is used, or a SHA256_ hash otherwise.
+A chunk is stored as an object as well, of course.
 
-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 (`0xFFF`), 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.
+|project_name| uses a rolling hash computed by the Buzhash_ algorithm, with a
+window size of 4095 bytes (`0xFFF`), with a minimum chunk size of 1024 bytes.
+It triggers (chunks) when the last 16 bits of the hash are zero, producing
+chunks of 64kiB on average.
+
+The buzhash table is altered by XORing it with a seed randomly generated once
+for the archive, and stored encrypted in the keyfile.
+
 
-Indexes
--------
+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.
 
-There are two main indexes: the chunk lookup index and the repository
-index. There is also the file chunk cache.
+The files cache is stored as a python associative array storing
+python objects, which generates a lot of overhead.
 
-The chunk lookup index is stored in ``cache/chunk`` and is indexed on
-the ``chunk hash``. It contains:
+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
-* ciphered size
+* encrypted/compressed size
 
-The repository index is stored in ``repo/index.%d`` and is also
-indexed on ``chunk hash`` and contains:
+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
-* offset
+* segment (that contains the chunk)
+* offset (where the chunk is located in the segment)
 
-The repository index files are random access but those files can be
-recreated if damaged or lost using ``check --repair``.
+The repository index file is random access.
 
-Both indexes are stored as hash tables, 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%.
+Hints are stored in a file (``repo/hints.%d``).
+It contains:
 
-The file chunk cache is stored in ``cache/files`` and is indexed on
-the ``file path hash`` and contains:
+* version
+* list of segments
+* compact
 
-* age
-* inode number
-* size
-* mtime_ns
-* chunks hashes
+hints and index can be recreated if damaged or lost using ``check --repair``.
 
-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 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|:
 
-The file chunk cache 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 chunk hash.
+  chunk_count ~= total_file_size / 65536
 
-Indexes memory usage
---------------------
+  repo_index_usage = chunk_count * 40
 
-Here is the estimated memory usage of |project_name| when using those
-indexes.
+  chunks_cache_usage = chunk_count * 44
 
-Repository index
-  40 bytes x N ~ 200MB (If a remote repository is
-  used this will be allocated on the remote side)
+  files_cache_usage = total_file_count * 240 + chunk_count * 80
 
-Chunk lookup index
-  44 bytes x N ~ 220MB
+  mem_usage ~= repo_index_usage + chunks_cache_usage + files_cache_usage
+             = total_file_count * 240 + total_file_size / 400
 
-File chunk cache
-  probably 80-100 bytes x N ~ 400MB
+All units are Bytes.
+
+It is assuming every chunk is referenced exactly once and that typical chunk size is 64kiB.
+
+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:
+
+  mem_usage  =  1 * 2**20 * 240  +  1 * 2**40 / 400  =  2.8GiB
+
+Note: there is a commandline option to switch off the files cache. You'll save
+some memory, but it will need to read / chunk all the files then.
 
-In the above we assume 350GB of data that we divide on an average 64KB
-chunk size, so N is around 5.3 million.
 
 Encryption
 ----------
 
-AES_ is used with CTR mode of operation (so no need for padding). A 64
-bits initialization vector is used, a `HMAC-SHA256`_ 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 |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 zeroes. 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
-``BORG_PASSPHRASE`` environment or prompted on the commandline, or
-stored in automatically generated key files.
+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
 ---------
@@ -274,22 +311,20 @@ 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)
+  the key used to HMAC the encrypted data (256 bits)
 
 id_key
-  the key used to HMAC the above chunks, the resulting hash is
-  stored out of band (256 bits)
+  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 phassphrase
-is encrypted with 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:
+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
@@ -315,3 +350,9 @@ 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
+-----------
+
+Currently, zlib level 6 is used as compression.